# Video Library

Since 2002 Perimeter Institute has been recording seminars, conference talks, and public outreach events using video cameras installed in our lecture theatres.  Perimeter now has 7 formal presentation spaces for its many scientific conferences, seminars, workshops and educational outreach activities, all with advanced audio-visual technical capabilities.  Recordings of events in these areas are all available On-Demand from this Video Library and on Perimeter Institute Recorded Seminar Archive (PIRSA)PIRSA is a permanent, free, searchable, and citable archive of recorded seminars from relevant bodies in physics. This resource has been partially modelled after Cornell University's arXiv.org.

## Dualities for chiral algebras from S-duality

Thursday Jun 01, 2017
Speaker(s):

During past few decades, string theory has been used as a source of conjectural dualities in various areas of physics and mathematics. We have extended these applications of string dualities to the study of chiral algebras in 2d CFT. In this talk, I will sketch how to use S-duality of D3-D5-NS5 systems to shed some light on already known dual constructions of chiral algebras and generate huge amount of new dualities.

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## Entanglement entropy of scalar fields in causal set theory

Thursday Jun 01, 2017
Speaker(s):

Entanglement entropy is now widely accepted as having deep connections with quantum gravity. It is therefore desirable to understand it in the context of causal sets, especially since they provide in a natural and covariant manner the UV cutoff needed to render entanglement entropy finite. Defining entropy in a causal set is not straightforward because the usual hypersurface data on which definitions of entanglement typically rely is not available.

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## The holographic dual to general covariance

Thursday Jun 01, 2017
Speaker(s):

One of the defining features of holography is the geometerization of the renormalization group scale. This means that when a quantum field theory is holographically dual to a bulk gravity theory, then the direction normal to the boundary in the bulk (the radial' direction) is to be interpreted as the energy scale of the dual quantum field theory. So this direction can be seen to be emergent', and the evolution of bulk fields along this direction is equated with the renormalization group flow of sources or couplings of boundary operators.

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## Lorentzian quantum cosmology

Thursday Jun 01, 2017
Speaker(s):

Using Picard-Lefschetz theory we show that the Lorentzian path integral forms a good starting point for quantum cosmology which avoids the conformal factor problem present in Euclidean gravity. We study the Lorentzian path integral for a homogeneous and isotropic model with a positive cosmological constant. Applied to the “no-boundary” proposal, we show that this leads to the inverse of the result obtained by Hartle and Hawking.

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## Entanglement structure and UV regularization in cMERA

Thursday Jun 01, 2017
Speaker(s):

We give an introduction to cMERA, a continuous tensor networks ansatz for ground states of QFTs. We also explore a particular feature of it: an intrinsic length scale that acts as an ultraviolet cutoff. We provide evidence for the existence of this cutoff based on the entanglement structure of a particular family of cMERA states, namely Gaussian states optimized for free bosonic and fermionic CFTs. Our findings reflect that short distance entanglement is not fully present in the ansatz states, thus hinting at ultraviolet regularization.

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## Gravity degrees of freedom on a null surface

Thursday Jun 01, 2017
Speaker(s):

A canonical analysis for general relativity is performed on a null surface without fixing the diffeomorphism gauge, and the canonical pairs of configuration and momentum variables are derived. Next to the well-known spin-2 pair, also spin-1 and spin-0 pairs are identified. The boundary action for a null boundary segment of spacetime is obtained, including terms on codimension two corners.
FH, Laurent Freidel arXiv:1611.03096, Phys. Rev. D 95, 104006 (2017)

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## Topological field theories and gapped phases of matter

Thursday Jun 01, 2017
Speaker(s):

I will introduce the idea that topological field theories describe the low-energy properties of gapped local quantum systems. This idea has proved fruitful in recent studies of gapped phases of matter.

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## Rise of non-perturbative effects below the upper critical dimension

Thursday Jun 01, 2017

Extracting low energy universal data of quantum critical systems is a task whose difficulty increases with decreasing dimension. The increasing strength of quantum fluctuations can be tamed by using renormalization group (RG) schemes based on dimensional regularization close to the upper critical dimension of the system. By presenting a non-perturbative approach that allows the reliable extraction of the low energy universal data for the antiferromagnetic quantum critical metal in \$2 \leq d

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## Quantum tunneling with a Lorentzian path integral

Thursday Jun 01, 2017
Speaker(s):

We describe the tunneling of a quantum mechanical particle with a Lorentzian (realtime) path integral. The analysis is made concrete by application to the inverted harmonic oscillator potential, where the path integral is known exactly. We apply Picard-Lefschetz theory to the time integral of the Feynmann propagator at fixed energy, and show that the Euclidean integration contour is obtained as a Lefschetz thimble, or a sum of them, in a suitable limit.

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## Quantum critical responses via holographic models and conformal perturbation theory

Thursday Jun 01, 2017
Speaker(s):

We investigate response functions near quantum critical points, allowing for finite temperature and a mild deformation by a relevant scalar. When the quantum critical point is described by a conformal field theory, we use conformal perturbation theory and holography to determine the two leading corrections to the scalar two-point function and to the conductivity. We build a bridge between the couplings fixed by conformal symmetry with the interaction couplings in the gravity theory.

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## Next Public Lecture

Check back for details on the next lecture in Perimeter's Public Lectures Series

## RECENT PUBLIC LECTURE

### Janna Levin: Black Hole Blues and Other Songs from Outer Space

Speaker: Janna Levin