MS Thesis Defense: Maxwell Weiner

"Explorations of Amplified Feedback in Quantum Circuits"

Abstract

The Josephson Traveling Wave Parametric Amplifier (TWPA) has emerged as a key technology for high-fidelity qubit readout in superconducting quantum computing. By leveraging the nonlinear inductance of an array of Josephson Junctions, the TWPA enables broadband, near-quantum-limited amplification with minimal added noise, significantly improving the signal-to-noise ratio in qubit measurements. Unlike traditional resonant parametric amplifiers, which suffer from bandwidth constraints, the traveling wave design of the TWPA allows for wideband operation, making it particularly suited for multiplexed readout of both simple qubits and large-scale quantum processors.

In this thesis, we explore how the TWPA can be integrated into a feedback loop with a 3D microwave cavity acting as a parametrically driven nonlinear oscillator. Rather than focusing on amplification alone, we use the TWPA to stabilize the photon number in the oscillator by providing directional gain and engineered dissipation. This configuration enables preparation of non-classical states of the cavity, with the loop dynamics supporting strong squeezing and nonlinear interactions reminiscent of Kerr-like behavior. We have estimated the system to have a Kerr nonlinearity J of about 1.3 MHz, much higher than non-loop experiments which measure nonlinearity on the order of tens of kilohertz.

We use Wigner tomography to reconstruct the quantum states generated in the oscillator. Unlike many traditional approaches, we do not rely on an ancillary probe (transmon) qubit to extract aspects of the state like the photon number parity; instead, we read out the cavity state directly. While this method simplifies the experimental setup, it is inefficient, as it requires repeated measurements over many quadrature angles to reconstruct the full Wigner function, and there is a significant amount of dissipation in the loop. This inefficiency leads to slower data acquisition and increased susceptibility to decoherence and drift during measurement. A potential solution is to use a higher-quality cavity with reduced photon loss, which would improve coherence and stability over the tomography timescale. Alternatively, coupling our setup to a transmon qubit in future designs could enable fast, parity-sensitive measurements and more efficient quantum state reconstruction.

Thesis Committee

• Mattias W. Fitzpatrick (Chair)
• Laura E. Ray
• Miles P. Blencowe

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu.