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Theory Seminar

Seminars are at 2pm in Room 179 CP Building unless otherwise indicated.

Theory Seminar: The Ryu-Takayanagi formula from Quantum Error Correction

I review the reformulation of the AdS/CFT correspondence as

a quantum error correcting code, and explain how this gives a new

perspective on several mysterious features of the correspondence, in

particular the Ryu-Takayanagi formula.

Date:
-
Location:
CP179
Event Series:

Theory Seminar: Scale Anomalies and States in Conformal Field Theory

In 4D CFT, the anomaly coefficient c is the coefficient of an anomaly in scale transformations. In this talk, we show how scale anomaly coefficients such as c can be computed in terms of CFT data, namely the operator spectrum and OPE coefficients. We obtain expressions for the anomaly coefficient using Euclidean position space and Minkowski momentum space. The latter gives the coefficient in terms of a positive-definite sum over states, but may suffer from UV and IR divergences. We confirm our expressions by explicit calculations for anomalies involving scalar operators. Along the way, we show how to do calculations in Minkowski momentum space using the state-operator correspondence, a technique which may have many other applications.

Date:
-
Location:
CP179
Event Series:

Theory Seminar: Quantum Entanglement of Local Operators in Various Field Theories

We have investigated the time evolution of (Renyi) entanglement entropies for a locally excited state defined by acting with a local operator. When the subsystem is given by a half of the total space, we have found that they approach finite constants in free field theories or solvable models. On the other hand, those entropies in holographic field theories logarithmically increase with t even at the late time. Thus they are expected to be a candidate which reveals the characteristic feature of holographic field theories. In the current works, we found that the entropies in free field theories are given by the probability with which the quasi-particle created by local operators located in the subsystem. We also introduce a toy model where a particle created by a local operator freely propagates. By using this model, we can reproduce the result which is obtained by the replica trick. I would like to explain the detail of our results as long as permitted.

Date:
-
Location:
***CP183***
Event Series:

Theory seminar: Information Loss in AdS3/CFT2

We discuss information loss from black hole physics in AdS3, focusing on two sharp signatures infecting CFT2 correlators at large central charge c: 'forbidden singularities' arising from Euclidean-time periodicity due to the effective Hawking temperature, and late-time exponential decay in the Lorentzian region. We study an infinite class of examples where forbidden singularities can be resolved by non-perturbative effects at finite c, and we show that the resolution has certain universal features that also apply in the general case. Analytically continuing to the Lorentzian regime, we find that the non-perturbative effects that resolve forbidden singularities qualitatively change the behavior of correlators at times t∼SBH, the black hole entropy. This may resolve the exponential decay of correlators at late times in black hole backgrounds. By Borel resumming the 1/c expansion of exact examples, we explicitly identify 'information-restoring' effects from heavy states that should correspond to classical solutions in AdS3. Our results suggest a line of inquiry towards a more precise formulation of the gravitational path integral in AdS3.

Date:
-
Location:
CP179
Event Series:

Theory Seminar: Monte Carlo calculations on Lefschetz thimbles and beyond

A possible solution to the notorious sign problem for systems with non-zero chemical potential is to deform the integration region in the complex plane to a Lefschetz thimble. We introduce an easy to implement Monte Carlo algorithm to sample the dominant thimble, based on a contraction map on a thimble. We point out that manifolds other than Lefschetz thimble could be useful for numerical simulations. We describe a family of such manifolds, using the contraction map, that interpolate between the tangent space at one critical point (where the sign problem could be severe) and the union of relevant thimbles (where the sign problem is mild but a multimodal distribution function complicates the Monte Carlo sampling). We show how this works in a couple of models.

Date:
-
Event Series:

Theory Seminar: New Constraints on Two-dimensional Conformal Field Theories

I will discuss nontrivial constraints on the spectrum and structure constants of 2D unitary CFTs from the associativity of OPE and modular invariance, and some implications for non-perturbative completions of gravity in AdS3.

Date:
-
Location:
CP179
Event Series:

Theory Seminar: AdS/CFT via Tensor Network

In this talk I will explore recently proposed tensor networks/AdS correspondence based on the quantum error correcting model. Coxeter group is a useful tool to describe tensor networks in a negatively curved space. Studying generic tensor network populated by perfect tensors, we will find that the physical wave function generically do not admit any connected correlation functions of local operators. As we know computation of correlation functions plays an important role in establishing AdS/CFT duality, so we will like to remedy this problem. In this talk, I will give a proposal for computing connected two and three point functions and we will see there is a possibility of the emergence of the analogues of Witten diagrams in the tensor network. Using the Coxeter construction, I will describe how to construct the tensor network counterpart of the BTZ black hole, by orbifolding the discrete lattice on which the network resides and show that the construction naturally reproduces some of the salient features of the BTZ black hole, such as the appearance of RT surfaces that could wrap the horizon, depending on the size of the entanglement region. Also I will briefly mention how to construct the bulk isometry generators in terms of Coxeter tessellations thereby taking a step towards establishing the bulk/boundary symmetry matching and solidifying the AdS/CFT dictionary.
 
 
 
Date:
-
Location:
CP179
Event Series:
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