Scientific Series

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.

I will present and motivate a program establishing, in full generality, the symmetries and charge analysis for gravitational theories near a generic null hypersurface without specifying any boundary condition.  I will illustrate the first steps of this program on three dimensional Einstein gravity. In this case, there are three charges which are generic functions over the codimension one null surface. The integrability of the charges and the charge algebra depend on the state-dependence of symmetry generators which is a priori not specified. I will establish the existence of infinitely many choices that render the surface charges integrable. Then, I will argue that one expects this result to be valid for d > 3 when there is no Bondi news by developing some aspects of the four dimensional case. Finally, I will put our results in the context of earlier constructions of near horizon symmetries.  

Based on: arxiv:2002.08346, arxiv:2005.06936 and arxiv:2007.12759
 

I will present recent work (to appear soon) on the homological mirror symmetry about the universal centralizers $J_G$, for any complex semisimple Lie group $G$. The A-side is a partially wrapped Fukaya category of $J_G$, and the B-side is the category of coherent sheaves on the categorical quotient of a dual maximal torus by the Weyl group action (with some modification if $G$ has a nontrivial center).

The Fermi-Hubbard model is of fundamental importance in condensed-matter physics, yet is extremely challenging to solve numerically. Finding the ground state of the Hubbard model using variational methods has been predicted to be one of the first applications of near-term quantum computers. In this talk, I will discuss recent work which carried out a detailed analysis and optimisation of the complexity of variational quantum algorithms for finding the ground state of the Hubbard model, including extensive numerical experiments for systems with up to 12 sites. The depth complexities we find are substantially lower than previous work. If our numerical results on small lattice sizes are representative of the somewhat larger lattices accessible to near-term quantum hardware, they suggest that optimising over quantum circuits with a gate depth less than a thousand could be sufficient to solve instances of the Hubbard model beyond the capacity of classical exact diagonalisation. I will also discuss a proof-of-principle implementation on Rigetti quantum computing hardware.

The talk is based on joint work with Chris Cade, Lana Mineh and Stasja Stanisic ( arXiv:1912.06007 , arXiv:2006.01179 ).

Gravitational waves provide a unique way to study the universe. From the initial direct detection of coalescing black holes in 2015, to the ground-breaking multimessenger observations of coalescing neutron stars in 2017, and continuing with the now routine detection of merging stellar remnants, gravitational wave astronomy has quickly matured into a key aspect of modern physics. After briefly discussing what we've begun to learn from the new gravitational-wave transient catalog published by the LIGO, Virgo, and KAGRA collaborations (GWTC-2), I will discuss novel tests of fundamental physics GWs enable. In particular, I will focus on our current understanding of matter effects during the inspiral of compact binaries and matter at supranuclear densities, including possible phase transitions, through tests of neutron star structure. Detailed knowledge of dynamical interactions between coalescing stars, observations of extreme relativistic astrophysical systems, terrestrial experiments, and nuclear theory provide complementary views of fundamental physics. I will show how combining aspects from all these will improve our understanding of dense matter through the example of how we can determine whether newly observed objects are neutron stars or black holes.