Qubit measurement and backaction in a multimode nonreciprocal system

TL;DR

This paper develops a theoretical framework for understanding and designing multimode nonreciprocal systems integrated with qubits, demonstrating its effectiveness through experimental implementation and analysis of qubit readout and amplification.

Contribution

It introduces a first-principles theoretical tool for analyzing multimode nonreciprocal systems with embedded qubits, bridging the gap between theory and experimental implementation.

Findings

Excellent agreement between theory and experiment in qubit measurement and dephasing rates.

Predicted high efficiency for integrated nonreciprocal amplification.

Validated design principles for scalable quantum readout devices.

Abstract

High fidelity qubit readout is a cornerstone for quantum information protocols. In traditional superconducting qubit readout, a chain of microwave amplifiers and nonreciprocal components aid in detecting the qubit's state with tolerable added noise and backaction. However, the loss, size, and magnetic field of standard nonreciprocal components have sparked a decades-long search for more efficient and scalable alternatives. One prominent approach employs networks of parametrically coupled modes to achieve nonreciprocity. While this class of devices can be directly integrated with the qubit's readout cavity, current understanding of the resulting single quantum system is substantially lacking. Here we provide a first-principles theoretical tool to understand and design networks of linear modes integrated with embedded qubits. We utilize this theory to inform and analyze the experimental implementation of a qubit readout with an integrated three-mode nonreciprocal system. In doing so, we achieve excellent agreement between the experimental and theoretical qubit measurement and dephasing rates. We then theoretically analyze the same system operated as an integrated nonreciprocal amplifier, predicting high efficiency for reasonable experimental parameters.