Talks

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.

Using a process-theoretic formalism, we introduce the notion of a causal-inferential theory: a triple consisting of a theory of causal influences, a theory of inferences (of both the Boolean and Bayesian varieties), and a specification of how these interact. Recasting the notions of operational and realist theories in this mold clarifies what a realist account of an experiment offers beyond an operational account. It also yields a novel characterization of the assumptions and implications of standard no-go theorems for realist representations of operational quantum theory, namely, those based on Bell’s notion of locality and those based on generalized noncontextuality. Moreover, our process-theoretic characterization of generalised noncontextuality is shown to be implied by an even more natural principle which we term Leibnizianity. Most strikingly, our framework offers a way forward in a research program that seeks to circumvent these no-go results. Specifically, we argue that if one can identify axioms for a realist causal-inferential theory such that the notions of causation and inference can differ from their conventional (classical) interpretations, then one has the means of defining an intrinsically quantum notion of realism, and thereby a realist representation of operational quantum theory that salvages the spirit of locality and of noncontextuality.

We have tentative evidence of massive stars that disappear without a bright transient. It is commonly argued that this massive stars have low angular momentum and can collapse into a black hole without significant feedback. In this talk I will make use of general-relativistic hydrodynamical simulations to understand the flow around a newly-formed black hole. I will discuss the angular momentum needed in order for the infalling material to be accreted into the black hole without forming a centrifugally supported structure, thus generating no effective feedback. If the feedback from the black hole is significant, the collapse can be halted and, as a result, it is likely followed by a bright transient. With the results from the simulation, I will constrain the maximum rotation rate for the disappearing massive progenitors know, and set a limit on the rate of expected disappearing stars.

"Analogue" Hamiltonian simulation involves engineering a Hamiltonian of
interest in the laboratory and studying its properties experimentally.
Large-scale Hamiltonian simulation experiments have been carried out in
optical lattices, ion traps and other systems for two decades. Despite
this, the theoretical basis for Hamiltonian simulation is surprisingly
sparse. Even a precise definition of what it means to simulate a
Hamiltonian was lacking.

AdS/CFT duality postulates that quantum gravity in a d-dimensional
anti-de-Sitter bulk space is equivalent to a strongly interacting field
theory on its d-1 dimensional boundary. Recently, connections between
AdS/CFT duality and quantum error-correcting codes have led (amongst
other things) to tensor network toy models that capture important aspects
of this holographic duality. However, these toy models struggle to
encompass dualities between bulk and boundary energy scales and dynamics.

On the face of it, these two topics seem to have nothing whatsoever to do
with one another.

In my talk, I will explain how we put analogue Hamiltonian simulation on
a rigorous theoretical footing, by drawing on techniques from Hamiltonian
complexity theory and Jordan algebras. I will show how this proved far
more fruitful than a mere mathematical tidying-up exercise, leading to
the discovery of universal quantum Hamiltonians [Science, 351:6 278,
p.1180, 2016], [Proc. Natl. Acad. Sci. 115:38 p.9497, 2018]. And I will
explain how this new Hamiltonian simulation formalism, together with
hyperbolic Coxeter groups, allowed us to extend the toy models of AdS/CFT
to encompass energy scales, dynamics, and even (toy models of) black hole
formation [arXiv:1810.08992].

A dark matter candidate lighter than about 30 eV exhibits wave behavior in a typical galactic environment. Examples include the QCD axion as well as other axion-like-particles. We review the particle physics motivations, and discuss experimental and observational implications of the wave dynamics, including interference substructures, vortices, soliton condensation and black hole hair.

I will present a holographic framework for reconstructing the experience of bulk observers in AdS/CFT. In particular, I will show how to recover the proper time and energy distribution measured along bulk worldlines, directly in the CFT via a universal, background-independent prescription. For an observer falling into an eternal AdS black hole, the proposal resolves a conceptual puzzle raised by Marolf and Wall. Notably, the prescription does not rely on an external dynamical Hamiltonian or the AdS boundary conditions and is, therefore, outlining a general framework for the emergence of time.

In order to infer cosmological parameters from galaxy survey data, we typically use summary statistics such as the power spectrum and we need an accurate estimate of their covariance matrix. The traditional process of obtaining the covariance involves simulating thousands of mocks. I will present an analytic approach for the covariance matrix which is more than four orders of magnitude faster than mocks and show its validation with an analysis of the BOSS DR12 data. Furthermore, our analytic approach is free of sampling noise which makes it useful for upcoming surveys like DESI and Euclid. Towards the end, I will change gears and talk about some recent work on the assembly bias of neutral hydrogen.
 

Quantum entanglement of pure states has led to new insights into a wide variety of topics. Entanglement of mixed states is however less well understood. In this talk I will focus on a few themes where mixed-state entanglement leads to new insights that are difficult to obtain otherwise. I will mainly focus on two topics: (i) Characterizing finite-temperature topological order (ii) Detecting presence/absence of quasiparticles. Time permitting, I will also discuss a relation between mixed-state entanglement and information obtained from projective measurements, and its application to characterizing multi-component systems, such as quantum disentangled liquids and Kondo lattice systems.