This online seminar takes place on Tuesdays, starting at UTC 14.

Current organisers are Margherita Disertori (Bonn), Wojciech Dybalski (Poznan), Ian Jauslin (Rutgers), and Hal Tasaki (Gakushuin).

Scientific committee: Nalini Anantharaman (Strassbourg), Mihalis Dafermos (Cambridge), Stephan De Bièvre (Lille), Jan Dereziński (Warsaw), Bernard Helffer (Nantes), Vojkan Jaksic (McGill), Flora Koukiou (Cergy), Antti Kupiainen (Helsinki), Mathieu Lewin (Paris Dauphine), Bruno Nachtergaele (UC Davis), Claude-Alain Pillet (Toulon), Marcello Porta (SISSA Trieste), Kasia Rejzner (York), Robert Seiringer (IST Austria), Jan Philip Solovej (Copenhagen), Daniel Ueltschi (Warwick).

If you would like to receive seminar announcements, please send an email to [email protected] with “subscribe” in the subject line; or “unsubscribe” to have your email address removed. You can also email comments or suggestions.

Our YouTube channel is https://www.youtube.com/@iamp_seminars.
Other *One World seminars* are listed on the website of the probability seminar, which initiated the series.
The researchseminars.org website lists further mathematical-physics seminars.

September 10, 2024 |
Sky Cao (MIT)Global well-posedness of stochastic Abelian-Higgs in two dimensionsLink will be available at https://researchseminars.org/talk/IAMP_seminars/162/ Show abstract There has been much recent progress on the local solution theory for geometric singular SPDEs. However, the global theory is still largely open. In my talk, I will discuss the global well-posedness of the stochastic Abelian-Higgs model in two dimensions, which is a geometric singular SPDE arising from gauge theory. The proof is based on a new covariant approach, which consists of two parts: First, we introduce covariant stochastic objects, which are controlled using covariant heat kernel estimates. Second, we control nonlinear remainders using a covariant monotonicity formula, which is inspired by earlier work of Hamilton. Joint work with Bjoern Bringmann. |

September 24, 2024 |
David Mitrouskas (ISTA)TBALink will be available at https://researchseminars.org/talk/IAMP_seminars/161/ |

July 30, 2024 |
Robert Wald (Chicago)The Memory Effect and Infrared DivergencesVideo link: https://youtu.be/BdC5q-sJL5s Show abstract The "memory effect" is the permanent relative displacement of test particles after the passage of gravitational radiation. It is associated with both the propagation of massive bodies out to timelike infinity ("ordinary memory") or the propagation of radiation out to null infinity ("null memory"). The memory effect can be characterized by the failure of the shear tensor at order 1/r to return to zero at late times, even though it is "pure gauge." Closely analogous effects occur in electromagnetism, where the vector potential at order 1/r fails to return to zero even though it is "pure gauge." In both cases, the Fourier transform of the radiative field has divergent behavior at low frequencies. This gives rise to infrared divergences (i.e., infinite numbers of "soft" gravitons/photons) in the quantum field theory description if one attempts to describe these states as vectors in the usual Fock Hilbert space representation. To obtain a mathematically sensible quantum scattering theory, one must allow states with nonvanishing memory in the "in" and "out" Hilbert spaces. An elegant solution to this problem in massive quantum electrodynamics was given by Kulish and Fadeev, who constructed a Hilbert space of incoming/outgoing charged particle states that are "dressed" with radiative fields of corresponding memory, so as to yield vanishing large gauge charges at spatial infinity. However, we show that this type of construction fails in quantum gravity. The primary underlying reason is that the ``dressing'' contributes to null memory, thereby invalidating the construction of eigenstates of large gauge charges. In quantum gravity, there does not appear to be any choice of (separable) Hilbert space of incoming/outgoing states that can accommodate all scattering states.etermining the set of all Wigner-positive states and of all Kirkwood-Dirac positive states. |

June 18, 2024 |
Stephan De Bièvre (Lille)Assessing optical nonclassicality with the quadrature coherence scaleVideo link: https://youtu.be/OZUcHEbLuxk Show abstract In quantum optics, a state is said to be ``classical’’ if it is a mixture of coherent states. Equivalently, such states have a positive Glauber-Sudarshan P-function. It is a longstanding problem in quantum optics, that has attracted renewed attention in the context of quantum information theory, to efficiently identify/characerize the convex set of all such states. After reviewing some of the history of this problem, we shall in this talk present the ``quadrature coherence scale’’, which provides a measure of non-classicality (in the above sense), and detail some of its merits. We will further briefly discuss the analogous problems of determining the set of all Wigner-positive states and of all Kirkwood-Dirac positive states. |

June 4, 2024 |
Niels Benedikter (Milan)Bosonization of Large Systems of Interacting FermionsVideo link: https://youtu.be/cPO9JXlyYmk Show abstract The behavior of electrons in a metal presents a wide variety of emergent behavior including a number of phase transitions. The mean-field scaling limit acts as a simplified model capturing part of this complexity. In this limit, results going beyond the precision of Hartree-Fock theory have recently been obtained by bosonization methods. I will review the expansion of the ground state energy and present results extending to the dynamics and momentum distribution of excitations. |

May 14, 2024 |
Roderich Tumulka (Tübingen)Time evolution of closed macroscopic quantum systems towards thermal equilibriumVideo link: https://youtu.be/aDNRdSc2y60 Show abstract Suppose we make a hot brick and a cold brick touch each other and isolate them from the rest of the world. Then, of course, energy will be transported from the hotter to the cooler until they reach the same temperature. This example illustrates the sense in which an isolated, macroscopic quantum system in a pure state will approach thermal equilibrium. Here, different macro-states correspond to mutually orthogonal subspaces of Hilbert space, each of very high dimension, thermal equilibrium to one of these subspaces, and entropy to the logarithm of the dimension. I will describe results concerning the unitary time evolution of such systems towards thermal equilibrium, the role played by a version of the eigenstate thermalization hypothesis, and some open questions. |

April 30, 2024 |
Aernout van Enter (Groningen)Dyson models: One-sided and two-sided aspectsVideo link: https://youtu.be/vvptE99DE5Y Show abstract I will discuss Dyson models, long-range one-dimensional Ising spin systems with ferromagnetic pair interactions. I first will discuss how they provide an example for non-equivalence of one-sided and two-sided regularity properties. The switching between one-sided and two-sided descriptions lies at the base of Thermodynamic Formalism, which describes various Dynamical Systems as one-dimensional statistical mechanics models, usually with fast decaying interactions.. In particular at low temperatures there can be differences in continuity properties between a measure's one-sided and two-sided conditional probabilities. Gibbs measures for summable interactions can be characterised by the continuity of their two-sided (from both left and right) conditional probabilities, The one-sided analogue, where a spin depends continuously on the left only (the past), forms the class of g-measures. It was known before that a g-measure need not be a Gibbs measure. We show the opposite, that a Gibbs measure for a summable interaction, and in particular for a low-temperature Dyson interaction, can fail to be a g-measure. In a somewhat different usage, one-sided versus two-sided systems describes models on a half-line as compared with the model on the whole line. One can consider the model on the whole line as a model on two half-lines with a coupling term between the half-lines. Properties of this coupling term, connecting either ordered or disordered half-lines, such as its boundedness, and integrability, explain the properties of the "metastates" -- the distribution over limit states- of the model with random boundary conditions at low temperatures. Also they explain the continuity, or the lack thereof, of the eigenfunctions of Ruelle transfer operators at high temperatures. Continuity of these eigenfunctions is a sufficient, but not necessary, condition for continuity of the one-sided conditional probabilities. We show that in certain circumstances the Radon-Nikodym density on the half-line, between coupled and decoupled Gibbs measures -- which is equal to the transfer operator eigenfunction--, can have stronger regularity properties than the analogous Radon-Nikodym density on the whole line. The talk is based on joint works with Rodrigo Bissacot, Eric Endo, Roberto Fernández, Arnaud Le Ny, Mirmukhsin Makhmudov and Evgeny Verbitskiy |

April 16, 2024 |
Kasia Rejzner (York)Renormalisation of gauge theories in perturbative algebraic quantum field theoryVideo link: https://youtu.be/Ne8wv2F7vv0 Show abstract In this talk I will briefly introduce the framework of perturbative algebraic quantum field theory (pAQFT) and explain how it can be used in quantisation of gauge theories. This requires methods from functional analysis as well as homological algebra. Some new developments in this area involve treatment of theories on manifolds with boundary, including the "boundary at infinity." The latter case allows the discussion of asymptotic symmetries, e.g. in quantum electrodynamics. |

April 2, 2024 |
Noé Cuneo (LPSM)Large deviations of return times and related entropy estimators on shift spacesVideo link: https://youtu.be/7U_z1IkVfuE Show abstract The study of return times in dynamical systems has a long history, and their role as entropy estimators is well established. While considerable attention has been devoted to the corresponding Law of Large Numbers, Central Limit Theorem and pressure function in the literature, surprisingly little was known about their large deviations. In fact, only local versions of the Large Deviation Principle (LDP) were obtained, under some rather restrictive assumptions. I will talk about a recent work in collaboration with Renaud Raquépas (arXiv:2306.05277, to appear in Commun. Math. Phys.), where we prove that return times satisfy the full LDP for a large class of invariant measures on shift spaces. Our assumptions cover in particular irreducible Markov chains, equilibrium measures for Bowen-regular potentials, invariant Gibbs states for absolutely summable interactions in statistical mechanics (including in the phase-transition regime), and some repeated quantum measurement processes. As we will see in a striking example, the rate function governing the LDP can be nonconvex. |

March 19, 2024 |
Claude-Alain Pillet (Toulon)Adiabatic Time Evolution and Quasi-Static Processes in Translation-Invariant Quantum SystemsVideo link: https://youtu.be/6v7M1sLxkzM Show abstract We study the slowly varying, non-autonomous dynamics of a translation-invariant quantum spin system on the lattice Z^d . This system is assumed to be initially in thermal equilibrium, and we consider realizations of quasi-static processes in the adiabatic limit. By combining the Gibbs variational principle with the notion of quantum weak Gibbs states, we establish a number of general structural results regarding such realizations. In particular, we show that such a quasi-static process is incompatible with the property of approach to equilibrium as discussed by V. Jaksic in his talk on March 22, 2022. This is a joint work with V. Jaksic and C. Tauber |

March 5, 2024 |
Rainer Verch (Leipzig)Moving away from equilibriumVideo link: https://youtu.be/1qV2lXQcSZY Show abstract The seminar takes up on several aspects related to the Unruh effect. In a nutshell, a mathematically rigorous treatment of the Unruh effect by de Biévre and Merkli has shown that a quantum mechanical, finite-energy level "detector" system which moves with constant acceleration with respect to an inertial system, and is locally coupled to a quantum field in the inertial vacuum state, will asymptotically for large times approximate a Gibbs (thermal equilibrium) state. In a new work with A.G. Passegger, we show that the behaviour of such a detector system is quite different if it moves with constant velocity with respect to an inertial system in which a quantum field is in a KMS state, and to which the detector is locally coupled. Concretely, combining previous results known in the literature, we show that for any quantum field theory in Minkowski spacetime, two primary states KMS states with respect to different inertial frames are disjoint (belong to inequivalent folia of states) if they fulfill a suitable clustering condition. The quantized free scalar field is an example where the conditions are fulfilled. Put differently, Lorentz symmetry is broken (not unitarily implementable) in quantum field theory in inertial thermal (KMS) representations. (This seems to belong to "general wisdom", but appears to have never been fully proved in the literature.) We argue, along the lines of G. Sewell, that the detector moving with constant velocity against the thermal background of a quantum field in an inertial KMS state, will not asymptotically thermalize - at least the return to equilibrium arguments are not applicable in this context because of the broken Lorentz symmetry in inertial KMS states. We also argue that this corroborates the view expressed by D. Buchholz and R. Verch, and others, that the Unruh effect is not due to thermal contact of the detector with a "heat bath" supplied by the quantum field, but is rather due to conversion of work to heat through the coupling of the detector to the quantum field and the externally driven motion of the detector. |

February 20, 2024 |
Alexander Stottmeister (Hannover)Embezzlement of entanglement, quantum fields, and the classification of von Neumann algebrasVideo link: https://youtu.be/XPAWK5qs60g Show abstract We discuss the embezzlement of entanglement in the setting of von Neumann algebras and its relation to the classification of the latter, as well as its application to relativistic quantum field theory. Embezzlement (of entanglement), introduced by van Dam and Hayden, denotes the task of producing any entangled state to arbitrary precision from a shared entangled resource state, the embezzling state, using local operations without communication while perturbing the resource arbitrarily little. We show that Connes' classification of type III von Neumann algebras can be given a quantitative operational interpretation in terms of embezzlement. In particular, this quantification implies that all type III factors, apart from some type III_0 factors, host embezzling states. In contrast, semifinite factors (type I or II) cannot host embezzling states. Specifically, type III_1 factors are characterized as "universal embezzlers“, meaning every normal state is embezzling. The latter observation provides a simple explanation as to why relativistic quantum field theories maximally violate Bell inequalities. Our results follow from a one-to-one correspondence between embezzling states and invariant states on the flow of weights. This is joint work with Lauritz van Luijk, Reinhard F. Werner, and Henrik Wilming. |

February 6, 2024 |
Wei-Min Wang (Shanghai / CNRS)On the dynamics of nonlinear random and quasi-periodic systemsVideo link: https://youtu.be/kKssHnmsuwQ Show abstract We discuss time evolution of nonlinear random and quasi-periodic systems and elaborate on their similarities. Examples of such systems include the nonlinear random and the nonlinear quasi-periodic Schr\"odinger equations on the lattice. These problems are related to and are partly motivated by many body localization. |

January 23, 2024 |
Yoh Tanimoto (Tor Vergata)Wightman fields for two-dimensional conformal field theoryVideo link: https://youtu.be/gT5RPq2yHV4 Show abstract For a class of two-dimensional full conformal field theory, we construct Wightman fields generating the whole theory. The construction is based on non-local charged primary fields that intertwine superselection sectors of the chiral components. We discuss the corresponding Schwinger functions that should satisfy the Osterwalder-Schrader axioms and relations with possibly massive models by the perturbation of the spacetime symmetry. (joint with M.S. Adamo, L. Giorgetti, C. Jäkel and Y. Moriwaki) |

January 9, 2024 |
Charles Newman (NYU -- Courant)Brownian Excursions, Ising Models and the Riemann Hypothesis -- Possible ConnectionsVideo link: https://youtu.be/AdmDRr5eOTs Show abstract The distribution of the maximum of a Brownian excursion (BE), known explicitly since 1976, is related to the Riemann zeta and ksi functions. We discuss how if one could *properly* relate either ksi or BE to an Ising model (IM), the Riemann Hypothesis would follow from the 1952 theorem of Lee and Yang about zeros of IM partition functions. |

December 12, 2023 |
Torben Krüger (Erlangen)Merging Singularity in the Normal Matrix ModelVideo link: https://youtu.be/9BtLzlqZrsg Show abstract The two-dimensional one-component plasma is a particle system in the plane with long-range logarithmic interactions. At a specific temperature the system is equivalent to the eigenvalue ensemble of a normal random matrix model. In equilibrium the particles form droplets when placed in an external potential. Using the Riemann-Hilbert approach we determine the local statistical behaviour of the particles at the merging point of droplets and observe an anisotropic scaling behaviour with particles being much further apart in the direction of merging than the perpendicular direction, effectively generating a one dimensional particle system. Very close to the point of merger this leads to correlations described by the Painleve II kernel that is also present in certain unitary matrix ensembles. Furthermore, in the vicinity of the singularity sine kernel statistics, Ginibre edge and bulk statistics, as well as the local statistics of a Coulomb gas confined to a small strip are observed. Our comprehensive description of the merging singularity lends support to the conjecture that the hierarchy of local particle statistics at singularities of the particle density within two-dimensional Coulomb gases can be characterised in terms of the corresponding hierarchy for one-dimensional invariant ensembles. This is joint work with Meng Yang and Seung-Yeop Lee |

