Dartmouth Events

Physics & Astronomy-PhD Thesis Defense - Bhargava Thyagarajan, Dartmouth College

Title: "The cavity-Embedded Cooper pair transistor as a charge detector operating in the nonlinear regime"

Monday, May 23, 2022
2:30pm – 4:30pm
Wilder 202 & Zoom
Intended Audience(s): Public
Categories: Lectures & Seminars

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 3μs and a detection fidelity
of 92%. 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 1μs 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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