Readout of a weakly coupled qubit through the use of an auxiliary mode

TL;DR

This paper demonstrates that using an auxiliary cavity mode significantly reduces the required dispersive coupling strength for qubit readout, enhancing measurement efficiency in quantum systems.

Contribution

It introduces a method employing an auxiliary mode to lower the dispersive coupling threshold for qubit readout, with analytical optimization and broader applicability.

Findings

Auxiliary mode reduces dispersive coupling requirements by orders of magnitude.

Derived analytical expressions for optimal parameters.

Applicable to longitudinal cavity-qubit interactions and sensing schemes.

Abstract

The dispersive coupling between a qubit and a cavity mode is widely used for performing non-destructive readout of the qubit state. In this approach, it is typically required that the dispersive strong coupling regime is achieved. Here we show that the use of an auxiliary cavity mode reduces by orders of magnitude the required value of the dispersive coupling, for a given decay rate of the cavity mode. The analysis is performed within the input-output formalism, in terms of the photon scattering matrix elements and of the signal-to-noise ratio. We derive simple analytical expressions for the optimal parameters and recover the standard single-mode result as a limiting case. The present results can also be applied to the qubit readout based on longitudinal cavity-qubit interactions, and to any sensing scheme where the cavity frequency is used as a probe to estimate some physical parameter of interest.