November 28, 2023 |
Sebastien Ott (EPFL)Asymptotic expansion for classical O(N) modelsVideo link: https://youtu.be/lD3c1BKnHmw Show abstract the spin O(N) model on the cubic lattice at small temperature can be seen (at least at a formal level) as a perturbation of a (N-1) vectorial GFF on the lattice. As a result, one expects that it is possible to extract precise informations about many observables from the GFF. A first result going in this direction is to establish that one can expand quantities such as the spontaneous magnetisation in Taylor series (in the temperature), and that the coefficients of this series are the one obtained by formal power series expansion. This was acheived in 1981 by Bricmont, Fontaine, Lebowitz, Lieb, and Spencer in the abelian case of the XY (O(2)) model, using a regularized version of Gaussian integration by part and infrared bounds coming from reflection positivity of the model. I will briefly review their approach, and will present an extension to general N in dimension 3 and higher, highlighting what are the requiered new inputs. Based on joint work with A.Giuliani |

November 14, 2023 |
Nguyen Viet Dang (Sorbonne University)The Phi43 measure on Riemannian manifolds.Video link: https://youtu.be/X0f8jiR3110 Show abstract I will describe a joint work with Bailleul, Ferdinand and To where we construct the $\phi^4_3$ quantum field theory measure on compact Riemannian 3-manifolds, as invariant measure of some stochastic partial differential equation, I will try to motivate the approach and show many examples. This gives an example of nonperturbative, interacting, non topological quantum field theory constructed on 3 manifolds. |

October 31, 2023 |
Horng-Tzer Yau (Harvard University)Spectral gap and two point function estimates for mean-field spin glass modelsVideo link: https://youtu.be/z5Mj6AtM0IU Show abstract In this lecture, we’ll review some recent results regarding spectral gaps and logarithmic Sobolev inequality for Glauber dynamics of mean-field spin glass models. In particular, we will present a method to prove that the spectral gap of the Glauber dynamics is of order one at sufficiently high temperature. In addition, we will review certain estimates on two point functions for the SK model satisfying a modified AT condition. |

October 17, 2023 |
Alberto Cattaneo (University of Zurich)A cohomological view of quantum field theoryVideo link: https://youtu.be/mCUA24yG6mc Show abstract I will review the Batalin–Vilkovisky formalism and its cognates which present classical and quantum field theory via a flexible formalism in which spaces of fields are presented as complexes whose cohomology returns the physical content. Different but equivalent complexes may be used, which turns out to be important conceptuallly and in practice. Examples will be discussed. |

October 3, 2023 |
Sourav Chatterjee (Stanford University)Spin glass phase in the Edwards-Anderson model at zero temperatureVideo link: https://youtu.be/9BSW1JsQl-Y Show abstract While the analysis of mean-field spin glass models has seen tremendous progress in the last twenty years, lattice spin glasses have remained largely intractable. I will talk about recent progress on this topic, giving the first proof of glassy behavior in the Edwards-Anderson model of lattice spin glasses. |

September 19, 2023 |
David Huse (Princeton University)Many-body localizationVideo link: https://youtu.be/19wpP3mas5o Show abstract Many-body localization (MBL) is Anderson localization of many interacting quantum degrees of freedom in highly-excited states at conditions that correspond to a nonzero entropy density at thermal equilibrium. The opposite of MBL is thermalization, where the isolated quantum many-body system successfully acts as a thermal bath for itself, bringing all of its small subsystems to thermal equilibrium with each other via the unitary quantum dynamics of the closed system. For finite systems with short-range interactions the transition from thermalization to MBL occurs in two stages as the interactions are reduced: First is a smooth crossover to a “glassy” prethermal MBL regime, where the thermalization time of a large system becomes extremely large but not infinite. Then, at still weaker interaction is the dynamical phase transition in to the MBL phase, which in many cases occurs at a strength of interactions that is so small that it is thermodynamically insignificant in the limit of large systems, even though it has strong long-time dynamical effects. |

September 5, 2023 |
Marcello Porta (SISSA)Universality of Transport in Many-Body Lattice ModelsVideo link: https://youtu.be/Q3yLcprGPcA Show abstract In this talk I will discuss rigorous results about the charge transport properties of gapless interacting fermionic lattice models, with a particular emphasys on the emergence of universality at the macroscopic scale. I will outline a strategy that has been used over the years to compute response functions for a class of gapless models, including one-dimensional metals and two and three-dimensional semimetals. The approach is based on the combination of: analytic continuation of real-time response functions to imaginary times; renormalization group analysis of imaginary-time correlations and resolution of the scaling limit; lattice conservation laws and Ward identities to prove universality of transport coefficients. I will focus on the application to the edge response function of interacting 2d quantum Hall systems, defined starting from the linear response ansatz. In the last part of the talk, I will discuss how the framework can be used go beyond linear response, for gapped systems at low temperatures. |

June 27, 2023 |
Marius Lemm (University of Tübingen)Light cones for open quantum systemsVideo link: https://youtu.be/qwD2-ya68eg Show abstract We consider non-relativistic Markovian open quantum dynamics (MOQD) in continuous space. We show that, up to small-probability tails, the supports of quantum states evolving under MOQD propagate with finite speed in any finite-energy subspace. More precisely, if the initial quantum state is localized in space, then any finite-energy part of the solution of the von Neumann-Lindblad equation is approximately localized inside an energy-dependent light cone. We also obtain an explicit upper bound on the slope of this light cone (i.e., on the maximal speed). The general method can be used to derive propagation bounds for a variety of other quantum systems including Lieb-Robinson bounds for lattice bosons. Based on joint works with S. Breteaux, J. Faupin, D.H. Ou Yang, I.M. Sigal, and J. Zhang. |

June 13, 2023 |
Scott Armstrong (New York University)Anomalous diffusion for passive scalar equations by fractal homogenizationVideo link: https://youtu.be/XHt9od3vSFE Show abstract In this talk I will describe the main ideas in a recent joint work with Vlad Vicol. We construct an explicit divergence-free vector field on the torus, which is Holder continuous with exponent almost 1/3 in space and time, such that the corresponding advection-diffusion equation admits anomalous diffusion, for all initial data, along a subsequence of diffusivities tending to zero. The construction builds into the vector field a countable sequence of "active scales" which have resonances with the scalar, resulting in a cascade of energy moving to smaller scales. The proof is a renormalization scheme, based on homogenization methods. |

May 30, 2023 |
Flora Koukiou (Cergy–Pontoise University)Freezing and Entropy of the Log-correlated Random fieldsVideo link: https://youtu.be/mvrjV3OFsuU Show abstract Freezing is a concept that was introduced more than thirty years ago and has since been studied in the context of statistical mechanics and, more recently, has been related to multifractality and extreme value statistics. In this talk, after a short survey, we define freezing in relation to the behaviour of the entropy of the Gibbs measures for several mean-field models. |

May 16, 2023 |
Cristina Toninelli (University of Paris Dauphine)Bootstrap percolation and kinetically constrained models: universality resultsVideo link: https://youtu.be/j97kh_XLabo Show abstract Recent years have seen a great deal of progress in understanding the behavior of bootstrap percolation (BP) models, a particular class of monotone cellular automata. In the initial configuration sites are occupied with probability p. The evolution of BP occurs at discrete times: empty sites stay empty and occupied sites are emptied iff a certain model dependent neighborhood is already empty. In the Euclidean lattice there is now a complete understanding of the large time evolution with a universality picture for the critical behavior. Much less is known for their non-monotone stochastic counterpart, namely kinetically constrained models (KCM). In KCM each vertex is updated independently at rate p (respectively 1-p) to occupied (respectively empty) iff it could be emptied in the next step by the bootstrap model. In the last two decades KCM have been the subject of intensive research both in physics and mathematics literature. The main motivation is that, for certain choices of the constraints, when p goes to zero KCM display some of the most striking features of the liquid/glass transition. Indeed, they were originally introduced in the 80's to support the free volume theories and later on used as the simplest lattice models reproducing the dynamical facilitation scenario. In this seminar I will discuss some recent rigorous results on the characteristic time scales of KCM as p goes to zero as well as the connection with the critical behavior of the corresponding BP models. |

May 2, 2023 |
Nikita Nekrasov (Simons Center for Geometry and Physics)Qouroboros: Quantum mechanics from supersymmetric quantum field theoryVideo link: https://youtu.be/oMJmeZR5f58 Show abstract Wavefunctions of the stationary states of some quantum mechanical systems can be exactly computed using supersymmetric gauge theory in dimension 4. I will explain theory and practice of the method. |

April 18, 2023 |
Michał Wrochna (Cergy Paris University)Thermodynamics of quantum fields on rotating black hole spacetimesVideo link: https://youtu.be/YiOm32G7u1I Show abstract What is the final quantum state arising from black hole collapse, and what are its thermodynamical properties? In somewhat idealized models like the Kerr metric, the conjectural candidate is the Unruh state. While it is not an equilibrium state, asymptotically one expects the occurrence of Hawking temperature. In this talk I will review recent results that give a rigorous definition and construction based on scattering theory. While the case of bosonic fields on Kerr spacetime is still open, it turns out that it is possible to prove many physical properties of the Unruh state for instance for massless fermions. The talk is based on joint work with Christian Gérard and Dietrich Häfner. |

April 4, 2023 |
Lingrui Ge (University of California, Irvine)Multiplicative Jensen’s formula and quantitative global theory for ID quasiperiodic operatorsVideo link: https://youtu.be/GT7d2_TG5NA Show abstract We prove multiplicative Jensen’s formula and establish quantitative global theory for analytic one-frequency Schrodinger operators. We will also discuss the motivations and further applications. This is based on a joint work with Jiangong You, Svetlana Jitomirskaya and Qi Zhou. |

March 21, 2023 |
Hermann Schulz-Baldes (Friedrich-Alexander-Universitaet Erlangen-Nuernberg)Pseudo-gaps for random hopping modelsVideo link: https://youtu.be/A95xKkSraw8 Show abstract For one-dimensional random Schrödinger operators, the integrated density of states is known to be given in terms of the (averaged) rotation number of the Prüfer phase dynamics. This paper develops a controlled perturbation theory for the rotation number around an energy at which all the transfer matrices commute and are hyperbolic. Such a hyperbolic critical energy appears in random hopping models. The main result is a Hölder continuity of the rotation number at the critical energy that implies the existence of a pseudo-gap. The proof uses renewal theory. The result is illustrated by numerics. |

March 7, 2023 |
Amanda Young (Technische Universität München)A bulk gap in the presence of edge states for a truncated Haldane pseudopotentialVideo link: https://youtu.be/wnOuDmR4gpQ Show abstract In this talk, we discuss the bulk gap for a truncated 1/3-filled Haldane pseudopotential for the fractional quantum Hall effect in the cylinder geometry. In the case of open boundary conditions, a lower bound on the spectral gap (which is uniform in the volume and particle number) accurately reflects the presence of edge states, which do not persist into the bulk. A uniform lower bound for the Hamiltonian with periodic boundary conditions is also obtain. Both of these bounds are proved by identifying invariant subspaces to which spectral gap and ground state energy estimating methods originally developed for quantum spin Hamiltonians are applied. Customizing the gap technique to the invariant subspace, however, we are able to avoid the edge states and establish a more precise estimate on the bulk gap in the case of periodic boundary conditions. The same approach can also be applied to prove a bulk gap for the analogously truncated Haldane pseudopotential with maximal half filling, which describes a strongly correlated system of spinless bosons in a cylinder geometry. This is based off joint work with S. Warzel. |

February 21, 2023 |
Christian Maes (KU Leuven)Nonequilibrium extension of Third LawVideo link: https://youtu.be/90dwNdnD3U8 Show abstract The Nernst-Planck Postulate (Third Law of Thermodynamics stating that the entropy becomes zero at zero temperature) has a different status from the other Laws: it is not a consequence of the general structure of statistical thermodynamics, it can fail, and it does not carry a straightforward dynamic or kinetic derivation when it does hold. In this talk we consider the latter question, by extending the Third Law to nonequilibrium jump processes. We give sufficient conditions in a Nonequilibrium Nernst Theorem, that the operationally defined excess heat vanishes at absolute zero, and from counter examples, we understand that those conditions are well on target. We give an example of a zero-temperature nonequilibrium transition where the heat capacity abruptly diverges as a function of the chemical potential. The main ingredients in the proof are a formulation of the quasistatic relaxation between nonequilibrium steady conditions,d a combined application of the matrix tree and the matrix forest theorem. Joint work with Faezeh Khodabandehlou and Karel Netocny. |

February 7, 2023 |
Alexander Elgart (Virginia Tech)Localization of the random XXZ spin chain in fixed energy intervalsVideo link: https://youtu.be/iyuSAuwPWdc Show abstract A Schrödinger operator H is known to exhibit quasi-locality: Matrix elements of analytic functions of H decay exponentially away from the diagonal. Localization for a random Schrödinger operator can be expressed as an extension of this feature to Borel measurable functions with support in the region of localization. We show that the spin-½ XXZ random chain Hamiltonian HXXZ manifests a suitably defined notion of quasi-locality as well. We exploit this property to prove that HXXZ exhibits localization in any fixed energy interval (expressed as quasi-locality for Borel measurable functions of HXXZ with support in the energy interval). Localization is proved in a nontrivial region of the parameter space, which includes weak interaction and strong disorder regimes, and is independent of the size of the system, depending only on the energy interval. Based on a joint work with Abel Klein, https://arxiv.org/abs/2210.14873 |

January 24, 2023 |
Christian Hainzl (LMU Munich)Correlation energy of an electron gas in the mean-field approximationVideo link: https://youtu.be/rCkIfhkDAtI Show abstract The correlation energy of a high density fermionic Coulomb gas, called Jellium, is expected to be given by the Gell-Mann Brueckner formula. I will discuss an analogue of this formula for the mean-field regime. I present a rigorous upper bound established by variational methods in the case of Coulomb interaction. I will further review similar results for more regular potentials, which is meanwhile well understood for particles in a fixed box. The talk is based on joint work with Martin Christiansen and P. T. Nam. |

January 10, 2023 |
Jan Dereziński (University of Warsaw)Dirac-Coulomb HamiltoniansVideo link: https://youtu.be/oRtT9r085AA Show abstract I will discuss the theory of Dirac operators perturbed by the Coulomb potential in an arbitrary dimension. These operators are interesting for several reasons: – They have obvious applications to atomic physics. – If one neglects the mass term, they are formally homogeneous. – Depending on the coupling constant and dimension, they exhibit several distinct scaling behaviors (or if you prefer the high energy jargon, various “renormalization group flows”). – Their definition involves subtle boundary conditions. – They are exactly solvable in terms of Whittaker functions. My talk will be based on recent joint work with Błażej Ruba. |

December 20, 2022 |
Massimiliano Gubinelli (University of Oxford)A stochastic analysis of EQFTs: the forward-backwards equation for Grassmann measuresVideo link: https://youtu.be/lMnynWDGJT4 Show abstract I will report on a research program to use ideas from stochastic analysis in the context of constructive quantum field theories. Stochastic analysis can be summarized as the study of measures on path space via push-forward from Gaussian measures. The basic example is the Ito map which sends Brownian motion to a Markov diffusion process solution to a stochastic differential equation. Parisi-Wu stochastic quantisation can be understood as a stochastic analysis of an Euclidean quantum field, in the above sense. In this talk I will focus on another way to introduce such an “Ito map” which has connection to the continuous renormalization group a la Polchinski and which uses a forward-backwards stochastic differential equation. In order to be able to give a full non-perturbative construction I will focus on the case of Grassmann measures seen as instances of non-commutative random fields. |

