Friday Quantum Seminar

Noisy Intermediate-Scale Quantum (NISQ) devices fail to produce outputs with sufficient fidelity for deep circuits with many gates today. Such devices suffer from read-out, multi-qubit gate and cross-talk noise combined with short decoherence times limiting circuit depth. This work develops a methodology to generate shorter circuits with fewer multi-qubit gates whose unitary transformations approximate the original reference one. It explores the benefit of such generated approximations under NISQ devices.

Quantum process tomography is a critical capability for building quantum computers, enabling quantum networks, and understanding quantum sensors. Like quantum state tomography, the process tomography of an arbitrary quantum channel requires a number of measurements that scale exponentially in the number of quantum bits affected. However, the recent field of shadow tomography, applied to quantum states, has demonstrated the ability to extract key information about a state with only polynomially many measurements.

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.