QuICS Special Seminar

Applying a chemical potential bias to a conductor drives the system out of equilibrium into a current carrying non-equilibrium state.  This current flow is associated with entropy production in the leads, but it remains poorly understood under what conditions the system is driven to local equilibrium by this process.  We investigate this problem using two toy models for coherent quantum transport of diffusive fermions:  Anderson models in the conducting phase and a class of random quantum circuits acting on a chain of qubits, which exactly maps to an interacting f

This talk will have quantum information connections.
 
Area laws were first discovered by Bekenstein and Hawking,
who found that the entropy of a black hole grows proportional to its
surface area, and not its volume. Entropy area laws have since become
a fundamental part of modern physics, from the holographic principle
in quantum gravity to ground state wavefunctions of quantum matter,
where entanglement entropy is generically found to obey area law

Multiphoton interference is at the very heart of quantum foundations and applications in quantum
sensing and information processing. In particular, boson sampling (BS) experiments and, in particular, scattershot boson sampling (SBS) schemes, have the potential to demonstrate quantum computational supremacy while only relying on multiphoton interference in linear optical interferometers. However, scalable experiments are challenged by the need to generate

I will talk about a notion of universality in spin systems by which certain spin systems can simulate all others. We provide sufficient and necessary conditions for a spin system to be universal, and show that the Ising model and many other “simple” systems are universal. This notion of universality is somewhat similar to the notion of universality in computer science (by which there exist universal Turing machines) — I will mention ongoing work in trying to make this connection precise. 

The observation of Bell nonlocality without signaling is a signature of quantum entanglement. Starting in 2007, this observation was made quantitative: nonlocality can be used to estimate the amount of entanglement, the number of extractable random bits, the length of a secret key... This type of estimates has been called "device-independent" (DI), since it relies only on the validity of quantum theory and on observation, without the need for any knowledge of the degrees of freedom under study. 
 

Following the works of Drineas et al. (2004), Randomised Numerical Linear Algebra (RNLA) applications have enjoyed a increasing surge of interest in the classical algorithms community. Following their success we recently demonstrated a classical algorithm based on the Nystroem method which allows us to efficiently simulate dynamics of quantum system with specific structural conditions. We briefly introduce the most basic RNLA algorithm by Drineas et al. and then outline our own recent work.

We describe a novel dynamical phase in which thermalization breaks down even on the level of the many-body eigenstates. This “trapped” phase is found in spin glass models of particular relevance for quantum computing applications. After briefly discussing the use of eigenstates & eigenstate phases to understand isolated many-body quantum systems, we contrast this trapped phase to the better-known many-body localized (MBL) phase.

The equilibrium states of quantum systems without a sign problem can in many cases be efficiently sampled using classical Markov chain Monte Carlo algorithms.   These classical algorithms challenge the possibility of obtaining quantum speedups using transverse-field quantum annealing, and this has motivated efforts to design next-generation quantum annealing architectures with a sign problem that cannot be removed by any change of the local basis.

We prove a characterization of t-query quantum algorithms in terms of the unit ball of a space of degree-2t polynomials. Based on this, we obtain a refined notion of approximate polynomial degree that equals the quantum query complexity, answering a question of Aaronson et al. (CCC’16). Our proof is based on a fundamental result of Christensen and Sinclair'87 that generalizes the well-known Stinespring representation for quantum channels to multilinear forms.

This talk will focus on the question of what precise signatures one should look for in an experiment to rule out the possibility that the experiment admits a well-defined classical model. By a “classical” model, we refer to a particular notion of classicality, namely, noncontextuality, inspired by the Kochen-Specker theorem.