December 13, 2022 |
Sinya Aoki (YITP, Kyoto University)Lattice QCD: Introduction, results and challengesVideo link: https://youtu.be/xf_4yPuPXHQ Show abstract Lattice QCD is a non-perturbative definition of QCD (Quantum ChromoDynamics) on a discrete spacetime. I first give a comprehensive review on lattice QCD from the continuum QCD to numerical simulations. As a representative of lattice QCD investigations, I introduce hadron mass calculations including an extraction of a hadron mass from the hadron propagator, its quark mass dependence and its extrapolation to a zero lattice spacing limit. I also present the latest results on hadron spectra including electromagnetic corrections. Finally I explain a recent challenge in lattice QCD, an extraction of hadron interactions such as nuclear forces, starting from a definition of potentials in QCD to numerical results on scattering phase shifts. Throughout my talk, I comment on degrees of rigorousness of the lattice QCD method from time to time. |

December 6, 2022 |
John Imbrie (University of Virginia)On the Many-Body Localization TransitionVideo link: https://youtu.be/PlUtM-RNEjU Show abstract A quantum system is said to be many-body localized (MBL) if it fails to thermalize. This can be understood as a consequence of the existence of an extensive set of local integrals of motion (LIOMs). In 2016, I showed that there is an MBL phase by constructing LIOMs at strong disorder (the proof depended on a certain assumption on level statistics). The nature of the transition out of the MBL phase has been a subject of study and conjecture. In recent work with Morningstar and Huse (PRB, 2020), we utilize a series of approximations to develop specific RG flow equations. These are similar to the Kosterlitz-Thouless (KT) flow equations, but there are important differences that place the MBL transition in a new universality class. |

November 29, 2022 |
Camillo De Lellis (IAS Princeton)Anomalous dissipation for the forced Navier-Stokes equationsVideo link: https://youtu.be/EUviqFwNepM Show abstract Consider smooth (or at least Leray) solutions to 3d Navier-Stokes \[ \left\{ \begin{array}{ll} \partial_t u^\varepsilon + {\rm div}\, (u^\varepsilon \otimes u^\varepsilon ) + \nabla p^\varepsilon = \varepsilon \Delta u^\varepsilon\\ \\ u^\varepsilon (0, \cdot) = u^\varepsilon_0\, . \end{array}\right. \] While the balance of the energy is \[ \frac{d}{dt} \frac{1}{2} \int |u|^2 (x,t)\, dx = - \varepsilon \int |Du|^2 (x,t)\, dx\, , \] it is a tenet of the theory of fully developed turbulence that in a variety of situations the left hand side should typically be independent of $\varepsilon$: the mechanism is not supposed to be ignited by high oscillations in the initial data, which would trigger an ``immediate'' dissipation through the viscosity, but it is rather thought to be an effect of the quadratic nonlinearity. It is on the other hand very challenging to produce rigorous examples. If the bounds on $u_0^\varepsilon$ are too strong, the well-posedness theory for Euler obstructs the anomalous dissipation up until the first blow-up time of Euler. With sufficiently coarse bounds Euler can be shown to have a variety of dissipative solutions, but it is very difficult to prove the convergence of Navier-Stokes to any of them. In a recent joint work with Elia Bruè we study the forced version of the equation and we can prove rigorously that, as soon as the regularity of the force drops below the known thresholds for the well-posedness of classical Euler, it is in fact possible to show anomalous dissipation. |

November 22, 2022 |
Ivan Avramidi (New Mexico Tech)Spectral Asymptotics of Elliptic Operators on ManifoldsVideo link: https://youtu.be/S5iK7A7338s Show abstract The study of spectral properties of natural geometric elliptic partial differential operators acting on smooth sections of vector bundles over Riemannian manifolds is a central theme in global analysis, differential geometry and mathematical physics. Instead of studying the spectrum of a differential operator $L$ directly one usually studies its spectral functions, that is, spectral traces of some functions of the operator, such as the spectral zeta function $\zeta(s)=\Tr L^{-s}$ and the heat trace $\Theta(t)=\Tr\exp(-tL)$. The kernel $U(t;x,x')$ of the heat semigroup $\exp(-tL)$, called the heat kernel, plays a major role in quantum field theory and quantum gravity, index theorems, non-commutative geometry, integrable systems and financial mathematics. We review some recent progress in the study of spectral asymptotics. We study more general spectral functions, such as $\Tr f(tL)$, that we call quantum heat traces. Also, we define new invariants of differential operators that depend not only on the their eigenvalues but also on the eigenfunctions, and, therefore, contain much more information about the geometry of the manifold. Furthermore, we study some new invariants, such as $\Tr\exp(-tL_+)\exp(-sL_-)$, that contain relative spectral information of two differential operators. Finally we show how the convolution of the semigroups of two different operators can be computed by using purely algebraic methods. |

November 15, 2022 |
Anton Alekseev (University of Geneva)Virasoro Hamiltonian spacesVideo link: https://youtu.be/97hCsjcDPoI Show abstract In this talk, we consider Hamiltonian actions of the group of diffeomorphisms of the circle. This is a semiclassical countepart of Virasoro algebra actions in Conformal Field Theory. The main examples include Virasoro coadjoint orbits (previously studied by Segal, Kirillov, Witten and others) and moduli spaces of hyperbolic metrics on surfaces with boundary. Our study is motivated by recent progress in Jackiw-Teitelboim quantum gravity. In particular, we give mathematical interpretation of some 2D - 1D duality phenomena. The talk is based on work in progress with Eckhard Meinrenken. |

November 8, 2022 |
Razvan Gurau (Universität Heidelberg)Melonic field theoriesVideo link: https://youtu.be/eYqmVc1nGwI Show abstract Like random matrices, random tensors exhibit a 1/N expansion, but their large N limit is more amenable to analytic computations. In particular tensor field theories provide a new class of large N theories, the so called melonic theories. In this talk I will present an overview of recent results on such theories and discuss their implications. |

November 1, 2022 |
Giovanni Felder (ETH Zürich)Boltzmann's billiard and table tennisVideo link: https://youtu.be/f_sx3NU3HWc Show abstract In Boltzmann's billiard a particle moves in a half-plane subject to a central force and is reflected elastically when it hits the boundary. Boltzmann took the central force to be the sum of a gravitational inverse-square-law force and a centrifugal term proportional to the inverse cube of the distance to the centre. He formulated the expectation that except for special values of the parameters the system would be chaotic and would obey his Ergodic Hypothesis. Recently Gallavotti and Jauslin showed that the system is integrable if the centrifugal term is omitted: it has a second conserved quantity besides the energy. I will review this result and show that this integrable Boltzmann system has the Poncelet property: if in a level set of the conserved quantities a trajectory is periodic then all trajectories on the level set are periodic. As for the classical Poncelet theorem on inscribed-circumscribed polygons in Jacobi's interpretation, the result relies on the theory of elliptic curves. I will also present some work in progress with Misha Feigin on Boltzmann's table tennis, the three dimensional version of Boltzmann's integrable system, and the relation to QRT maps on biquadratic plane curves. |

October 25, 2022 |
Sohei Ashida (Gakushuin University)Mathematics of molecular structures and dynamicsVideo link: https://youtu.be/1A8bkmVRDPU Show abstract I will talk about mathematically rigorous results concerned with studies of molecules. The subject corresponds to N-body problems of the Schrödinger equation from the standpoint of mathematics. First, we will consider the spectrum and long-time asymptotic behaviors, focusing on scattering matrices of N-body Schrödinger equation. Next we will consider quantitative studies based on the Born-Oppenheimer approximation. Results related to estimates of electronic energy levels such as the upper bound of electronic density and results concerned with the Hartree-Fock equation will be presented. References: [1] Kyushu J. Math. 75, 277-294 (2021) (arXiv:2002.07426) [2] Hiroshima Math. J., 52, 177-216 (2022) (arXiv:1811.07527) [3] arXiv:2012.09397 to appear in Tohoku Math. J. [4] arXiv:2112.09521 to appear in Hokkaido Math. J. [5] arXiv:2205.10300 |

October 18, 2022 |
Karol Kozlowski (ENS Lyon)Towards an exact rigorous construction of the Sinh-Gordon quantum field theory in 1+1 dimensionsVideo link: https://youtu.be/1M-H9fkouOQ Show abstract The bootstrap program was devised in the late 70s/early 80s as a possible path for a fully explicit construction of numerous massive integrable quantum field theories in 1+1 dimensions. Ultimately, it allows one to express the physically pertinent observables in these models --- the multi-point correlation functions also called Wightman functions --- in terms of form factor series expansion. On technical grounds, the latter correspond to fully explicit series of multiple integrals in which the $n^{\rm th}$ summand is given by a $n$-fold integral. The well definiteness of such a construction amounts to proving the convergence of these series expansions and this was a basically an open problem. In this talk, I will review the boostrap approach to the construction of the Sinh-Gordon quantum field theory. In particular, I will discuss the type of representations one gets for the multi-point correlation functions in this model. Then, I will discuss the recent progress I achieved on the convergence problem for the form factor series arising in the description of space-like separated two-point functions. The proof of the convergence of amounts to obtaining a sufficiently sharp estimate on the leading large-$n$ behaviour of the $n$-fold integral arising in those series. This appeared possible by refining some of the techniques that were fruitful in the analysis of the large-$n$ behaviour of integrals over the spectrum of $n\times n$ random Hermitian matrices. |

October 11, 2022 |
Christoph Kopper (Ecole Polytechnique)On asymptotically free scalar fieldsVideo link: https://youtu.be/EBITAh3XJDY Show abstract I present recent work (Annales Henri Poincaré 2022) analysing the flow equations of the renormalisation group in the mean field approximation. There it is shown that these equations for scalar fields in four dimensional euclidean space, have solutions which are UV asymptotically free. I comment on this result, also in view of the literature, in particular with regard to the triviality of \varphi^4_4-theory. We close with an outlook on future perspectives. |

October 4, 2022 |
Paweł Duch (Adam Mickiewicz University)Flow equation approach to singular stochastic PDEsVideo link: https://youtu.be/fuS1DPThwwU Show abstract Most stochastic PDEs arising in physics, such as the KPZ equation describing the motion of a growing interface or the stochastic quantization equation of the $\Phi^4$ Euclidean QFT, are ill-posed in the classical analytic sense due to irregular nature of random terms. Equations of this type are called singular. Regularization and renormalization are usually necessary to give mathematical meaning to such equations. In the talk, I will present a novel technique of solving singular stochastic PDEs. The technique is based on the renormalization group flow equation. It is applicable to a large class of parabolic or elliptic SPDEs with fractional Laplacian, additive noise and polynomial non-linearity. It covers equations in the whole super-renormalizable regime. A nice feature of the method is that it avoids the algebraic and combinatorial problems arising in different approaches. Based on arXiv:2109.11380 and arXiv:2201.05031. |

September 27, 2022 |
Dorothea Bahns (Georg-August-Universität Göttingen)The Sine Gordon Model in hyperbolic signatureShow abstract I will review some recent results in the construction of the Sine Gordon model in hyperbolic signature in the setting of perturbative algebraic quantum field theory. One focus is on the construction of a Haag Kastler net of observables (of operators on a Hilbert space), the other will be to explain how the adiabatic switching function can be removed in expectation values in thermal states. The methods presented are a mixture of "old and new" in constructive and algebraic quantum field theory. This is joint work with Kasia Rejzner, Klaus Fredenhagen, and Nicola Pinamonti. |

September 20, 2022 |
Slava Rychkov (IHES)Tensor Renormalization Group at Low TemperaturesVideo link: https://youtu.be/e02QNafITIs Show abstract Tensor RG is a real-space approach to renormalization in lattice models. It shows impressive numerical results, but its rigorous theory is still in its infancy. I will start by reminding the basics of Tensor RG, and what makes it potentially more powerful than Wilson-Kadanoff approach. I will then explain how Tensor RG can be used to understand the Ising model in nonzero magnetic field at low temperatures. This gives an alternative to the Pirogov-Sinai theory and to the renormalization group methods in the contour representation from the 1980’s. Joint work with Tom Kennedy, to appear soon. |

August 30, 2022 |
Jian Ding (Peking University)Repeated emergence of 4/3-exponentVideo link: https://youtu.be/ErHRtFE7fY4 Show abstract In this talk, I will describe the emergence of the 4/3-exponent in two seemingly unrelated models: random distance of Liouville quantum gravity and correlation length for the two-dimensional random field Ising model. I will then explain that such 4/3-exponent, while being unexpected among respective communities even from a physics perspective, has in fact been hinted in Leighton-Shor (1989) and Talagrand (2014) where the 4/3-exponent emerges in a random matching problem. Finally, I will present the heuristic computation which leads to the emergence of the 4/3-exponent. While I will review related progress on these topics, the two papers featuring 4/3-exponent are with Subhajit Goswami and with Mateo Wirth. |

August 16, 2022 |
Jonathan Dimock (SUNY at Buffalo)Ultraviolet Stability for Quantum Electrodynamics in d=3Video link: https://youtu.be/pfcKXB1z_eQ Show abstract We report on results for quantum electrodynamics on a finite volume Euclidean spacetime in dimension d=3. The theory is formulated as a functional integral on a fine toroidal lattice involving both fermion fields and abelian gauge fields. The main result is that, after renormalization, the partition function is bounded uniformly in the lattice spacing. This is a first step toward the construction of the model. The result is obtained by renormalization group analysis pioneered by Balaban. A single renormalization group transformation involves block averaging, a split into large and small field regions, and an iden- tification of effective actions in the small field regions via cluster expansions. This leads to flow equations for the parameters of the theory. Renormalization is accomplished by fine-tuning the initial conditions for these equations. Large field regions need no renormalization, but are shown to give a tiny contribution. |

August 2, 2022 |
Martin Fraas (UC Davis)Projections, parallel transport and adiabatic theoryVideo link: https://youtu.be/wBYsgKbavz8 Show abstract I will give an overview of adiabatic theory with the focus on a geometric point of view. The talk will cover traditionally adiabatic theory, adiabatic theory in many-body systems, and a recent work with W. De Roeck and A. Elgart on the adiabatic theory for disordered systems. |

July 26, 2022 |
Alexander Strohmaier (University of Leeds)A mathematical analysis of Casimir interactions and determinant formulaeVideo link: https://youtu.be/DkhJFZf-aH4 Show abstract I will explain a mathematical treatment of Casimir interactions of perfect conductors in which the Casimir energy is written as the trace of an operator without the need for regularisations. I will also show some consequences of this approach, relations to microlocal analysis, and will prove a trace formula that allows to compute the Casimir energy in terms of determinants of single layer operators. Such formulae have been derived by other methods in the physics literature and we will show that all these approaches give the same well defined Casimir energy. (Based on joint work with F. Hanisch and A. Waters, as well as numerical work with T. Betcke and X. Sun). |

