Nonlinear Parity Readout with a Microwave Photodetector

TL;DR

This paper introduces a nonlinear parity measurement method in circuit-QED that achieves high contrast and efficiency using a strongly driven nonlinear microwave cavity, improving robustness over linear schemes.

Contribution

It demonstrates a new nonlinear dispersive approach for high-fidelity, single-shot parity readout in circuit-QED systems, applicable to multiple qubits.

Findings

High contrast parity measurement achieved

Robust tuning conditions based on nonlinearity

Extension to N-qubit parity measurements

Abstract

Robust high-fidelity parity measurment is an important operation in many applications of quantum computing. In this work we show how in a circuit-QED architecture, one can measure parity in a single shot at very high contrast by taking advantage of the nonlinear behavior of a strongly driven microwave cavity coupled to one or multiple qubits. We work in a nonlinear dispersive regime treated in an exact dispersive transformation. We show that appropriate tuning of experimental parameters leads to very high contrast in the cavity and therefore to a high efficiency parity readout with a microwave photon counter or another amplitude detector. These tuning conditions are based on nonlinearity and are hence more robust than previously described linear tuning schemes. In the first part of the paper we show in detail how to achieve this for two qubit parity measurements and extend this to $N$ qubits in the second part of the paper. We also study the QNDness of the protocol.