Friday Quantum Seminar

Theoretical models of quantum computation usually assume that 2-qubit gates can be performed between arbitrary pairs of qubits. However, in practice, scalable quantum architectures have qubit connectivity constraints, which can introduce polynomial depth overheads. Compiling quantum algorithms to work on scalable architectures therefore requires optimizing arrangements of gates and qubits to minimize these overheads.

In this special Friday Quantum Seminar, Dr. Allyson O'Brien, a Quantum Technology Scientist at Leidos, will share stories from her career path and a broader perspective on the field.

Pizza and drinks served after the talk.

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.

To realize a digital quantum processor based on superconducting qubits, gate error rates must be further reduced by raising coherence times and increasing anharmonicity. I report our group's progress in improving coherence and control of fluxonium superconducting qubits by optimizing the circuit's spectrum and enhancing fabrication methods.

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.

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.

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.

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.

We derive a family of optimal protocols, in the sense of saturating the quantum Cramér-Rao bound, for measuring a linear combination of d field amplitudes with quantum sensor networks, a key subprotocol of general quantum sensor networks applications. We demonstrate how to select different protocols from this family under various constraints via linear programming. Focusing on entanglement-based constraints, we prove the surprising result that highly entangled states are not necessary to achieve optimality for many problems.

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.