July 19, 2022 |
Tomohiro Sasamoto (Tokyo Institute of Technology)Mapping macroscopic fluctuation theory for the symmetric simple exclusion process to a classically integrable systemVideo link: https://youtu.be/bfkgsn6R_3s Show abstract The large deviation principle for symmetric simple exclusion process(SEP) had been established by Kipnis, Olla, Varadhan in 1989 [1]. A somewhat different formulation, known as the macroscopic fluctuation theory (MFT), was initiated and developed by Jona-Lasinio et al in 2000’s [2]. The basic equations of the theory, MFT equations, are coupled nonlinear partial differential equations and have resisted exact analysis except for stationary situation. In this talk we introduce a novel generalization of the Cole-Hopf transformation and show that it maps the MFT equations for SEP to the classically integrable Ablowitz-Kaup-Newell-Segur(AKNS) system. This allows us to solve the equations exactly in time dependent regime by adapting standard ideas of inverse scattering method. The talk is based on a joint work with Kirone Mallick and Hiroki Moriya [3]. References [1] C. Kipnis, S. Olla, S. R. S. Varadhan, Hydrodynamics and large deviations for simple exclusion processes, Comm. Pure Appl. Math., 42:115–137, 1989. [2] L. Bertini, A. De Sole, D. Gabrielli, G. Jona-Lasinio, and C. Landim, Macroscopic fluctuation theory, Rev. Mod. Phys., 87:593–636, 2015. [3] Kirone Mallick, Hiroki Moriya, Tomohiro Sasamoto, Exact solution of the macroscopic fluctuation theory for the symmetric exclusion process, arXiv: 2202.05213, to appear in Phys. Rev. Lett. |

July 12, 2022 |
Stefan Teufel (University of Tübingen)A slightly different look at the bulk-edge correspondence in quantum Hall systemsVideo link: https://youtu.be/-XX8mn3d8ec Show abstract The bulk-edge correspondence of transport coefficients in quantum Hall systems is usually shown for systems with a (mobility) gap in the bulk and at zero temperature. I will present recent results showing equality of magnetization in the bulk and edge current in microscopic models without and with interaction between electrons. This equality holds at positive temperature and without assuming a (mobility) gap in the bulk, and is robust to perturbations near the edges. I then discuss how the equality of transport coefficients can be recovered from this form of bulk-edge correspondence, at least in some cases. My talk is based on joint work with Horia Cornean, Jonas Lampart, Massimo Moscolari, and Tom Wessel. |

July 5, 2022 |
Michele Correggi (Politecnico di Milano)Ground State Properties in the Quasi-Classical RegimeVideo link: https://youtu.be/NAFRt6-m1uQ Show abstract We study the ground state energy and ground states of systems coupling non-relativistic quantum particles and force-carrying Bose fields, such as radiation, in the quasi-classical approximation. The latter is very useful whenever the force-carrying field has a very large number of excitations, and thus behaves in a semiclassical way, while the non-relativistic particles, on the other hand, retain their microscopic features. We prove that the ground state energy of the fully microscopic model converges to the one of a nonlinear quasi classical functional depending on both the particles' wave function and the classical configuration of the field. Equivalently, this energy can be interpreted as the lowest energy of a Pekar-like functional with an effective nonlinear interaction for the particles only. If the particles are confined, the ground state of the microscopic system converges as well, to a probability measure concentrated on the set of minimizers of the quasi classical energy. |

June 28, 2022 |
Owen Gwilliam (University of Massachusetts Amherst)Observables in the Batalin-Vilkovisky formalism: from Feynman diagrams to commutative diagramsVideo link: https://youtu.be/UAvvpjdDhAc Show abstract This talk will discuss a convergence between perturbative QFT and higher algebra, in the sense of homological algebra, operads, and higher categories, although we do not expect the listener to have any expertise in those topics. A key notion here is a factorization algebra, which captures the local-to-global nature of the observables of a field theory and which is due to Beilinson and Drinfeld. With a focus on the example of Chern-Simons theory, we will discuss how the BV/factorization package offers a new view on the emergence of algebraic structures, like braided monoidal categories, from physics. |

June 21, 2022 |
Chiara Saffirio (University of Basel)Mean-field evolution and semiclassical limit of many interacting fermionsVideo link: https://youtu.be/-Z2XcmTJxgI Show abstract We will review recent progresses in the derivation of effective evolution equations for the dynamics of many weakly interacting fermions. We will focus on the mean-field regime and couple it with a semiclassical limit to obtain, as the number of particles goes to infinity, the Hartree-Fock and the Vlasov equations. We will comment on the class of singular interactions and quantum states that we are able to treat, drawing a comparison with the PDE theory of the limiting equations. Based on joint works with J. Chong and L. Laflèche. |

June 7, 2022 |
Michael Aizenman (Princeton University)A new perspective on a pair of two dimensional phenomena: delocalization in random height functions and the Berezinskii-Kosterlitz-Thouless phase of O(2) symmetric spin modelsVideo link: https://youtu.be/if5kMzJtrlw Show abstract Delocalization in integer-restricted Gaussian field, and other random height functions formulated over planar doubly-periodic lattices, is shown to imply slow (power law) decay of correlations in the corresponding dual O(2) symmetric two-component spin model. The link proceeds through a lower bound on the spin-spin correlation in terms of the probability of their sites being on a common level loop of the dual random height function. Motivated by this observation, we have extended the recent proof by P. Lammers of delocalization transition in two dimensional graphs of degree three, to all doubly periodic graphs, in particular to Z^2. The extension is established through a monotonicity argument based on lattice inequalities of O. Regev and N. Stephens-Davidowitz. Taken together the results yield a new perspective on the BKT phase transition in O(2)-invariant models and complete a new proof of delocalization in two-dimensional integer-valued height functions. (Both phenomena are unique to two dimension, and have been previously proven and studied by other means). That talk is based on a joint work by M. Aizenman, M. Harel J. Shapiro and R. Peled. |

May 31, 2022 |
Benjamin Doyon (King's College London)Ergodicity, large-scale correlations and hydrodynamics in many-body systemsVideo link: https://youtu.be/yTGFLC9IB-A Show abstract Long-time behaviours in statistical ensembles of many-body systems are notoriously difficult to access. Hydrodynamics, which is the theory for the emergent large-scale dynamics, gives a lot of information, such as exact asymptotics of correlation functions. It turns out that at the Euler scale, the emergent theory for extended systems is largely universal. I will discuss a number of such universal results in one dimension of space. Some can be shown rigorously: a notion of ergodicity at all frequencies hold for correlation functions in stationary states of all short-range quantum spin chains. The Boltzmann-Gibbs principle, where local observables project onto hydrodynamic modes in two-point functions, and the linearised Euler equations, are also established. The complete space of hydrodynamic modes is the space of ``extensive conserved quantities”, which is defined unambiguously. I will illustrate these concepts in integrable systems, using generalised hydrodynamics. For correlations in non-stationary states, much less is established. I will describe a macroscopic fluctuation theory for the Euler scale which provides a framework for these. In particular, I will explain how a new type of long-range correlations, hitherto not observed, appear when the system is subject to fluid motion, which breaks the paradigm that separate fluid cells are not correlated. |

May 24, 2022 |
Kevin Costello (Perimeter Institute)Form factors of gauge theory as correlators of a vertex algebraVideo link: https://youtu.be/fbSlnXn5sPs Show abstract Form factors are scattering amplitudes in the presence of a local operator. I will explain that, for certain gauge groups, form factors of self-dual Yang-Mills theory (with some additional fields) are the correlators of a vertex algebra. This is joint work with Natalie Paquette, and is closely related to the celestial holography program. |

May 17, 2022 |
Wojciech Dybalski (Adam Mickiewicz University)Interacting massless excitations in 1+1 dimensional QFTVideo link: https://youtu.be/iiueQ7UtWJc Show abstract Massless 1+1 dimensional quantum field theories have a peculiar scattering theory. Since the motion of the excitations is dispersionless, only the distinction between left-movers and right-movers is meaningful. Thus complete particle interpretation (asymptotic completeness) can be expected already at the level of two-body scattering. This effect will be illustrated by certain wedge-local models of interacting Wigner particles. In the second part of the talk I will move to the more exotic case of infraparticles, i.e., excitations which are not of Wigner type. It is known that infraparticles abound in 1+1 massless QFT, even in the familiar case of free field theory. A natural definition of a scattering amplitude for infraparticles will be proposed and tested in this latter model. Paradoxically, infraparticles in free field theory exhibit non-trivial scattering. The talk is based on joint works with Yoh Tanimoto (CMP 305, 427-440 (2011)) and Jens Mund (arXiv:2109.02128). |

May 10, 2022 |
Vieri Mastropietro (University of Milan)Anomalies and non-perturbative QFTVideo link: https://youtu.be/XoPMlMpmadg Show abstract Adler and Bardeen in 1969 established the non-renormalization of the chiral anomaly, writing it as a perturbative expansion which is order by order vanishing; since then, this property has found uncountable applications, including the Standard Model consistence via the anomaly cancellation condition (Bouchiat, Iliopoulos, Meyer 1972). After reviewing briefly this notion, we prove the anomaly non renormalization at a non-perturbative level in the case of lattice vector boson-fermion models in d=1+1 (uniformly in the lattice) and in d=3+1 (up to a cut-off of the order of the inverse coupling). The proof relies on bounds on the large distance decay of correlations and Ward Identities, both exact and emerging. In the case of chiral models, like the effective electroweak theory with quartic Fermi interaction in d=3+1, some Ward Identites are violated and the anomaly can be proved to vanish up to subdominant corrections which are rigorously bounded, under the cancellation condition on charges and with a cut-off of the order of the inverse coupling. Open problems and conjectures will be finally discussed. |

May 3, 2022 |
Daniel Ueltschi (University of Warwick)Random loop models and their universal behaviour in dimension 3+Video link: https://youtu.be/dHV2D0MOuc8 Show abstract I will discuss several loop soup models that represent classical or quantum systems of statistical physics. These systems undergo phase transitions that are characterised by the occurrence of macroscopic loops. As it turns out, the joint distribution of the loop lengths exhibits a universal behaviour: In 3+ dimension it is always given by a Poisson-Dirichlet distribution. The heuristics is based on the fact that the loops are so intertwined that they behave effectively in mean-field fashion. Hence the connection to the split-merge process, whose invariant measure is Poisson-Dirichlet. I will also discuss consequences about symmetry breaking in certain quantum spin systems. This talk is based on collaborations over the years with D. Gandolfo, J. Ruiz, C. Goldschmidt, P. Windridge, V. Betz, S. Grosskinsky, A. Lovisolo, C. Benassi, J. Björnberg, J. Fröhlich. |

April 26, 2022 |
Deepak Dhar (IISER Pune)Hard rigid rods on hypercubical latticesVideo link: https://youtu.be/-7INKbcMd_0 Show abstract I consider a system of hard rigid rods of length $k$ and width $1$, on $d$-dimensional hypercubic lattices. I will show that in the limit of large $k$, the entropy per site at full packing $ s(d,k)$ for $d=2$ satisfies $$ \lim_{k \rightarrow \infty} \frac { s(d,k) k^2}{\log k} =1, $$ and give heuristic arguments that this result would be true also for all dimensions $d>2.$ If $k$ is large enough ($ k> 6 $ in $2$-d, and $k>4$ in $3$-d), this model is known to undergo two phase transitions when chemical potential is increased: from a low density isotropic phase to an intermediate density nematic phase, and on further increase to a high-density phase with no orientational order. I will present evidence to suport the conjecture that, for large $k$, the second phase transition is a first order transition with a discontinuity in density as a function of the chemical potential, in all dimensions greater than 1, and if the chemical potential per rod at the transition is $\mu^*(k,d)$, and the density of holes jumps from a value $\epsilon_1(k,d)$ to $\epsilon_2(k,d)$, we have for all $d \geq 2$ $$\lim_{ k \rightarrow \infty} \frac{ \epsilon_1(d, k) k^2}{\log k} =1,$$ $$\lim_{k \rightarrow \infty} \frac{ \exp\left[ \frac{\mu^*(d, k)}{k} \right] \log k}{k} =1,$$ $$\lim_{k \rightarrow \infty} \frac{\epsilon_2(d,k) k ^m}{\epsilon_1(d,k)} =0, ~{\rm for~ all}~m>0.$$ References: Deepak Dhar and R. Rajesh, Phys. Rev. E 103, 042130 (2021); Aagam Shah, D. Dhar and R. Rajesh, Phys. Rev. E. 105, 034103 (2022). |

April 19, 2022 |
Hirosi Ooguri (Caltech & Kavli IPMU)Symmetry in QFT and GravityVideo link: https://youtu.be/T0x4WClZ4SI Show abstract I will review aspects of symmetry in quantum field theory and combine them with the AdS/CFT correspondence to derive constraints on symmetry in quantum gravity. The quantum gravity constraints to be discussed include the no-go theorem on global symmetry, the completeness of gauge charges, and the decomposition of high energy states into gauge group representations. |

April 12, 2022 |
Edward Witten (IAS)Black Hole Entropies and AlgebrasVideo link: https://youtu.be/jAA_5_jQMbI Show abstract I will review the idea of black hole entropy, as originally discovered by Bekenstein and Hawking roughly half a century ago, and then explain how to interpret black hole entropy in terms of an appropriate algebra of observables. |

April 5, 2022 |
Jacob Shapiro (Princeton University)Continuum and strongly disordered topological insulatorsVideo link: https://youtu.be/ienfExpEshg Show abstract A natural question to ask in the field of topological insulators is whether continuum descriptions on L^2(R^d) and effective discrete descriptions on l^2(Z^d) agree, at the level of the long time dynamics and at the level of the topological indices. This is shown to be the case at least for a class of models of the integer quantum Hall effect. The basic tool to approach the problem is homotopies of Fredholm operators. This same tool is also shown to apply to other problems in topological insulators, such as the bulk-edge correspondence for Z_2 time-reversal invariant strongly disordered discrete systems. This talk is based on joint works with Michael I. Weinstein and with Jeff Schenker and Alex Bols. |

March 29, 2022 |
Serena Cenatiempo (GSSI L'Aquila)Trial states for the zero temperature dilute Bose gasVideo link: https://youtu.be/I5YY-MfhD4g Show abstract Non-relativistic interacting bosons at zero temperature, in two and three dimensions, are expected to exhibit a fascinating critical phase, famously known as condensate phase. Even though a proof of Bose-Einstein condensation in the thermodynamic limit is still beyond reach of the current available methods, the mathematical physics community has recently gained an enhanced comprehension of other aspects of the macroscopic behavior of low temperature Bose gases, in several interesting regimes. In this talk we review part of these advances, by describing trial states for the dilute three dimensional Bose gas, capturing the celebrated Lee-Huang-Yang sub-leading correction to the ground state energy. We conclude with some open questions and perspectives. Based on joint works with G. Basti, C. Boccato, C. Brennecke, C. Caraci, A. Olgiati, G. Pasqualetti and B. Schlein. |

March 22, 2022 |
Vojkan Jaksic (McGill University)Approach to equilibrium in translation-invariant quantum systems: some structural resultsVideo link: https://youtu.be/tkdzzK_MK1w Show abstract We formulate the problem of approach to equilibrium in algebraic quantum statistical mechanics and study some of its structural aspects, focusing on the relation between the zeroth law of thermodynamics (approach to equilibrium) and the second law (increase of entropy). Our main result is that approach to equilibrium is necessarily accompanied by a strict increase of the specific (mean) entropy. In the course of our analysis, we introduce the concept of quantum weak Gibbs state which is of independent interest. This is a joint work with C. Tauber and C.-A. Pillet |

