Date of Award

Spring 5-5-2022

Document Type

Thesis (Ph.D.)

Department or Program

Physics and Astronomy

First Advisor

Alexander Rimberg

Second Advisor

Miles Blencowe

Abstract

Mesoscopic quantum systems currently serve as essential building blocks in many quantum information and metrology devices. This thesis investigates the potential of quantum-limited detection in a mesoscopic electrometer named the cavity-embedded Cooper pair transistor (cCPT). As one application, this charge detector can act as the basis for an optomechanical system in the single-photon strong coupling regime. The realization of this scheme would entail near quantum-limited, ultra-sensitive electrometry at the single-photon level, the feasibility of which is studied at length in this thesis.

On the one hand, we approach this question using a fundamental, first-principles study, where an operator scattering model is used to analyze the quantum dynamics of this device. While the cCPT is inherently a tunable, strongly nonlinear system affording diverse functionalities, we restrict our analysis to a necessary first investigation of its linear charge sensing capabilities, limiting to low pump powers corresponding to an average cavity photon number <1. Assuming realizable cCPT parameters, we predict the fundamental, photon shot noise-limited charge sensitivity to be 0.12 μe/√Hz, when the pumped cavity has an average of one photon.

In practice, this lower bound is difficult to achieve using conventional detection approaches, owing mainly to the low-frequency noise caused by the coupling of two-level systems to the cCPT. Hence we further employ a top-down approach where the gate-dependent tunability of the cCPT is used to implement a feedback scheme derived from the Pound-Drever-Hall locking technique. This scheme effectively reduces the fluctuations due to intrinsic charge noise. In particular, we report a reduction in the resonant frequency fluctuations caused by the internal charge noise over a bandwidth of ~1.4 kHz when the cavity is driven at an average photon number n=10, and a bandwidth of 11 Hz for average n=1. Our technique can be generalized to achieve frequency stabilization in tunable microwave resonators that play a vital role in today's quantum computing architectures, thereby moderating the limitations in detection caused by the intrinsic 1/f-noise on such circuit devices. As a concluding study, we incorporate these feedback techniques to improve the charge sensitivity of the cCPT, thus demonstrating the potential of near quantum-limited charge detection using this device.

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