June 2021

Date: June 7

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

Speaker: Subir Sachdev (Harvard University)

Title: The SYK model: a window into non-Fermi liquids and black holes

Abstract: Many modern materials feature a “strange metal”: a phase of electronic quantum matter without particle-like excitations. I will describe recent progress in the theory of strange metals by drawing upon insights from the solvable Sachdev-Ye-Kitaev model. This model features all-to-all entanglement of electrons, and chaos and thermalization in the shortest possible time, which is of order(Planck’s constant)/(absolute temperature). Insights from the SYK model have also led to many exciting advances in the quantum theory of black holes, which also thermalize in a time of order (Planck’s constant)/(black hole Hawking temperature). I will describe an example: the universal, leading, low temperature correction to the Bekenstein-Hawking entropy of charged black holes in Einstein’s theory of general relativity.

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Date: June 10

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

Speaker: Johanna Erdmenger (University of Wurzburg)

Title: Complexity and modular flows: Relating quantum information, quantum field theory and gravity

• Abstract: I present recent work of our group on realizing computational complexity in conformal field theories in view of relating it holographically to complexity proposals in AdS gravity. Gates and cost functions in the CFT are introduced and used for a complexity definition based on geometric actions on Virasoro and Kac-Moody orbits. We discuss a relation to the Euler-Arnold approach in Hamiltonian dynamics. In a complementary project, we developed a new method for evaluating modular flows for 1+1-dimensional chiral fermions, working directly from the resolvent in complex analysis. Both projects provide new ingredients for exploring the connection between spacetime and entanglement.

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Date: June 14

Time: 18:00 IST (UTC + 5:30)

Speaker: Ning Bao (Caltech)

Title: Hypergraph Theory and Entanglement Entropy

• Abstract: Entanglement is a precious resource in quantum information/computing, and the quantification thereof is of great importance. In this talk, I will focus on a method of characterizing a measure of the amount of entanglement, namely the entanglement entropy, in holographic systems using techniques from graph theory and convex optimization. In doing so, we will prove novel inequalities that these systems must obey, and construct the "holographic entropy cone" defined by these inequalities for small numbers of systems. Following this, I will generalize the techniques described before to apply to more generic quantum states, in particular the stabilizer states, via the extension of graph theory to hypergraph theory, and will comment on the new constraints there.

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Date: June 17

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

Speaker: Sukrut Mondkar (IIT Madras)

Title: Quasinormal modes of a semi-holographic black hole and thermalization

• Abstract: We study a simplified model of semi-holography which consistently integrates mutually interacting perturbative and strongly coupled holographic degrees of freedom. We study the quasinormal modes and non-linear dynamics of such a semi-holographic black hole system in which the total conserved energy-momentum tensor is simply the sum of those of the perturbative and holographic components. Contrary to naive expectations, we show that the thermalisation of the full system can be parametrically slow when the mutual coupling is weak. For typical homogeneous initial states, we find reverse transfer of energy from the black hole to the boundary at the initial stage, which later gives way to complete transfer of energy to the black hole at a slow and constant rate, while the entropy grows monotonically for all time. We establish that when the mutual coupling is below a critical value, there exists a hybrid mode with finite momentum and zero energy. Along with other characteristics of the quasinormal mode spectrum, this could imply turbulent equipartioning of energy between the boundary and the holographic degrees of freedom. Additionally, we find emergence of conformal symmetry at infinite mutual coupling in accordance with previous results.

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