March 15, 2022 |
Jerzy Lewandowski (University of Warsaw)Horizon equationsVideo link: https://youtu.be/ofHsa8A8p8o Show abstract The extremity assumptions imposes a set of non-trivial equations on the intrinsic and extrinsic geometry of the horizon. Some of them already got more attention since they are applied in the near horizon geometries (NHG), some other are less know. All of them are very important for the existence and uniqueness of extremal black hole solutions to Einstein's equations. In 4d spacetime, an integrability condition for the NHG equation is available that is satisfied by the family of non-extremal horizon geometries, namely by those that are of the Petrov type D. What is special about that equation, is that its solutions exhibit similar properties to those proved in the global black hole spacetime theory: spherical topology of horizon cross sections, rigidity, no-hair. Extension of the research to the horizons that had a Hopf bundle structure led to interesting results about the global structure of the Kerr-NUT-(A)dS spaces. Misner's construction was generalised to the case when none of the three parameters (NUT, Kerr, and the cosmological constant) vanishes. The resulting family contains also exact solutions to Einstein's equations that admit no horizon or nowhere time like Killing vector and describe a topologically spherical universe evolving from the past scry to the future scry in a non-singular manner. |

March 8, 2022 |
Makiko Sasada (University of Tokyo)Topological structures and the role of symmetry in the hydrodynamic limit of nongradient modelsVideo link: https://youtu.be/TD2ePNrkwhI Show abstract Recently, we introduce a general framework in order to systematically investigate hydrodynamics limits of various microscopic stochastic large scale interacting systems in a unified fashion. In particular, we introduced a new cohomology theory called the uniformly cohomology to investigate the underlying topological structure of the interacting system. Our theory gives a new interpretation of the macroscopic parameters, the role played by the group action on the microscopic system, and the origin of the diffusion matrix associated to the macroscopic hydrodynamic equation. Furthermore, we rigorously formulate and prove for a relatively general class of models Varadhan’s decomposition of closed forms, which plays a key role in the proof of hydrodynamic limits of nongradient models. Our result is applicable for many important models including generalized exclusion processes, multi-species exclusion processes, exclusion processes on crystal lattices and so on. Based on joint papers with Kenichi Bannai and Yukio Kametani |

March 1, 2022 |
Marek Biskup (UCLA)Extremal points of random walks on planar and tree graphsVideo link: https://youtu.be/9CDB-A__nWk Show abstract I will review recent progress on the description of points heavily visited by paths of random walks. The focus will be on the situations where the random walk has an approximate scale-invariant structure and the associated local time process is thus logarithmically correlated in space. Two geometric settings will be considered: the simple random walk on finite subsets of the square lattice and the random walk on homogeneous trees of finite depth. In the former case, a full description will be given of the scaling limit of thick, thin and avoided points for the walk run up to the times proportional to the cover time. For the latter setting, the law of the most frequently visited leaf-vertex, and the time spent there, will be given in the limit of the tree depth tending to infinity. In both cases, the spatial laws will be determined by a corresponding multiplicative chaos measure. Based on joint papers with Y. Abe, S. Lee and O. Louidor. |

February 22, 2022 |
Lorenzo Taggi (Sapienza Università di Roma)Macroscopic loops in the interacting Bose gas, Spin O(N) and related modelsVideo link: https://youtu.be/s6Nko0_ZTBc Show abstract We consider a general system of interacting random loops which includes several models of interest, such as the spin O(N) model, the double dimer model, random lattice permutations, and is related to the loop O(N) model and to the interacting Bose gas in discrete space. We present an overview on these models, introduce some of the main open questions about the size and the geometry of the loops, and present some recent results about the occurrence of macroscopic loops in dimensions d > 2 as the inverse temperature is large enough. |

February 15, 2022 |
Martin Zirnbauer (University of Cologne)Field theory of Anderson transitions reviewed: Color-Flavor TransformationVideo link: https://youtu.be/OOSZ8gS8N0U Show abstract This talk is in two parts. First, the field-theoretic approach to (de-)localization in Anderson-type models for disordered electrons is reviewed, with emphasis placed on the presence of a hyperbolic target-space sector and the expected pattern of spontaneous symmetry breaking. The second part is a review of the "Color-Flavor (CF) Transformation" (MZ, 1996). Conceived as a variant of the Efetov-Wegner supersymmetry method, the CF Transformation is tailored to quantum systems with disorder distributed according to Haar measure for any compact Lie group of classical type (A, B, C, or D). It has been applied to Dyson's Circular Ensembles, random-link network models, quantum chaotic graphs, disordered Floquet dynamics, and more. It is reviewed here as a rigorous tool to derive the effective field theory for systems in the metallic regime. |

February 8, 2022 |
Erik Skibsted (Aarhus University)Stationary scattering theory, the N-body long-range caseVideo link: https://youtu.be/KU02QnM1S10 Show abstract Within the class of Dereziński-Enss pair-potentials which includes Coulomb potentials and for which asymptotic completeness is known, we show that all entries of the N-body quantum scattering matrix have a well-defined meaning at any given non-threshold energy. As a function of the energy parameter the scattering matrix is weakly continuous. This result generalizes a similar one obtained previously by Yafaev for systems of particles interacting by short-range potentials. As for Yafaev’s works we do not make any assumption on the decay of channel eigenstates. The main part of the proof consists in establishing a number of Kato-smoothness bounds needed for justifying a new formula for the scattering matrix. Similarly we construct and show strong continuity of channel wave matrices for all non-threshold energies. The set of so-called stationary complete energies has full measure. We show that the scattering and channel wave matrices constitute a well-defined ‘scattering theory’ at such energies, in particular at any stationary complete energy the scattering matrix is unitary, strongly continuous and characterized by asymptotics of minimum generalized eigenfunctions. |

February 1, 2022 |
Daisuke Shiraishi (Kyoto University)Recent progress on loop-erased random walk in three dimensionsVideo link: https://youtu.be/XhtoyR196hs Show abstract Loop-erased random walk (LERW) is a model for a random simple path, which is created by running a simple random walk and, whenever the random walk hits its path, removing the resulting loop and continuing. LERW was originally introduced by Greg Lawler in 1980. Since then, it has been studied extensively both in mathematics and physics literature. Indeed, LERW has a strong connection with other models in statistical physics, especially the uniform spanning tree which arises in statistical physics in conjunction with the Potts model. In this talk, I will talk about some recent progress on LERW while focusing on the three-dimensional case. This is joint work with Xinyi Li. |

January 25, 2022 |
Michele Schiavina (ETH Zürich)Ruelle Zeta Function from Field TheoryVideo link: https://youtu.be/fFQsMaDBIiU Show abstract I will discuss a field-theoretic interpretation of Ruelle's zeta function, which "counts" prime geodesics on hyperbolic manifolds, as the partition function for a topological field theory (BF) with an unusual gauge fixing condition available on contact manifolds. This suggests a rephrasing of a conjecture due to Fried, on the equivalence between Ruelle's zeta function (at zero) and the analytic torsion, as gauge-fixing independence in the Batalin--Vilkovisky formalism. |

January 18, 2022 |
Marcin Lis (University of Vienna)An elementary proof of phase transition in the planar XY modelVideo link: https://youtu.be/x3yUrbMfojI Show abstract Using elementary methods we obtain a power-law lower bound on the two-point function of the planar XY spin model at low temperatures. This was famously first rigorously obtained by Fröhlich and Spencer and establishes a Berezinskii-Kosterlitz-Thouless phase transition in the model. Our argument relies on a new loop representation of spin correlations, a recent result of Lammers on delocalisation of integer-valued height functions, and classical correlation inequalities. This is joint work with Diederik van Engelenburg. |

January 11, 2022 |
Claudio Dappiaggi (Università di Pavia)Stochastic Partial Differential Equations and Renormalization à la Epstein-GlaserVideo link: https://youtu.be/sFSiNu4tgIo Show abstract We present a novel framework for the study of a large class of nonlinear stochastic partial differential equations, which is inspired by the algebraic approach to quantum field theory. The main merit is that, by realizing random fields within a suitable algebra of functional-valued distributions, we are able to use specific techniques proper of microlocal analysis. These allow us to deal with renormalization using an Epstein-Glaser perspective, hence without resorting to any specific regularization scheme. As a concrete example we shall use this method to discuss the stochastic $\Phi^3_d$ model and we shall comment on its applicability to the stochastic nonlinear Schrödinger equation -- Joint work with Nicolò Drago, Paolo Rinaldi and Lorenzo Zambotti. |

December 21, 2021 |
Christoph Schweigert (Universität Hamburg)More about CFT correlatorsVideo link: https://youtu.be/dfau-m5usck Show abstract I will explain two recent developments concerning bulk fields in two-dimensional rational conformal field theories: the importance of the relative Serre functor to study bulk fields for logarithmic conformal field theories and the use of stringnet techniques to simplify the construction of correlators for semisimple modular tensor categories. |

December 14, 2021 |
Nicolai Reshetikhin (University of California, Berkeley)On the asymptotic behavior of character measures in large tensor powers of finite dimensional representations of simple Lie algebrasVideo link: https://youtu.be/n1iT80P8YFo Show abstract Let $V$ be a finite dimensional representation of a simple Lie algebra and $H$ be an positive element of its Cartan subalgebra ("magnetic field"). On the space $V^{\otimes N}$ we have a natural density matrix $N\exp(-H)$where $H$ acts diagonally: $H(x\otimes y\otimes z\dots)=Hx\otimes y\otimes z\dots +x\otimes y\otimes z\dots+x\otimes y\otimes z\dots$. The space $V^{\otimes N}$ decomposes into a direct sum of irreducible subrepresentations: \[ V^{\otimes N}\simeq \oplus_{\lambda} V_\lambda^{\oplus m(\lambda, N)} \] where $\lambda$ is the highest weight of the representation $V_\lambda$ and $m(\lambda, N)$ is its multiplicity in the tensor product. The character distribution assigns the probability \[ p_{\lambda}(N,H)=\frac{m(\lambda, N)Tr_{V_\lambda}(e^{-H)})}{(Tr_V(e^{-H}))^N} \] to each $\lambda$ in the decomposition of the tensor product. One of the natural problems for this distribution is to find its asymptotic in the limit $N\to \infty$ and $\lambda\to \infty$ in the appropriate way. When $H=0$ the character distribution becomes a uniform distribution. In this case, in such generality the asymptotic was studied by Ph. Biane, 1993 by T. Tate and S. Zelditch, 2004. For tensor powers of vector representations the asymptotic was derived by S. Kerov in 1986. When $H$ is generic, i.e. when $H$ is strictly inside of the principal Weyl chamber it was computed by O.Postnova and N.R. in 2018. This talk is based on a joint work with O. Postnova and V. Serganova (to appear on the arxiv). |

December 7, 2021 |
Nina Holden (ETH Zurich)Conformal welding in Liouville quantum gravity: recent results and applicationsVideo link: https://youtu.be/R534M3YAx7g Show abstract Liouville quantum gravity (LQG) is a natural model for a random fractal surface with origins in conformal field theory and string theory. A powerful tool in the study of LQG is conformal welding, where multiple LQG surfaces are combined into a single LQG surface. The interfaces between the original LQG surfaces are typically described by variants of the random fractal curves known as Schramm-Loewner evolutions (SLE). We will present a few recent conformal welding results for LQG surfaces and applications to the integrability of SLE and LQG partition functions. Based on joint work with Ang and Sun and with Lehmkuehler. |

November 30, 2021 |
Niklas Beisert (ETH Zurich)Planar Integrability and Yangian Symmetry in N=4 Supersymmetric Yang–MillsVideo link: https://youtu.be/jIMPJCzBqXk Show abstract The discovery and utilisation of integrability in the planar limit of certain gauge and string theory models has made available many exciting non-perturbative results and insights for the AdS/CFT correspondence. In this talk I will introduce the prototypical AdS/CFT duality between string theory on AdS₅×S⁵ and N=4 supersymmetric Yang–Mills theory, and highlight the usefulness of planar integrability through some achievements. I will then present a formal notion of integrability as a hidden Yangian symmetry of the action. Importantly, this symmetry is realised on field theory objects in a non-standard fashion for which a rigorous framework to deal with its implications is lacking. I will demonstrate that this symmetry indeed leads to non-trivial relations for quantum correlations functions of this model. In order to make them work out, I also show in what sense the symmetry is compatible with BRST gauge fixing. |

November 23, 2021 |
Julian Sonner (University of Geneva)Random matrices and black holes: new connections from holographic dualityVideo link: https://youtu.be/ezir3kAHBYQ Show abstract Quantum chaotic systems are often defined via the assertion that their spectral statistics coincides with, or is well approximated by, random matrix theory (RMT). In turn, the late-time ergodic behaviour of chaotic quantum systems is expected to fall into a small number of universality classes of RMT dynamics. Recent developments in holographic duality have made it possible to characterise the ergodic late-time behaviour of black holes in asymptotically anti de Sitter space in similar terms. In this talk I will start by introducing the basic ideas and notions of holographic duality (also known as AdS/CFT), before moving on to these more recent developments, mostly focussing on low-dimensional examples. |

November 16, 2021 |
Paweł Nurowski (Polish Academy of Sciences, Warsaw)Split real form of Lie group G2: simple, exceptional and naturally appearing in physicsVideo link: https://youtu.be/0XSGTOPTnzQ Show abstract A simple exceptional Lie group G2 was predicted to exist by Wilhelm Killing in 1887. Its geometric realization appeared later, in 1893; it is due to Friedrich Engel and Elie Cartan. They realized this group as a symmetry group of two quite different geometric structures on five manifolds. In my talk I will try to explain how the idea of these G2 symmetric geometric structures could came to minds of Cartan and Engel. My explanation will be based on properties of the root diagram of G2. I will keep my lecture self contained, so I will define the G2 root diagram and will explain how to use it. After nailing down the two 5-dimensional G2 -symmetric geometries of Cartan and Engel, I will show how they appear naturally in constrained classical mechanics. |

November 9, 2021 |
Gaultier Lambert (University of Zurich)Universality for free fermions point processesVideo link: https://youtu.be/AsC1eJq315E Show abstract I report on recent results obtained with A. Deleporte on universality of local statistics for the ground state of a free Fermi gas confined in a generic potential on ℝ^n. This model was introduced by Macchi in 1975 and it forms a determinantal point process whose kernel is the spectral projector associated to a Schrödinger operator −ℏ^2Δ+V. I will explain how to obtain the asymptotics of the kernel of this projector as ℏ→0 using methods from semiclassical analysis and discuss some probabilistic consequences. In particular, this implies universality of microscopic fluctuations, both in the bulk and around regular boundary points. |

November 2, 2021 |
Lea Bossmann (IST Austria)Asymptotic expansion of low-energy excitations for weakly interacting bosonsVideo link: https://youtu.be/QHmbRNvI_wU Show abstract We consider a system of N bosons in the mean-field scaling regime in an external trapping potential. We derive an asymptotic expansion of the low-energy eigenstates and the corresponding energies, which provides corrections to Bogoliubov theory to any order in 1/N. We show that the structure of the ground state and of the non-degenerate low-energy eigenstates is preserved by the dynamics if the external trap is switched off. This talk is based on joint works with Sören Petrat, Peter Pickl, Robert Seiringer and Avy Soffer. |

October 26, 2021 |
Tomaž Prosen (University of Ljubljana)Random Matrix Spectral Fluctuations in Quantum Lattice SystemsVideo link: https://youtu.be/ryerbRMYgEI Show abstract I will discuss the problem of unreasonable effectiveness of random matrix theory for description of spectral fluctuations in extended quantum lattice systems. A class of interacting spin systems has been recently identified - specifically, the so-called dual unitary circuits - where the spectral form factor is proven to match with circular ensembles of random matrix theory. The key ideas of novel methodology needed in the proofs will be discussed which are very different than the standard periodic-orbit based methods in quantum chaos of few body semiclassical systems. |

