Scientific Series

There is a deep relation between classical error-correcting codes, Euclidean lattices, and chiral 2d CFTs. We show this relation
extends to include quantum codes, Lorentzian lattices, and non-chiral CFTs. The relation to quantum codes provides a simple way to solve
modular bootstrap constraints and identify interesting examples of conformal theories. In particular we construct many examples of physically distinct isospectral theories, examples of "would-be" CFT partition function -- non-holomorphic functions satisfying all constraints of the modular bootstrap, yet not associated with any

known CFT, and find theory with the maximal spectral gap among all Narain CFTs with the central charge c=4. At the level of code theories the problem of finding maximal spectral gap reduces to the problem of finding optimal code, leading to "baby bootstrap" program. We also discuss averaging over the ensemble of all CFTs associated with quantum codes, and its possible holographic interpretation. The talk is based on arXiv:2009.01236 and arXiv:2009.01244.

I will study quantum error correcting codes that model aspects of the AdS/CFT correspondence. In an algebraic approach I will demonstrate the existence of a consistent assignment, to each boundary region, of conditional expectations that preserve the code subspace. This allows us to give simple derivations of well known results for these holographic code, and also to derive a few new results. 

I will also make a connection to the theory of QFT super-selection sectors.

In the first part of the talk I will review some recent progress in large-scale structure theory and show how it can be used to measure cosmological parameters from current and future redshift surveys. Then I will discuss some ongoing challenges in the modeling of galaxy clsutering data and covariance matrices. Finally, I will present a systematic calculation of the probability distribution function for the dark matter density field and discuss its potential as a cosmological probe.

Axion-like particles (ALPs) are one of the most attractive solutions to the dark matter issue. In this talk, I will discuss new ideas in the search for ALPs in astrophysical structures. In particular, I will focus on the emission associated with their decay into photons. The discussion will involve different ALP mass ranges and wavelength bands.

Quantum circuits, relevant for quantum computing applications, present a new kind of many-body problem. Recently it was discovered that the quantum state evolved by random unitary gates, interrupted by occasional local measurements undergoes a phase transition from a highly entangled (volume law) state at small measurement rate to an area law state above a critical rate. I will review the current understanding of this transition from the statistical mechanics and the information perspectives. I will then argue that a circuit with intrinsic symmetries admits more phases, which represent distinct patterns of protection and flow of quantum information. These states can be studied and classified by mapping to an effective ground state problem of a Hamiltonian with enlarged effective symmetry. I will give two simple examples to illustrate these ideas: (i) a circuit with intrinsic Z2 spin symmetry; (ii) A circuit with Gaussian Majorana fermion gates showing a surprising Kosterlitz-Thouless transition in the entanglement content.

A proposal is made for a fundamental theory,  in which the history of the universe is constituted of views of itself.  Views are attributes of events, and the theory's only be-ables; they comprise information about energy and momentum transferred to an event from its causal past. 

The  theory is called the causal theory of views (CTV) and is a candidate for a completion of QM.  It is partly based on energetic causal sets (ECS),   an approach developed  with Marina Cortes.  A key result that applies also here is that spacetime is emergent from the ECS dynamics.  This implies that the fundamental dynamics involve no notion of space, distance or derivatives.  Instead I propose that a measure of similarity of views replaces derivatives as the basic measure of change and difference.

A measure of the diversity of views in a  causal network is introduced, called the variety (originally invented with Julian  Barbour).  I postulate a dynamics for CTV based on an action involving the variety and show that in an appropriate limit, it reduces to Schrodinger quantum mechanics.  A key result is that  the variety reduces to Bohm's quantum potential.

Based on arXiv:1307.6167,  arXiv:1308.2206 , arXiv:1712.0479 and a paper in preparation.

The subject of equivariant elliptic cohomology finds itself at the interface of topology, string theory, affine representation theory, singularity theory and integrable systems. These connections were already known to the founders of the discipline, Grojnowski, Segal, Hopkins, Devoto, but have come into sharper focus in recent years with a number of remarkable developments happening simultaneously: First, there are the geometric constructions, due to Kitchloo, Rezk and Spong, Berwick-Evans and Tripathy, building on the older program of Stolz-Teichner, and of course Segal. These are now all known to be equivalent to Grojnowski's original definition, thanks to work of Spong. Second, there are new applications, to integrable systems (Aganagic-Okounkov, Felder-Rimanyi-Varchenko-Tarasov) and to representation theory (Yang-Zhao-Zhong, G-Ram). Finally, there is the work of Weber-Rimanyi-Kumar, who build on the techniques of Borisov and Libgober to define Schubert classes in equivariant elliptic cohomology (with an added dynamical parametre), recovering the stable envelopes in type A. This talk will give a gentle introduction to the topic and attempt an overview over these recent developments.

Compact binaries may be formed dynamically in globular clusters, with large (close to unity) orbital eccentricity and emitting gravitational waves within the detection band of ground based detectors. The gravitational waves from such sources resemble more a discrete set of bursts than the continuous signal of their quasi-circular counterparts. I here discuss the construction of new analytic waveforms for such systems, which rely on treating the problem as a perturbation of a parabolic fly-by. I will discuss the results of parameter estimation with these waveforms for binary black holes, and the inclusion of tidal effects for binary neutron stars.

In a quantum measurement process, classical information about the measured system spreads through the environment. In contrast, quantum information about the system becomes inaccessible to local observers. In this talk, I will present a result about quantum channels indicating that an aspect of this phenomenon is completely general. We show that for any evolution of the system and environment, for everywhere in the environment excluding an O(1)-sized region we call the "quantum Markov blanket," any locally accessible information about the system must be approximately classical, i.e. obtainable from some fixed measurement. The result strengthens the earlier result of arXiv:1310.8640 in which the excluded region was allowed to grow with total environment size.  I will also discuss applications to many-body physics.

Rolling scalar fields play an important role in understanding cosmology within a particle physics framework. Coupling a rolling scalar field to light degrees of freedom gives rise to a thermal friction which, if large enough, induces a thermal bath. In the context of inflation the presence of such a thermal bath has compelling consequences as it significantly alters the usual observables, leading to a suppression of the tensor-to-scalar ratio r and a unique prediction for non-gaussianities. In my talk, I will illuminate why the axion of a non-Abelian gauge group is the ideal candidate for generating the thermal friction and how it sets the stage for a minimal setup of warm inflation, as well as a potential solution to the Hubble tension.

Gravitational waves (GWs) have already proved immensely powerful for constraining cosmological extensions of GR, both from data-driven and theoretical perspectives. However, GWs really come into their own when used in combination with complementary electromagnetic data. I’ll start by reviewing some of the bounds on extended gravity theories from GW detections to date. I'll introduce the formalism, the phenomenology, and the astrophysical pitfalls of these tests. Finally, we'll explore the impact of future experiments like LISA and accompanying galaxy surveys on the remaining parameter space of modified gravity theories.

This talk starts by reviewing known examples of how topological materials generate new kinds of electrodynamic couplings and effects.  Three-dimensional topological insulators realize a particular electromagnetic coupling known as “axion electrodynamics”, and understanding this leads to an improved understanding of magnetoelectricity in all materials.  We then turn to how topological Weyl and Dirac semimetals can show unique electromagnetic responses; we argue that in linear response the main observable effect solves an old problem via the orbital moment of Bloch electrons, and how in nonlinear optics there should be a new quantized effect, which may have been seen experimentally.  This nonlinear effect has a natural quantum e^3/h^2 and appears in chiral Weyl semimetals over a finite range of frequencies.  We discuss interaction and disorder corrections to nonlinear responses in closing.