Date of Award

Spring 1-1-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Physics

First Advisor

Schoelkopf, Robert

Abstract

A single high-Q harmonic oscillator with a fixed `dispersive’ coupling to an ancillary qubit provides a remarkably hardware-efficient platform for a wide range of quantum technologies, capable of acting as a dark matter detector, a simulator of quantum chemistry or a quantum memory with a lifetime longer than its underlying components. The strength of this platform lies in the linearity and favorable decoherence rates of the high-Q oscillator mode. The question then arises: how can we scale this oscillator-based platform to practically useful sizes without compromising a) the speed of operations or b) the properties that make oscillators an attractive platform in the first place? The addition of a tunable oscillator-oscillator coupling, equivalent to an optical beamsplitter, has extended the power of this platform to enable multi-mode entanglement, a key element for quantum computation, but until now, implementations have been limited to low interaction strengths and introduced unwanted oscillator nonlinearity. Inspired by advances in parametric amplification, we show how a three-wave mixing element solves this challenge by acting as a switch, with beamsplitter interaction strengths exceeding those of the dispersive coupling when turned on, and the ability to fully decouple the modes when turned off. We then demonstrate how this regime unlocks a powerful new toolbox of high-fidelity multimode operations which are the analogs of established single-mode control techniques. In particular, we show how these techniques can be leveraged to perform a mid-circuit erasure check, the vital building block for a newly-proposed quantum computer made out of superconducting cavity dual-rail qubits.

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