October 19, 2021 |
Rupert L. Frank (Caltech and LMU Munich)Eigenvalue bounds for Schrödinger operators with complex potentialsVideo link: https://youtu.be/3fAtjx_t96M Show abstract We discuss open problems and recent progress concerning eigenvalues of Schrödinger operators with complex potentials. We seek bounds for individual eigenvalues or sums of them which depend on the potential only through some L^p norm. While the analogues of these questions are (almost) completely understood for real potentials, the complex case leads to completely new phenomena, which are related to interesting questions in harmonic and complex analysis. |

October 12, 2021 |
Christophe Garban (Université Lyon 1)Continuous symmetry breaking along the Nishimori lineVideo link: https://youtu.be/qnqQ1c2V6eE Show abstract I will discuss a new way to prove continuous symmetry breaking for (classical) spin systems on Z^d, d\geq 3 which does not rely on "reflection positivity". Our method applies to models whose spins take values in S^1, SU(n) or SO(n) in the presence of a certain quenched disorder called the Nishimori line. The proof of continuous symmetry breaking is based on two ingredients |

October 5, 2021 |
Michael Loss (Georgia Institute of Technology)Which magnetic fields support a zero mode?Video link: https://youtu.be/8G0Er7tP4ys Show abstract I present some results concerning the size of magnetic fields that support zero modes for the three dimensional Dirac equation and related problems for spinor equations. The critical quantity, is the $3/2$ norm of the magnetic field $B$. The point is that the spinor structure enters the analysis in a crucial way. This is joint work with Rupert Frank at LMU Munich. |

September 28, 2021 |
Rostyslav Hryniv (Ukrainian Catholic University)Generalized soliton solutions of the Korteweg-de Vries equationVideo link: https://youtu.be/73jYxzUDBZc Show abstract The Korteweg--de Vries (KdV) equation is a non-linear dispersive equation describing shallow-water waves and possessing many intriguing properties. One of them is existence of the so-called soliton solutions representing solitary waves travelling with constant speed and shape, as well as a special way in which several such solitons interact. Another interesting fact is that solutions of the KdV equation can be obtained as solutions of the inverse scattering problem for the family of associated Schroedinger operators, as discovered by S.Gardner, J.Green, M.Kruskal and R.Miura in 1967, and the classical soliton solutions of the KdV correspond precisely to the so-called reflectionless potentials (I.Kay and H.Moses, 1956). |

September 21, 2021 |
Klaus Fredenhagen (Universität Hamburg)Towards an algebraic construction of Quantum Field TheoryVideo link: https://youtu.be/3hQBDzgODAA Show abstract Algebraic quantum field theory provides a conceptual and mathematically precise framework for the analysis of structural features of quantum field theory. During the last decades, it has in addition provided a generalization of quantum field theory to generic Lorentzian spacetimes, and, moreover, an elegant reformulation of renormalized perturbative quantum field theory. The principles used there actually determine also a C*-algebraic formulation which is surprisingly rich and leads to some nonperturbative results. In particular, the time slice axiom can be derived, a version of Noether's theorem is obtained, and the renormalization group flow caused by anomalies becomes visible in the algebraic structure. The talk is based on joint work with Detlev Buchholz (2020) and on joint work with Romeo Brunetti, Michael Dütsch and Kasia Rejzner (2021). |

September 14, 2021 |
Yvan Velenik (Université de Genève)Fluctuations of a layer of unstable phase in the planar Ising modelVideo link: https://youtu.be/cq3ivs4F8rs Show abstract I'll first review some of the known results about phase separation, fluctuations of interfaces and (equilibrium) aspects of metastability in the planar Ising model. This will naturally lead us to consider a setup in which an interface separates a layer of unstable phase along the boundary of the system from the stable phase occupying the bulk. I'll then describe a recent work, in which we prove that, after a 1/3:2/3 scaling, the distribution of this interface weakly converges to the distribution of the stationary trajectories of an explicit Ferrari-Spohn diffusion. The proof relies on a rigorous reduction to an effective interface model, which I'll briefly sketch. |

September 7, 2021 |
Pierre Germain (New York University)Derivation of the kinetic wave equationVideo link: https://youtu.be/zHKP2KpG6Ac Show abstract The kinetic wave equation (KWE) is to nonlinear waves what the Boltzmann equation is to classical particles. The KWE is also very closely related to quantum versions of the Boltzmann equation. Finally, the KWE provides an entrypoint into turbulent phenomena, since it is aimed at describing weakly turbulent flows. For all these reasons, the KWE is an interesting object, and the question of its validity a fundamental one. I will review recent progress on this question. |

August 31, 2021 |
Antti Knowles (University of Geneva)The Euclidean phi^4_2 theory as a limit of an interacting Bose gasVideo link: https://youtu.be/J9Oe-p4xpWs Show abstract Local Euclidean field theories over d-dimensional space have been extensively studied in the mathematical literature since the sixties, motivated by high-energy physics and statistical mechanics. For d=2, I explain how the complex scalar field theory with quartic interaction arises as a limit of an interacting Bose gas at positive temperature, when the density of the gas becomes large and the range of the interaction becomes small. The proof is based on a quantitative analysis of a new functional integral representation of the interacting Bose gas combined with a Nelson-type argument for a general nonlocal field theory. Joint work with Jürg Fröhlich, Benjamin Schlein, and Vedran Sohinger. |

August 24, 2021 |
Ron Peled (Tel Aviv University)Quantitative estimates for the effect of disorder on low-dimensional lattice systemsVideo link: https://youtu.be/gzfsy3jgCoo Show abstract The addition of an arbitrarily weak random field to low-dimensional classical statistical physics models leads to the "rounding" of first-order phase transitions at all temperatures, as predicted in 1975 by Imry and Ma and proved rigorously in 1989 by Aizenman and Wehr. This phenomenon was recently quantified for the two-dimensional random-field Ising model (RFIM), proving that it exhibits exponential decay of correlations at all temperatures and all positive random-field strengths. The talk will present new results on the quantitative aspects of the phenomenon for general systems with discrete and continuous symmetries, including Potts, spin O(n) and spin glass models. The discussion is further extended to real- and integer-valued height function models driven by a random field, for which we study the fluctuations of the gradient and height variables. Among the challenges presented by the latter setup is a conjectured roughening transition in the random-field strength for the three-dimensional integer-valued random-field Gaussian free field. Joint work with Paul Dario and Matan Harel. |

August 17, 2021 |
Sasha Sodin (Queen Mary University London)Lower bounds on the eigenfunctions of random Schroedinger
operators in a stripVideo link: https://youtu.be/oF3lB8UCqjo Show abstract It is known that the eigenfunctions of a random Schroedinger operator in a strip decay exponentially. In some regimes, the same is true in higher dimensions. It is however not clear whether the eigenfunctions have an exact rate of exponential decay. In the strip, it is natural to expect that the rate should be given by the slowest Lyapunov exponent, however, only the upper bound has been previously established. |

August 10, 2021 |
Daniela Cadamuro (University of Leipzig)Operator-algebraic construction of quantum integrable models with bound statesVideo link: https://youtu.be/VBoS-5uRjXU Show abstract The rigorous construction of quantum field theories with self-interaction is one of the longstanding problems of Mathematical Physics. Progress in this respect has been made in integrable field theories in 1+1 spacetime dimensions. These are characterized by a factorizing scattering matrix, where two-particle interaction determines scattering completely. Specifically, some of these theories (so-called scalar models without bound states) have been successfully treated in the operator-algebraic approach, which is based on quantum fields localized in infinitely extended wedge regions. The existence of strictly localized observables is then obtained by abstract W*-algebraic arguments. This avoids dealing with the functional analytic properties of pointlike interacting fields, which are difficult to control due to the convergence problem of the infinite series of their form factors. In extension of these results, we consider S-matrices with poles in the physical strip (corresponding to the notion of `bound states’ in the quantum mechanical sense). We exhibit wedge-local fields in these models, which arise as a deformation of those in the non-boundstate models by an additive term, the so called ``bound state operator''. This technique applies to a variety of theories, e.g., the Bullough-Dodd model, the Z(N)-Ising model, the affine Toda field theories and the Sine-Gordon model. The link between these wedge-local fields and strictly local operator algebras is subject to ongoing research. |

August 3, 2021 |
ICMP 2021 (in Geneva) |

July 27, 2021 |
Eveliina Peltola (University of Bonn)On large deviations of SLEs, real rational functions, and zeta-regularized determinants of LaplaciansVideo link: https://youtu.be/T5XR9iKVKsQ Show abstract When studying large deviations (LDP) of Schramm-Loewner evolution (SLE) curves, we recently introduced a ''Loewner potential'' that describes the rate function for the LDP. This object turned out to have several intrinsic, and perhaps surprising, connections to various fields. For instance, it has a simple expression in terms of zeta-regularized determinants of Laplace-Beltrami operators. On the other hand, minima of the Loewner potential solve a nonlinear first order PDE that arises in a semiclassical limit of certain correlation functions in conformal field theory, arguably also related to isomonodromic systems. Finally, and perhaps most interestingly, the Loewner potential minimizers classify rational functions with real critical points, thereby providing a novel proof for a version of the now well-known Shapiro-Shapiro conjecture in real enumerative geometry. |

July 20, 2021 |
Phan Thành Nam (LMU Munich)Bogoliubov excitation spectrum of dilute trapped Bose gasesVideo link: https://youtu.be/AmlTpUfQI6w Show abstract In 1947, Bogoliubov proposed an approximation for the low-lying eigenvalues of weakly interacting Bose gases and used that to explain Landau’s criterion for superfluidity. We will discuss a rigorous derivation of Bogoliubov’s approximation for a general trapped Bose gas in the Gross-Pitaevskii regime, where the two-body scattering process of particles leads to an interesting nonlinear effect. The talk is based on joint work with Arnaud Triay. |

July 13, 2021 |
Mark Malamud (Peoples Friendship University of Russia, Moscow)On the spectral theory of Schrodinger and Dirac operators with point interactions and quantum graphsVideo link: https://youtu.be/nC6x6k5QeyY Show abstract Modern concepts of extension theory of symmetric operators, including concepts of boundary triples, corresponding Weyl functions, and boundary operators will be discussed. Applications to Schrodinger and Dirac operators with point interactions, as well as to quantum graphs, will be demonstrated. It turns out that certain spectral properties of each of these operators (deficiency indices, selfadjointness, discreteness, lower semiboundedness, etc) strictly correlate to that of a special difference operator. In the first two cases the corresponding difference operator is generated by a special Jacobi matrix. This matrix appears as a boundary operator of the corresponding Schrodinger and Dirac realization in an appropriate boundary triple. In the case of quantum graphs a similar role is played by a certain weighted discrete Laplacian on the underlying discrete graph, which also appears as a boundary operator. |

July 6, 2021 |
Nikolaos Zygouras (University of Warwick)On the 2d-KPZVideo link: https://youtu.be/t0JmfPHV0oo Show abstract The Kardar-Parisi-Zhang (KPZ) equation in dimension 1 is by now fairly well understood, both in terms of its solution theory and its phenomenology. But in the critical dimension 2, the first steps of progress have only recently been made, which shows signs of a rich structure. In particular, a phase transition is observed when the noise is suitably scaled. Below the critical scaling, the 2d KPZ coincides with the Edwards-Wilkinson universality, while at the critical scaling and beyond, its behaviour is still mysterious. We will review joint works with F. Caravenna and R. Sun as well as contributions by other groups, and expose some of the tools used, including (discrete) stochastic analysis, renewal theory, and mathematical physics ideas from the study of disorder relevance and Schrodinger operators with point interactions. |

June 29, 2021 |
Mathieu Lewin (Paris Dauphine University)Density Functional Theory: some recent resultsVideo link: https://youtu.be/iKXKN6E6ktI Show abstract The quantum electrons in an atom or a molecule are in principle described by the Schrödinger multi-particle linear equation. Unfortunately, in most cases it is essentially impossible to find a sufficiently precise numerical approximation of the solution, due to the very high dimensionality of the problem. It is absolutely necessary to resort to approximate models, most of which being nonlinear. Density Functional Theory (DFT) is probably the most successful and widely used method in Chemistry and Physics. In this talk I will explain what DFT is and outline its mathematical formulation. I will then describe some recent results obtained in collaboration with Elliott H. Lieb (Princeton) and Robert Seiringer (IST Austria). |

June 22, 2021 |
Clément Tauber (University of Strasbourg)Topological indices for shallow-water waves.Video link: https://youtu.be/b5ktNPe5Ohw Show abstract I will apply tools from topological insulators to a fluid dynamics problem: the rotating shallow-water wave model with odd viscosity. The bulk-edge correspondence explains the presence of remarkably stable waves propagating towards the east along the equator and observed in some Earth oceanic layers. The odd viscous term is a small-scale regularization that provides a well defined Chern number for this continuous model where momentum space is unbounded. Equatorial waves then appear as interface modes between two hemispheres with a different topology. However, in presence of a sharp boundary there is a surprising mismatch in the bulk-edge correspondence: the number of edge modes depends on the boundary condition. I will explain the origin of such a mismatch using scattering theory and Levinson’s theorem. This talk is based on a series of joint works with Pierre Delplace, Antoine Venaille, Gian Michele Graf and Hansueli Jud. |

June 15, 2021 |
Gordon Slade (UBC Vancouver)Mean-field tricritical polymersVideo link: https://youtu.be/5dHuklnysPQ Show abstract We provide a full description of a tricritical phase diagram in the setting of a mean-field random walk model of a polymer density transition. The model involves a continuous-time random walk on the complete graph in the limit as the number of vertices N in the graph grows to infinity. The walk has a repulsive self-interaction, as well as a competing attractive self-interaction whose strength is controlled by a parameter g. A chemical potential ν controls the walk length. We determine the phase diagram in the (g,ν) plane, as a model of a density transition for a single linear polymer chain. The proof uses a supersymmetric representation for the random walk model, followed by a single block-spin renormalisation group step to reduce the problem to a 1-dimensional integral, followed by application of the Laplace method for an integral with a large parameter. The talk is based on joint work with Roland Bauerschmidt, Probability and Mathematical Physics 1, 167-204 (2020); arXiv:1911.00395 . |

June 8, 2021 |
Laszlo Erdös (IST Austria)Eigenstate thermalisation hypothesis and Gaussian fluctuations for Wigner matricesVideo link: https://youtu.be/GtrvMt6r974 Show abstract We prove that any deterministic matrix is approximately the identity in the eigenbasis of a large random Wigner matrix with an optimal error inversely proportional to the square root of the dimension. This verifies a strong form of Quantum Unique Ergodicity with an optimal convergence rate and we also prove Gaussian fluctuations around this convergence. The key technical tool is a new multi-resolvent local law for Wigner ensemble and the Dyson Brownian motion for eigenvector overlaps. |

June 1, 2021 |
Benjamin Schlein (University of Zürich)Correlation energy of weakly interacting Fermi gasesVideo link: https://youtu.be/r-KiTPFUgdI Show abstract We consider Fermi gases in a mean-field regime. To leading order, the ground state energy is given by Hartree-Fock theory. The correction to the Hartree-Fock energy, produced by the many-body interaction, is known as correlation energy. In this talk, we obtain a formula for the correlation energy, based on the observation that, energetically, the main excitations of the Fermi sea are particle-hole pairs that behave, approximately, as bosons and can therefore be dealt with through (bosonic) Bogoliubov theory. The talk is based on joint works with N. Benedikter, P.T. Nam, M. Porta and R. Seiringer. |

May 25, 2021 |
Constanza Rojas-Molina (Cergy Paris University)Fractional random Schrödinger operators, integrated density of states and localizationVideo link: https://youtu.be/IUrzalS1tqs Show abstract We will review some recent results on the fractional Anderson model, a random Schrödinger operator driven by a fractional laplacian. The interest of the latter lies in its association to stable Levy processes, random walks with long jumps and anomalous diffusion. We discuss the interplay between the non-locality of the fractional laplacian and the localization properties of the random potential in the fractional Anderson model, in both the continuous and discrete settings. In the discrete setting we study the integrated density of states and show a fractional version of Lifshitz tails. This coincides with results obtained in the continuous setting by the probability community. This talk is based on joint work with M. Gebert (LMU Munich). |

