Date of Award

2022

Document Type

Thesis (Ph.D.)

Department or Program

Physics and Astronomy

First Advisor

Alexander J. Rimberg

Abstract

The cavity-embedded Cooper pair transistor (cCPT) has been shown to be a nearly quantum limited charge detector operating with only a single intracavity photon. Here, we use the inherent Kerr nonlinearity to demonstrate a dispersive charge sensing technique inspired by the Josephson bifurcation amplifier. Operating in the bistable regime close to a bifurcation edge, the cCPT is sensitive to charge shifts of 0.09e in a single-shot readout scheme with a detection time of 3 μs and a detection fidelity of 94%. The readout is implemented with only ∼ 25 intracavity photons in the high oscillation amplitude state, still several orders of magnitude lower than drives used in state-of-the-art radio frequency single electron transistors (rf-SETs). We find that a major limitation to the charge sensitivity of the device is fluctuation-induced switching between the metastable oscillation states in the bistable region. We study the lifetimes of these states across the gate and flux range of the cCPT and find that the switching properties depend on the strength of the Kerr nonlinearity at the cCPT bias point.

We also explore a second nonlinear detection scheme where we parametrically pump the cCPT using a time-varying flux close to twice its resonance frequency to induce parametric oscillations. Flux pumping at a detuning on the edge of the parametric oscillation threshold makes the amplitude of oscillations sensitive to the charge environment. With no input drive, we are able to distinguish charge states ∼ 0.1e apart in a measurement time of 1 μs with a fidelity of 83%.

The cCPT is a rich nonlinear system in which we observe sub-harmonic oscillations and phase coherent degenerate parametric amplification which could potentially be used to enhance the dispersive charge sensing of the device operating with a single intracavity photon level drive.

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