April 2021

Date: April 1

Time: 17:30 IST (UTC + 5:30)

Speaker: Toby Wiseman (Imperial College London)

Title: Quantum black holes on the lattice

• Abstract: Holography apparently relates certain quantum field theories to quantum theories of gravity. The original instance of this is Maldacena's duality between maximally supersymmetric gauge theories and type II string theory in the presence of D-branes. In the ’t Hooft limit, and at appropriate temperatures, the dual to the gauge theory is described by certain black holes (or rather black p-branes). This allows the remarkable opportunity to perform calculations of quantum black holes through the equivalent gauge theory, and to check the consistency of holographic duality. I will discuss such calculations, and the progress that has been made over the last decade in showing that the predicted gravity behaviour does indeed emerge from thermal gauge theory.

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Date: April 5

Time: 17:30 IST (UTC + 5:30)

Speaker: Diptarka Das (IIT Kanpur)

Title: Symmetry restoration via quantum quench

• Abstract: We consider smooth, global quantum quenches for two dimensional, massless interacting fermions with large number of fermions. We find that the spontaneously broken symmetry (in equilibrium) gets restored via the smooth quench. We identify the scaling of the symmetry restoration time with the Kibble-Zurek scaling in the problem. We point out a connection with Liouville quantum mechanics in a suitable scaling limit. We end with comments on thermalization and generalizations.

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Date: April 8

Time: 17:30 IST (UTC + 5:30)

Speaker: Prabha Mandayam (IIT Madras)

Title: The Petz map: from open quantum systems to holography (Part 1)

• Abstract: Originally introduced as a map that approximately preserves quantum information in open quantum systems, the Petz map has today emerged an important tool in the context of holographic models. In this talk we will review the various manifestations of the Petz map as a universal recovery channel: from approximate quantum error correction to bulk reconstruction in random tensor networks. Along the way, we will also touch upon a key information-theoretic application of the Petz construction, namely, in identifying the class of states that saturates the strong subadditivity inequality of the von Neumann entropy.

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Date: April 12

Time: 17:30 IST (UTC + 5:30)

Speaker: Wolfgang Lerche (CERN)

Title: Fluxes, Holomorphic Anomalies and Elliptic Genera in d=4

• Abstract: Motivated by tests of Weak Gravity Conjectures, we investigateproperties of elliptic genera of string theories that arise in large distance limits of the moduli space of F-theory compactifications. This allows for a non-perturbative definition of elliptic genera that goes beyond the ordinary world-sheet description. While this construction has been known for six-dimensional theories, we find the four-dimensional variant to be surprisingly complex. The essential new ingredient is background fluxes, and these lead to different sectors of the elliptic genus with different modular properties. Crucial is the appearance of derivatives which leads to novel modular (and hence, holomorphic) anomalies that are much worse than expected. We also give a physical interpretation of these phenomena.

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Date: April 15

Time: 9:30 IST (UTC + 5:30)

Speaker: Vincent Bouchard (University of Alberta)

Title: Whittaker vectors for W-algebras from topological recursion

• Abstract: Gaiotto vectors, describing the fundamental class in the equivariant cohomology of a suitable compactification of the moduli space of G-bundles over P^2, for G a complex simple Lie group, are Whittaker vectors for modules of W-algebras. In this work we identify these Whittaker vectors with partition functions of Airy structures, which means that they can be calculated by topological recursion methods. On the physics side, it means that the Nekrasov partition function for pure N=2 4d supersymmetric gauge theories can be accessed via topological recursion. We formulate the connection for Gaiotto vectors of type A, B, C, and D. For those interested in topological recursion, the type A case at arbitrary level gives rise to a new non-commutative formulation of topological recursion.

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Date: April 19

Time: 9:30 IST (UTC + 5:30)

Speaker: Hersh Singh (University of Washington, Seattle)

Title: Qubit regularization of asymptotic freedom

• Abstract: We provide strong evidence that the asymptotically free (1+1)-dimensional non-linear O(3) sigma model can be regularized using a quantum lattice Hamiltonian, referred to as the "Heisenberg-comb", that acts on a Hilbert space with only two qubits per spatial lattice site. The Heisenberg-comb consists of a spin-half anti-ferromagnetic Heisenberg-chain coupled anti-ferromagnetically to a second local spin-half particle at every lattice site. Using a world-line Monte Carlo method we show that the model reproduces the universal step-scaling function of the traditional model up to correlation lengths of 200,000 in lattice units and argue how the continuum limit could emerge. We provide a quantum circuit description of time-evolution of the model and argue that near-term quantum computers may suffice to demonstrate asymptotic freedom.

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Date: April 19

Time: 17:30 IST (UTC + 5:30)

Speaker: Suddhasattwa Brahma (McGill University)

Title: Hints for UV-completion of early universe cosmology

• Abstract: Recently, significant progress has been made towards imposing universal features of quantum gravity as consistency conditions on viable low-energy effective field theories. In this talk, I shall outline some of these swampland conditions, as necessary in guiding us in our quest for a UV-complete description of the early universe, and show additional evidence for them arising from considering new quantum effects in QFTs on curved spacetimes. In particular, the effects of quantum entanglement in cosmology shall be focussed on to show how they play a significant role in constraining early-universe physics. I shall present a new program aimed towards isolating universal properties that a cosmological model must satisfy to be consistent with our current understanding of UV physics, leading to remarkable consequences such as having novel interpretations for de Sitter space or advocating alternate paradigms for the early universe.

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Date: April 22

Time: 17:30 IST (UTC + 5:30)

Speaker: Arpita Mitra (IISER Bhopal)

Title: Soft theorem in presence of a small negative cosmological constant

• Abstract: At the classical level, the soft factor can be obtained from radiative profiles arising from the gravitational and electromagnetic bremmstrahlung in the vanishing frequency limit. We have studied scattering of a probe particle by a four dimensional AdS black hole background at large impact factor with the small negative cosmological constant. Our analysis is consistent perturbatively to leading order in the AdS radius and black hole mass parameter. We define a proper "soft limit" of the radiation and extract out the "soft factor" from it. We find the correction to the well known flat space soft factors due to the presence of a small negative cosmological constant. The leading soft factor can be expected to be universal and holds beyond tree level. This allows us to derive a correction to the Ward identity, following the known equivalence between large gauge Ward identities and soft photon theorems inCollective spin motion has been playing an increasingly prominent role both in high energy and condensed matter physics, especially after the observation of global polarization of $\Lambda$ hyperons in heavy ion collisions and generation of spin currents in liquid metals. In this talk I will introduce a novel construct of spin hydrodynamics, that is, relativistic hydrodynamics of the spin current coupled to the stress tensor. We then apply this to heavy ions and find surprisingly good agreement with data. If time permits, I will also present a holographic approach to spin currents employing the Lovelock-Chern-Simons theory in five dimensions. asymptotically flat spacetimes.

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Date: April 26

Time: 17:30 IST (UTC + 5:30)

Speaker: Umut Gursoy (Utrecht University)

Title: Hydrodynamics of spin currents

• Abstract: Collective spin motion has been playing an increasingly prominent role both in high energy and condensed matter physics, especially after the observation of global polarization of $\Lambda$ hyperons in heavy ion collisions and generation of spin currents in liquid metals. In this talk I will introduce a novel construct of spin hydrodynamics, that is, relativistic hydrodynamics of the spin current coupled to the stress tensor. We then apply this to heavy ions and find surprisingly good agreement with data. If time permits, I will also present a holographic approach to spin currents employing the Lovelock-Chern-Simons theory in five dimensions.

VIDEO: Click here