May 18, 2021 |
Marcel Griesemer (University of Stuttgart)From Short-Range to Contact Interactions in Many-Body Quantum SystemsVideo link: https://youtu.be/Xu6li-1bdfo Show abstract Many-body quantum systems with short-ranged two-body interactions, such as ultracold quantum gases are often described by simplified models with contact interactions (sometimes called delta-potentials). The merit of such models is their simplicity, solvability (to some extent) and lack of irrelevant or unknown detail. They are also toy models of renormalization. On the other hand, in dimensions $d\geq 2$ contact interactions are not small perturbations of the free energy, which makes their self adjoint implementation in the case of $N>2$ particles a subtle business. This talk gives an introduction to the mathematics of many-particle quantum systems with two-body contact interactions and their approximation by Schrödinger operators. |

May 11, 2021 |
David Brydges (University of British Columbia)Lace expansions and spin modelsVideo link: https://youtu.be/KAuRLtSuX9c Show abstract I will review lace expansions starting with their relation to the other expansions in statistical mechanics and then discussing their application to spin models. Akira Sakai opened this avenue in 2007 with his proof that in sufficiently high dimension $d$ the two point function for the critical Ising model is asymptotic to $c_d |x-y|^{-(d-2)}$. In |

May 4, 2021 |
Chris Fewster (University of York)Measurement of quantum fields in curved spacetimesVideo link: https://youtu.be/sdnYBumtkeM Show abstract A standard account of the measurement chain in quantum mechanics involves a probe (itself a quantum system) coupled temporarily to the system of interest. Once the coupling is removed, the probe is measured and the results are interpreted as the measurement of a system observable. Measurement schemes of this type have been studied extensively in Quantum Measurement Theory, but they are rarely discussed in the context of quantum fields and still less on curved spacetimes. |

April 27, 2021 |
Sabine Jansen (LMU Munich)Virial expansion for mixtures: some examples & recent resultsVideo link: https://youtu.be/YEKH8dsrcpY Show abstract Mayer's expansion is an expansion of the pressure in powers of the activity (fugacity) in equilibrium statistical mechanics; the virial expansion is an expansion in powers of the density. Rigorous convergence results have been available for decades, nevertheless for mixtures the theory of density expansions is less advanced than the theory of activity expansions. I will review the setting, and discuss some recent results for the virial expansion for mixtures (multi-species systems) and illustrate them with three concrete models: mixtures of hard spheres of different sizes, non-overlapping rods of different lengths, and a hierarchical mixture of cubes in Z^d. The last two examples are helpful toy models for which concrete formulas are available and phase transitions can be studied. Based in part on joint works with Tobias Kuna, Stephen Tate, Dimitrios Tsagkarogiannis and Daniel Ueltschi. |

April 20, 2021 |
Eric Carlen (Rutgers University)Kac Master Equations, Classical and QuantumVideo link: https://youtu.be/Q6WcqC0aOMM Show abstract This lecture will review recent progress and open problems concerning Kac Master Equations in both the classical and quantum setting. It will be based largely on recent papers written in collaboration with Maria Carvalho, Michael Loss and Bernt Wennberg. |

April 13, 2021 |
Wojciech De Roeck (KU Leuven)Impurities and boundaries for a class of gapped ground statesVideo link: https://youtu.be/z7aammyADlg Show abstract In the last two decades, a lot of rigorous results have been proven for quantum ground states protected by a spectral gap in their Hamiltonian. Think for example of decay of correlations, the topological classification of states, and area laws of entanglement. For all of those results, the spectral gap is a necessary input, both on the conceptual and on the technical level. Imagine now that we consider such a gapped Hamiltonian that is perturbed locally by a term that is not small, such that we certainly cannot hope to preserve the global gap. Common sense dictates that, far away from the perturbation, the ground state should not be seriously affected by this perturbation. While we cannot prove this in the generality in which it should be true, I will describe some results that go in this direction. |

April 6, 2021 |
Bernard Helffer (Paris-Sud / Nantes)Semiclassical methods and tunneling effects: old and newVideo link: https://youtu.be/szMl4Kv1-ek Show abstract In 1982-1983 the so-called symmetric double well problem was rigorously analyzed in any dimension in the semi-classical context by B. Simon and Helffer-Sjöstrand. This involves semi-classical Agmon estimates, Harmonic approximation, WKB constructions and a very fine analysis of the so-called tunneling effect in order to establish the splitting between the lowest eigenvalues. |

March 30, 2021 |
Jan Wehr (University of Arizona)Aggregation and deaggregation of interacting micro swimmersVideo link: https://youtu.be/-7TrciUd5w4 Show abstract A swarm of light-sensitive robots is moving in a planar region, changing the direction of motion randomly. Each robot emits light; in turn, it adapts the speed of its motion to the total intensity of the light shed on it by the other robots. The adaptation takes time---the sensorial delay. Using a natural approximation of the equations of motion, the robots may be programmed to make the sensorial delay negative. In this case they are, in some sense, predicting the future. In a series of experiments by the Giovanni Volpe group and the University of Gothenburg (Sweden) it was shown that at a certain negative value of the delay, the collective behavior of the robots changes qualitatively from aggregation to deaggregation. I am going to explain this phenomenon by an asymptotic analysis of the system of stochastic differential equations, describing the motion of a single robot in an inhomogeneous landscape. |

March 23, 2021 |
Sylvia Serfaty (Courant Institute)Ginzburg-Landau vortices, old and newVideo link: https://youtu.be/ayfWbNW16kA Show abstract We present a review of results on vortices in the 2D Ginzburg-Landau model of superconductivity (also relevant for superfluidity and Bose-Einstein condensates), their onset at critical fields, interaction and patterns. We also report on recent joint work with Carlos Roman and Etienne Sandier in which we study the onset of vortex lines in the 3D model and derive an interaction energy for them. |

March 16, 2021 |
Gianluca Panati (University of Rome Sapienza)The Localization Dichotomy in Solid State PhysicsVideo link: https://youtu.be/KbuVtavr0Qs Show abstract Solid state systems exhibit a subtle intertwining between the topology of the "manifold" of occupied states and the localization of electrons, if the latter is appropriately defined. |

March 9, 2021 |
Simone Warzel (TU Munich)An invitation to quantum spin glassesVideo link: https://youtu.be/2vv9ryssLR4 Show abstract Quantum spin glass models of mean-field type are prototypes of quantum systems exhibiting phase transitions related to the spead of the eigenstates in configuration space. Originally motivated by spin glass physics, they are discussed in relation to many-body localisation phenomena, quantum adiabatic algorithms as well as in the context of models in mathematical biology. Quantum effects transform the rich phase diagram of classical spin glasses as described by Parisi theory into an even more colourful landscape of phases which range from purely quantum paramagnetic to intermediate behavior in which eigenstates occupy only a fraction of configuration space. In this talk, I will give an introduction to the multifaceted motivations and challenges behind the study of these quantum glasses and explain existing and expected results. |

March 2, 2021 |
Margherita Disertori (University of Bonn)Supersymmetric transfer operatorsVideo link: https://youtu.be/lRtSNMix1Rs Show abstract Transfer matrix approach is a powerful tool to study one dimensional or quasi 1d statistical mechanical models. Transfer operator kernels arising in the context of quantum diffusion and the supersymmetric approach display bosonic and fermionic components. For such kernels, the presence of fermion-boson symmetries allows to drastically simplify the problem. I will review the method and give some results for the case of random band matrices. |

February 23, 2021 |
Tyler Helmuth (University of Durham)Efficient algorithms for low-temperature spin systemsVideo link: https://youtu.be/btFWDlne7kU Show abstract Two fundamental algorithmic tasks associated to discrete statistical mechanics models are approximate counting and approximate sampling. At high temperatures Markov chains give efficient algorithms, but at low temperatures mixing times can become impractically large, and Markov chain methods may fail to be efficient. Recently, expansion methods (cluster expansions, Pirogov--Sinai theory) have been put to use to develop provably efficient low-temperature algorithms for some discrete statistical mechanics models. I’ll introduce these algorithmic tasks, outline how expansion algorithms work, and indicate some open directions. |

February 16, 2021 |
Gerald Dunne (University of Connecticut)Resurgent Asymptotics and Non-perturbative PhysicsVideo link: https://youtu.be/i1iNUMurUrM Show abstract There are several important conceptual and computational questions concerning path integrals, which have recently been approached from new perspectives motivated by Ecalle's theory of resurgent functions, a mathematical formalism that unifies perturbative and non-perturbative physics. I will review the basic ideas behind the connections between resurgent asymptotics and physics, with examples from quantum mechanics and matrix models. |

February 9, 2021 |
Herbert Spohn (TU Munich)Hydrodynamic equations for the classical Toda latticeVideo link: https://youtu.be/1n8RuSod9-g Show abstract For arbitrary size, the Toda lattice is an integrable system with an extensive number of local conservation laws. To then build the respective hydrodynamic equations, one has to appropriately generalize the notion of local equilibrium and to obtain average fields together with their currents in such states. In my presentation I will explain how these goals can be achieved. The unexpected relation to the repulsive log gas in one dimension is discussed. |

February 2, 2021 |
Wei Wu (NYU Shanghai)Massless phases for the Villain model in d>=3Video link: https://youtu.be/XsYk9LV-gxk Show abstract The XY and the Villain models are mathematical idealization of real world models of liquid crystal, liquid helium, and superconductors. Their phase transition has important applications in condensed matter physics and led to the Nobel Prize in Physics in 2016. However we are still far from a complete mathematical understanding of the transition. The spin wave conjecture, originally proposed by Dyson and by Mermin and Wagner, predicts that at low temperature, large scale behaviors of these models are closely related to Gaussian free fields. I will review the historical background and discuss some recent progress on this conjecture in d>=3. Based on the joint work with Paul Dario (Tel Aviv). |

January 26, 2021 |
Jürg Fröhlich (ETH Zürich)The Evolution of States as the Fourth Pillar of Quantum MechanicsVideo link: https://youtu.be/I83mvV8FOZo Show abstract I present ideas about how to extend the standard formalism of Quantum Mechanics in such a way that the theory actually makes sense. My tentative extension is called |

January 19, 2021 |
Robert Seiringer (IST Austria)Quantum fluctuations and dynamics of a strongly coupled polaronVideo link: https://youtu.be/eR9b9hmssv0 Show abstract We review old and new results on the Fröhlich polaron model. The discussion includes the validity of the (classical) Landau--Pekar equations for the dynamics in the strong coupling limit, quantum corrections to this limit, as well as the divergence of the effective polaron mass. |

January 12, 2021 |
Fabio Toninelli (Technical University Vienna)The anisotropic KPZ equation and logarithmic super-diffusivityVideo link: https://youtu.be/d6-mN_x8ZO8 Show abstract The AKPZ equation is an anisotropic variant of the celebrated (two-dimensional) KPZ stochastic PDE, which is expected to describe the large-scale behavior of (2+1)-dimensional growth models whose average speed of growth is a non-convex function of the average slope (AKPZ universality class). Several interacting particle systems belonging to the AKPZ class are known, notably a class of two-dimensional interlaced particle systems introduced by A. Borodin and P. Ferrari. In this talk, I will show that the non-linearity of the AKPZ equation is marginally relevant in the Renormalization Group sense: in fact, while the 2d-SHE is invariant under diffusive rescaling, for AKPZ the diffusion coefficient diverges (logarithmically) for large times, implying marginal super-diffusivity. [Based on joint work with G. Cannizzaro and D. Erhard] |

January 5, 2021 |
Søren Fournais (Aarhus University)The ground state energy of dilute Bose gasesVideo link: https://youtu.be/JCa_n9nUK2U Show abstract The rigorous calculation of the ground state energy of dilute Bose gases has been a challenging problem since the 1950s. In particular, it is of interest to understand the extent to which the Bogoliubov pairing theory correctly describes the ground state of this physical system. In this talk I will report on a recent proof of the second term in the energy expansion for dilute gases, the so-called Lee-Huang-Yang term, and its relation to Bogoliubov theory. |

December 15, 2020 |
Yoshiko Ogata (University of Tokyo)Classification of symmetry protected topological phases in quantum spin systemsVideo link: https://youtu.be/cXk6Fk5wD_4 Show abstract A Hamiltonian is in a SPT phase with a given symmetry if it cannot be smoothly deformed into a trivial Hamiltonian without a phase transition, if the deformation preserves the symmetry, while it can be smoothly deformed into a trivial Hamiltonian without a phase transition, if the symmetry is broken during the deformation. We consider this problem for one- and two-dimensional quantum spin systems with on-site finite group symmetries. |

December 8, 2020 |
Katrin Wendland (Albert-Ludwigs-Universität Freiburg)On invariants shared by geometry and conformal field theoryVideo link: https://youtu.be/zJ-zF1KBYM0 Show abstract I will recall how some conformal field theories can be given geometric interpretations. This can be useful from a practical point of view, when geometric methods are transferred from geometry to conformal field theory. I will in particular focus on certain invariants that are shared by geometry and conformal field theory, including the complex elliptic genus. As we shall see, this invariant is also useful from a theoretical viewpoint, since it captures information about generic properties of certain theories. |

December 1, 2020 |
Alessandro Pizzo (University of Rome Tor Vergata)Stability of gapped quantum chains under small perturbationsVideo link: https://youtu.be/S5lRLi3W6-E Show abstract We consider a family of quantum chains that has attracted much interest amongst people studying topological phases of matter. Their Hamiltonians are perturbations, by interactions of short range, of a Hamiltonian consisting of on-site terms and with a strictly positive energy gap above its ground-state energy. We prove stability of the spectral gap, uniformly in the length of the chain. |

November 24, 2020 |
Roland Bauerschmidt (University of Cambridge)The Coleman correspondence at the free fermion pointVideo link: https://youtu.be/uckzDglTbcM Show abstract Two-dimensional statistical and quantum field theories are special in many ways. One striking instance of this is the equivalence of certain bosonic and fermionic fields, known as bosonization. I will first review this correspondence in the explicit instance of the massless Gaussian free field and massless Euclidean Dirac fermions. I will then present a result that extends this correspondence to the non-Gaussian `massless' sine-Gordon field on $\R^2$ at $\beta=4\pi$ and massive Dirac fermions. This is an instance of Coleman's prediction that the `massless' sine-Gordon model and the massive Thirring model are equivalent. We use this correspondence to show that correlations of the `massless' sine-Gordon model decay exponentially for $\beta=4\pi$. This is joint work with C. Webb (arXiv:2010.07096). |

November 17, 2020 |
Stefan Hollands (University of Leipzig)(In)determinism Inside Black HolesVideo link: https://youtu.be/BmZxDseHnT0 Show abstract In classical General Relativity, the values of fields on spacetime are uniquely determined by their initial values within its "domain of dependence". However, it may occur that the spacetime under consideration extends beyond this domain, and fields, therefore, are not entirely determined by their initial data. In fact, such a naive failure of determinism occurs inside all physically relevant black holes. |

November 10, 2020 |
Peter Hintz (MIT)Linear stability of slowly rotating Kerr spacetimesVideo link: https://youtu.be/iTBpM_DlGaI Show abstract I will describe joint work with Dietrich Häfner and András Vasy in which we study the asymptotic behavior of linearized gravitational perturbations of Schwarzschild or slowly rotating Kerr black hole spacetimes. We show that solutions of the linearized Einstein equation decay at an inverse polynomial rate to a stationary solution (given by an infinitesimal variation of the mass and angular momentum of the black hole), plus a pure gauge term. The proof uses a detailed description of the resolvent of an associated wave equation on symmetric 2-tensors near zero energy. |

