Anomalous subdiffusion in quantum chains
In typical quantum systems with conservation laws, the approach to equilibrium at finite temperature is governed by classical hydrodynamics in which charge and energy diffuse. In this talk, I will discuss some one dimensional quantum systems with anomalous hydrodynamic behavior — that is, systems where diffusion of charge is replaced by subdiffusion or superdiffusion.
Turbocharging quantum computing through active and passive error suppression
In this talk I will give an overview of various strategies we have developed for suppressing the inevitable errors occurring during quantum computations. These tools work at the gate level and thus can be effective even through a cloud API exposing only elementary gates to the end-user. I will demonstrate the effectiveness of these tools with experimental results across multiple hardware architectures.
Training Variational Quantum Algorithms Is NP-Hard
Variational quantum algorithms are proposed to solve relevant computational problems on near term quantum devices. Popular versions are variational quantum eigensolvers and quantum approximate optimization algorithms that solve ground state problems from quantum chemistry and binary optimization problems, respectively. They are based on the idea of using a classical computer to train a parametrized quantum circuit. We show that the corresponding classical optimization problems are NP-hard.
Large-N solvable models of measurement-induced criticality
Competition between unitary dynamics that scrambles quantum information non-locally and local measurements that probe and collapse the quantum state can result in a measurement-induced entanglement phase transition. Here we introduce analytically tractable models of measurement-induced criticality in large-N Brownian hybrid circuit model composed of qubits [1]. The system is initially entangled with an equal sized reference, and the subsequent hybrid system dynamics either partially preserves or totally destroys this entanglement depending on the measurement rate.
Classification of (2+1)D invertible fermionic topological phases with symmetry
The integer quantum Hall states, the quantum spin Hall insulator, and the (2+1)D p-wave topological superconductor each have an important place in condensed matter physics due to their quantized symmetry-protected topological invariants. These systems have a unique ground state on any closed manifold in (2+1) dimensions, and are examples of 'invertible' topological phases of fermions. Here I will describe a general theory which fully encodes the universal properties of such invertible phases, and classifies them based on their symmetries.
Experimental simulation of para-particle dynamics
Quantum mechanics allows for a consistent formulation of particles that are neither bosons nor fermions. In this talk, I’ll present a particular example of those particles, the so-called para-particles, which arise as a generalization of the usual bosons and fermions. Even though these particles are unlikely to be present in nature, a quantum system involving a spin-½ degree of freedom coupled to two bosonic modes yields a Hamiltonian that describes para-bosons and para-fermions.
Observation of measurement-induced quantum phases in a trapped-ion quantum computer
Many-body open quantum systems balance internal dynamics against decoherence from interactions with an environment. In this talk, I describe an experiment that explores this balance via random quantum circuits implemented on a trapped-ion quantum computer, where the system evolution is represented by unitary gates with interspersed projective measurements. As the measurement rate is varied, a purification phase transition is predicted to emerge at a critical point akin to a fault-tolerant threshold.
Energy absorption in chaotic billiards under rapid periodic driving
In this talk, I will discuss chaotic billiard systems subject to a rapid periodic driving force, with driving frequency ω. Classically, the energy of such systems changes by small, effectively random increments associated with collisions with the billiard wall, leading to a random walk in energy space, or “energy diffusion.” I will present a Fokker-Planck description of this process. This model displays several notable features, including a 1/ω² scaling of the energy absorption rate, and (in certain special cases) an exact analytical solution.
Quantum simulations of non-ergodic phenomena with trapped ions
Recent years have seen the development of isolated quantum simulator platforms capable of exploring interesting questions at the frontiers of many-body physics. We describe our platform, based on a chain of Ytterbium ions in a linear trap, and describe its capabilities, which include long-range spin-spin interactions and single-site manipulation and readout. We then describe some recent studies undertaken with this machine, focusing on two.
Nonequilibrium phases of matter on NISQ hardware
Recent progress on noisy, intermediate scale quantum (NISQ) devices opens exciting opportunities for many-body physics. NISQ platforms are indeed not just computers, but also interesting laboratory systems in their own right, offering access to large Hilbert spaces with exceptional capabilities for control and measurement. I will argue that nonequilibrium phases in periodically-driven (Floquet) systems are a particularly good fit for such capabilities in the near term.