Josh Mutus (Google Quantum AI)
Beyond classical computation with superconducting qubits

I will discuss how the Google Quantum AI team has outperformed the largest supercomputers in the world in a well defined computational challenge. I'll explain the working principle of superconducting qubits and the hardware and software we have built to control them. Next, I will discuss the nature of our experiment to demonstrate beyond-classical performance by sampling random circuits and how we validated the experiment using cross-entropy benchmarking to inform a full-system model. Lastly, I will discuss the current limits to performance in superconducting qubits and the opportunities in material science and characterization to overcome them.

Louis Taillefer (University of Sherbrooke & CIFAR)
New signatures of the pseudogap phase of cuprate superconductors

The pseudogap phase of cuprate superconductors is arguably the most enigmatic phase of quantum matter. We aim to shed new light on this phase by investigating the non-superconducting ground state of several cuprate materials at low temperature across a wide doping range, suppressing superconductivity with a magnetic field. Hall effect measurements across the pseudogap critical doping p* reveal a sharp drop in carrier density n from n = 1 + p above p* to n = p below p*, signaling a major transformation of the Fermi surface. Angle-dependent magneto-resistance (ADMR) directly reveals a change in Fermi surface topology across p*. From specific heat measurements, we observe the classic thermodynamic signatures of quantum criticality: the electronic specific heat C_el shows a sharp peak at p*, where it varies in temperature as C_el ~ – T logT. At p* and just above, the electrical resistivity is linear in T at low T, with an inelastic scattering rate that obeys the Planckian limit. Finally, the pseudogap phase is found to have a large negative thermal Hall conductivity, which extends to zero doping. We show that the pseudogap phase makes phonons become chiral.

Understanding the mechanisms responsible for these various new signatures will help elucidate the nature of the pseudogap phase.

Marcel Franz (University of British Columbia)
From solids with topology to black holes and back

Inclusion of topological phenomena in condensed matter physics over the past 10 years ushered a revolution in this field. As a result of the new theoretical insights entire classes of materials with exotic properties have been discovered, including topological insulators, Dirac and Weyl semimetals as well as topological superconductors containing Majorana fermions. In this talk I will review these developments and discuss an intriguing connection noticed recently by Kitaev between one such topological system – the Sachdev-Ye-Kitaev model – and the horizon of a black hole. This connection furnishes a rare example of holographic duality between a solvable quantum-mechanical model and Einstein gravity, and may have simple physical realization in a tabletop experiment.

Joseph Thywissen (University of Toronto)
Transport dynamics in ultracold atoms

The world around us is not in equilibrium, but slowly (or quickly) relaxing through transport of conserved quantities such as energy, charge, and momentum. However, transport is challenging to calculate ab initio, leaving many open questions (such as high-temperature superconductivity) and room for new theoretical paradigms (such as holographic duality). Ultracold atoms provide an ideal platform for the study of non-equilibrium quantum physics, since samples are isolated from the environment, and the strength of interactions can be tuned.

Time permitting, I will discuss two experiments that use cold atoms to explore transport. In the first experiment, we explore how particle-current is dissipated in a perfect and rigid crystal, due to interactions in a system with broken Galilean invariance. In the second experiment, we measure spin diffusion in a strongly interacting Fermi gas. We observe a kind of quantum “speed limit” on the transport rate.

Seen from another perspective, these experiments implement a quantum simulation, which is a very specialized quantum computation. Although neither error-corrected nor universal, quantum simulators can be built now, can exceed the computational power of a classical computer, and are ready to be applied to important open questions and challenges.