November 3, 2020 |
Nalini Anantharaman (University of Strasbourg)The bottom of the spectrum of a random hyperbolic surfaceVideo link: https://youtu.be/MThzUIIZx2Y Show abstract I will report on work in progress with Laura Monk, where we study the bottom of the spectrum of the laplacian, on a compact hyperbolic surface chosen at random, in the limit of growing genus. We pick combinatorial ideas from the study of random regular graphs, to propose a strategy to prove that, with high probability, there are no eigenvalues below $1/4-\epsilon$. |

October 27, 2020 |
Bruno Nachtergaele (UC Davis)Gapped ground state phases of lattice systems - Stability of the bulk gapVideo link: https://youtu.be/rKzw0mr9xg4 Show abstract I will give an overview of stability results for the spectral gap above the ground states of quantum lattice systems obtained in the past decade and explain the overall strategy introduced by Bravyi, Hastings, and Michalakis. A new result I will present proves the stability of the bulk gap for infinite systems with an approach that bypasses possible gapless boundary modes (joint work with Bob Sims and Amanda Young). |

October 13, 2020 |
Jeremy Quastel (University of Toronto)The landscape law for the integrated density of statesVideo link: https://youtu.be/0f4vaNuLbK8 Show abstract Complexity of the geometry, randomness of the potential, and many other irregularities of the system can cause powerful, albeit quite different, manifestations of localization, a phenomenon of sudden confinement of waves, or eigenfunctions, to a small portion of the original domain. In the present talk we show that behind a possibly disordered system there exists a structure, referred to as a landscape function, which can predict the location and shape of the localized eigenfunctions, a pattern of their exponential decay, and deliver accurate bounds for the corresponding eigenvalues. In particular, we establish the "landscape law", the first non-asymptotic estimates from above and below on the integrated density of states of the Schroedinger operator using a counting function for the minima of the localization landscape. The results are deterministic, and rely on a new uncertainty principle. Narrowing down to the context of disordered potentials, we derive the best currently available bounds on the integrated density of states for the Anderson model. |

October 6, 2020 |
Clotilde Fermanian Kammerer (Université Paris Est - Créteil Val de Marne)Nonlinear quantum adiabatic approximationVideo link: https://youtu.be/kY-IrHnlw54 Show abstract We will discuss generalization of the quantum adiabatic theorem to a nonlinear setting. We will consider evolution equations where the Hamiltonian operator depends on the time variable and on a finite number of parameters that are fixed on some coordinates of the unknown, making the equation non-linear. Under natural spectral hypotheses, there exist special functions that we call « Instantaneous nonlinear eigenvectors » such that, in the adiabatic limit, the solutions of the nonlinear equations with those initial data remain close to them, up to a rapidly oscillating phase. We will explain why this question arises, discuss the proof of this result and show how it can fail (works in collaboration with Alain Joye and Rémi Carles). |

September 29, 2020 |
Alessandro Giuliani (University Roma Tre)Non-renormalization of the `chiral anomaly' in interacting lattice Weyl semimetalsVideo link: https://youtu.be/zUhM1k5nPcc Show abstract Weyl semimetals are 3D condensed matter systems characterized by a degenerate Fermi surface, consisting of a pair of `Weyl nodes'. Correspondingly, in the infrared limit, these systems behave effectively as Weyl fermions in 3+1 dimensions. We consider a class of interacting 3D lattice models for Weyl semimetals and prove that the quadratic response of the quasi-particle flow between the Weyl nodes, which is the condensed matter analogue of the chiral anomaly in QED4, is universal, that is, independent of the interaction strength and form. Universality, which is the counterpart of the Adler-Bardeen non-renormalization property of the chiral anomaly for the infrared emergent description, is proved to hold at a non-perturbative level, notwithstanding the presence of a lattice (in contrast with the original Adler-Bardeen theorem, which is perturbative and requires relativistic invariance to hold). The proof relies on constructive bounds for the Euclidean ground state correlations combined with lattice Ward Identities, and it is valid arbitrarily close to the critical point where the Weyl points merge and the relativistic description breaks down. Joint work with V. Mastropietro and M. Porta. |

September 22, 2020 |
Ian Jauslin (Princeton University)An effective equation to study Bose gasses at both low and high densitiesVideo link: https://youtu.be/HyRG-PzvpyY Show abstract I will discuss an effective equation, which is used to study the ground state of the interacting Bose gas. The interactions induce many-body correlations in the system, which makes it very difficult to study, be it analytically or numerically. A very successful approach to solving this problem is Bogolubov theory, in which a series of approximations are made, after which the analysis reduces to a one-particle problem, which incorporates the many-body correlations. The effective equation I will discuss is arrived at by making a very different set of approximations, and, like Bogolubov theory, ultimately reduces to a one-particle problem. But, whereas Bogolubov theory is accurate only for very small densities, the effective equation coincides with the many-body Bose gas at both low and at high densities. |

September 15, 2020 |
Victor Ivrii (University of Toronto)Scott and Thomas-Fermi approximations to electronic densityVideo link: https://youtu.be/O25BT_-XNNE Show abstract In heavy atoms and molecules, on the distances $ a \ll Z^{-1/2}$ from one of the nuclei (with a charge $Z_m$), we prove that the ground state electronic density $\rho_\Psi (x)$ is approximated in $\sL^p$-norm by the ground state electronic density for a single atom in the model with no interactions between electrons. |

September 8, 2020 |
Antti Kupiainen (University of Helsinki)Integrability of Liouville Conformal Field TheoryVideo link: https://youtu.be/0ms4gEUT2Nw Show abstract A. Polyakov introduced Liouville Conformal Field theory (LCFT) in 1981 as a way to put a natural measure on the set of Riemannian metrics over a two dimensional manifold. Ever since, the work of Polyakov has echoed in various branches of physics and mathematics, ranging from string theory to probability theory and geometry. In the context of 2D quantum gravity models, Polyakov’s approach is conjecturally equivalent to the scaling limit of Random Planar Maps and through the Alday-Gaiotto- Tachikava correspondence LCFT is conjecturally related to certain 4D Yang-Mills theories. Through the work of Dorn,Otto, Zamolodchikov and Zamolodchikov and Teschner LCFT is believed to be to a certain extent integrable. |

July 28, 2020 |
Nicolas Rougerie (University of Grenoble Alpes)Two modes approximation for bosons in a double well potentialVideo link: https://youtu.be/ylb6BWewlpI Show abstract We study the mean-field limit for the ground state of bosonic particles in a double-well potential, jointly with the limit of large inter-well separation/large potential energy barrier. Two one-body wave-functions are then macroscopially occupied, one for each well. The physics in this two-modes subspace is usually described by a Bose-Hubbard Hamiltonian, yielding in particular the transition from an uncorrelated "superfluid" state (each particle lives in both potential wells) to a correlated "insulating" state (half of the particles live in each potential well). |

July 21, 2020 |
Hugo Duminil-Copin (IHES / University of Geneva)Marginal triviality of the scaling limits of critical 4D Ising and φ_4^4 modelsVideo link: https://youtu.be/DtLKEQran_Y Show abstract In this talk, we will discuss the scaling limits of spin fluctuations in four-dimensional Ising-type models with nearest-neighbor ferromagnetic interaction at or near the critical point are Gaussian and its implications from the point of view of Euclidean Field Theory. Similar statements will be proven for the λφ4 fields over R^4 with a lattice ultraviolet cutoff, in the limit of infinite volume and vanishing lattice spacing. The proofs are enabled by the models' random current representation, in which the correlation functions' deviation from Wick's law is expressed in terms of intersection probabilities of random currents with sources at distances which are large on the model's lattice scale. Guided by the analogy with random walk intersection amplitudes, the analysis focuses on the improvement of the so-called tree diagram bound by a logarithmic correction term, which is derived here through multi-scale analysis. |

July 14, 2020 |
Hal Tasaki (Gakushuin University)'Topological' index and general Lieb-Schultz-Mattis theorems for quantum spin chainsVideo link: https://youtu.be/q0k1sch56Dk Show abstract A Lieb-Schultz-Mattis (LSM) type theorem states that a quantum many-body system with certain symmetry cannot have a unique ground state accompanied by a nonzero energy gap. While the original theorem treats models with continuous U(1) symmetry, new LSM-type statements that only assume discrete symmetry have been proposed recently in close connection with topological condensed matter physics. Here we shall prove such general LSM-type theorems by using the "topological" index intensively studied in the context of symmetry protected topological phase. Operator algebraic formulation of quantum spin chains plays an essential role in our approach. Here I do not assume any advanced knowledge in quantum spin systems or operator algebra, and illustrate the ideas of the proof (which I believe to be interesting). |

July 7, 2020 |
Bruno Després (Sorbonne University)Spectral-scattering theory and fusion plasmasVideo link: https://youtu.be/lmnm1D3NFp8 Show abstract Motivated by fusion plasmas and Tokamaks (ITER project), I will describe recent efforts on adapting the mathematical theory of linear unbounded self-adjoint operators (Kato, Lax, Reed-Simon, ....) to problems governed by kinetic equations coupled with Maxwell equations. Firstly it will be shown that Vlasov-Poisson-Ampere equations, linearized around non homogeneous Maxwellians, can be written in the framework of abstract scattering theory (linear Landau damping is a consequence). Secondly the absorption principle applied to the hybrid resonance will be discussed. All results come from long term discussions and collaborations with many colleagues (Campos-Pinto, Charles, Colas, Heuraux, Imbert-Gérard, Lafitte, Nicolopoulos, Rege, Weder, and many others). |

June 30, 2020 |
Laure Saint-Raymond (ENS Lyon)Fluctuation theory in the Boltzmann-Grad limitVideo link: https://youtu.be/fLDFA7ZCagA Show abstract In this talk, I will discuss a long term project with T. Bodineau, I. Gallagher and S. Simonella on hard-sphere dynamics in the kinetic regime, away from thermal equilibrium. In the low density limit, the empirical density obeys a law of large numbers and the dynamics is governed by the Boltzmann equation. Deviations from this behavior are described by dynamical correlations, which can be fully characterized for short times. This provides both a fluctuating Boltzmann equation and large deviation asymptotics. |

June 23, 2020 |
Nilanjana Datta (University of Cambridge)Discriminating between unitary quantum processesVideo link: https://youtu.be/gHEjszXSjMQ Show abstract Discriminating between unknown objects in a given set is a fundamental task in experimental science. Suppose you are given a quantum system which is in one of two given states with equal probability. Determining the actual state of the system amounts to doing a measurement on it which would allow you to discriminate between the two possible states. It is known that unless the two states are mutually orthogonal, perfect discrimination is possible only if you are given arbitrarily many identical copies of the state. |

June 16, 2020 |
Nicola Pinamonti (University of Genova)Equilibrium states for interacting quantum field theories and their relative entropyVideo link: https://youtu.be/excgcO7loj0 Show abstract During this talk we will review the construction of equilibrium states for interacting scalar quantum field theories, treated with perturbation theory, recently proposed by Fredenhagen and Lindner. We shall in particular see that this construction is a generalization of known results valid in the case of C*-dynamical systems. We shall furthermore discuss some properties of these states and we compare them with known results in the physical literature. In the last part of the talk, we shall show that notions like relative entropy or entropy production can be given for states which are of the form discussed in the first part of talk. We shall thus provide an extension to quantum field theory of similar concepts available in the case of C*-dynamical systems. |

June 9, 2020 |
Andreas Winter (Universitat Autònoma de Barcelona)Energy-constrained diamond norms and the continuity of channel capacities and of open-system dynamicsVideo link: https://youtu.be/05ZQPFB0aAc Show abstract The channels, and more generally superoperators acting on the trace class operators of a quantum system naturally form a Banach space under the completely bounded trace norm (aka diamond norm). However, it is well-known that in infinite dimension, the norm topology is often "too strong" for reasonable applications. Here, we explore a recently introduced energy-constrained diamond norm on superoperators (subject to an energy bound on the input states). Our main motivation is the continuity of capacities and other entropic quantities of quantum channels, but we also present an application to the continuity of one-parameter unitary groups and certain one-parameter semigroups of quantum channels. |

June 2, 2020 |
Mihalis Dafermos (Cambridge University)The nonlinear stability of the Schwarzschild metric without symmetryVideo link: https://youtu.be/6Vh62H0rPiA Show abstract I will discuss an upcoming result proving the full finite-codimension non-linear asymptotic stability of the Schwarzschild family as solutions to the Einstein vacuum equations in the exterior of the black hole region. No symmetry is assumed. The work is based on our previous understanding of linear stability of Schwarzschild in double null gauge. Joint work with G. Holzegel, I. Rodnianski and M. Taylor. |

May 26, 2020 |
Sven Bachmann (University of British Columbia)Adiabatic quantum transportVideo link: https://youtu.be/ErgMuxMR_1A Show abstract In the presence of a spectral gap above the ground state energy, slowly driven condensed matter systems may exhibit quantized transport of charge. One of the earliest instances of this fact is the Laughlin argument explaining the integrality of the Hall conductance. In this talk, I will discuss transport by adiabatic processes in the presence of interactions between the charge carriers. I will explain the central role played by the locality of the quantum dynamics in two instances: the adiabatic theorem and an index theorem for quantized charge transport. I will also relate fractional transport to the anyonic nature of elementary excitations. |

May 19, 2020 |
Pierre Clavier (University of Potsdam)Borel-Ecalle resummation for a Quantum Field TheoryVideo link: https://youtu.be/EzRoLEZhono Show abstract Borel-Ecalle resummation of resurgent functions is a vast generalisation of the well-known Borel-Laplace resummation method. It can be decomposed into three steps: Borel transform, averaging and Laplace transform. I will start by a pedagogical introduction of each of these steps. To illustrate the feasability of the Borel-Ecalle resummation method I then use it to resum the solution of a (truncated) Schwinger-Dyson equation of a Wess-Zumino model. This will be done using known results about this Wess-Zumino model as well as Sauzin's analytical bounds on convolution of resurgent functions. |

May 12, 2020 |
Jan Philip Solovej (University of Copenhagen)Universality in the structure of Atoms and MoleculesVideo link: https://youtu.be/FCxkP7CqtQQ Show abstract The simplest approximate model of atoms and molecules is the celebrated Thomas-Fermi model. It is known to give a good approximation to the ground state energy of heavy atoms. The understanding of this approximation relies on a beautiful and very accurate application of semi-classical analysis. Although the energy approximation is good, it is, unfortunately, far from being accurate enough to predict quantities relevant to chemistry. Thomas-Fermi theory may nevertheless tell us something surprisingly accurate about the structure of atoms and molecules. I will discuss how a certain universality in the Thomas-Fermi model, indeed, holds approximately in much more complicated models, such as the Hartree-Fock model. I will also show numerical and experimental evidence that the approximate universality may hold even for real atoms and molecules. |

May 5, 2020 |
Martin Hairer (Imperial College London)The Brownian CastleVideo link: https://youtu.be/Ve_EFZDbXTU |

Jan Dereziński, May 2020--December 2022

Marcello Porta, April 2021--March 2023

Kasia Rejzner, April 2021--December 2023

Hal Tasaki, January 2022--present

Ian Jauslin, January 2023--present

Margherita Disertori, April 2023--present

Wojciech Dybalski, January 2024--present

Tribute to the IAMP seminar, by Robert Seiringer, Bruno Nachtergaele, Elliott Lieb, and Hal Tasaki (March 2021): youtu.be/uFpegN8ALYw.