# Titles and Abstracts

- Costas Bachas (LPTENS, Paris)

**H****olographic Interface****s**

I will discuss some recent results and open problems in the study of holographic interfaces between conformal field theories.

- Chris Herzog (King's College, London)

**Conformal Surface Defects in Maxwell Theory are Trivial**

I consider a free Maxwell field in four dimensions in the presence of a codimension two defect. Reflection positive, codimension two defects which preserve conformal symmetry in this context are very limited. Only generalized free fields can exist on the defect and interact with the free Maxwell field in the bulk. This result stands in stark contrast to the codimension one case where interacting conformal boundary conditions can be found for free bulk fields, producing systems with physical relevance, for example for graphene.

- Kristan Jensen (University of Victoria)

**Large N fractons**

I will discuss some aspects of coarse-grained models of fractons. These theories have been of recent interest to condensed matter and high energy theorists. On the one hand they are candidates for new phases of quantum matter, and on the other, they have features which seem impossible to describe in continuum field theory, including quasiparticles of restricted mobility and a ground state degeneracy which depends sensitively on the ultraviolet. I will showcase some soluble interacting large N versions of these models, and some lessons they teach us for building a theory of transport.

- Karl Landsteiner (UAM Madrid)

**PT-Holography **

Abstract: One of the basic axioms of quantum mechanics is that the Hamiltonian operator is a Hermitian matrix. Somewhat surprisingly, non-Hermitian but PT symmetric Hamiltonians can have a real eigenvalue spectrum and describe open quantum systems with balanced gain/loss couplings to the environment. After a brief review and introduction to non-Hermitian PT symmetric quantum mechanics.

I will show how this concept can be generalized to Holography. I will develop a simple model, show that it has a PT-symmetric and a PT-broken regime separated by an exceptional point. Then I will go on and study what happens if one makes the couplings time dependent (quantum quenches). A rich "phenomenology" arises in such holographic PT-quenches. In particular one can quite explicitly contrast non-unitary against unitary time evolution. I'll also try to draw some lessons for conventional, weakly coupled PT-symmetric quantum field theories.

- Ayan Mukhopahyay (IIT Madras)

**The horizon cap out of equilibrium and the correlation functions of the Bjorken flow**

Abstract: A precise definition of the holographic duality in real time, particularly the computation of the Schwinger-Keldysh correlation functions, has remained one of the outstanding challenges of this approach to out-of-equilibrium many-body physics. We prove the consistency of the Crossley-Glorioso-Liu (CGL) prescription for the implementation of the Schwinger-Keldysh contour at thermal equilibrium via a horizon cap, and extend it beyond equilibrium in the context of the Bjorken flow. Our out-of-equilibrium generalization passes several additional consistency tests. In particular, we show that the horizon cap should be pinned to the out-of-equilibrium event horizon to recover non-trivial identities in the field theory. In the perfect fluid limit, we show that all the correlation functions can be mapped to a thermal form via appropriate spacetime reparamatrizations, generalizing the results of Janik and Peschanski in the context of homogeneous transients. We show how viscous corrections can be captured by a late-time expansion, and how the correlation finctionctions can be Borel resummed like one-point functions, and thus matched to initial conditions. We will also discuss hints about how special types of residual gauge transformations could play a crucial role in reconstructing islands (behind the horizon) from the Hawking radiation.

- Jan Zaanen (Lorentz Institute, Leiden University)

**Holographic strange metals as densely entangled generalizations of the Fermi liquid. **

All along the holographic strange metals appeared as eerily familiar to the eyes of a condensed matter physicists like myself. There are good reasons to suspect that the laboratory variety are forms of “quantum supreme matter” characterized by dense many body entanglement rooted in the fermion sign problem. I believe I have deciphered the “whispers of the holographic oracle” in terms of an RG rooted in the EMD scaling geometries that describe the flow associated with Fermi-liquids but generalized in the universality sense of “stoquastic” strongly interacting quantum critical states. Among others this is characterized by an emergent charge conjugation symmetry that may be the key to understand the “incoherent” currents playing an important role in holographic hydrodynamics.