Dispersive Charge and Flux Qubit Readout as a Quantum Measurement Process

TL;DR

This paper investigates the dispersive readout of superconducting charge and flux qubits, demonstrating that large detuning enables fast, efficient measurement with minimal back-action, and deriving analytic expressions for measurement dynamics.

Contribution

It provides a theoretical analysis of dispersive qubit readout beyond the rotating wave approximation, showing high quantum efficiency and small non-adiabatic corrections.

Findings

Quantum efficiency is unity within the approximation.

Analytic expressions for measurement time and back-action are derived.

Large detuning allows for fast, strong coupling readout.

Abstract

We analyze the dispersive readout of superconducting charge and flux qubits as a quantum measurement process. The measurement oscillator frequency is considered much lower than the qubit frequency. This regime is interesting because large detuning allows for strong coupling between the measurement oscillator and the signal transmission line, thus allowing for fast readout. Due to the large detuning we may not use the rotating wave approximation in the oscillator-qubit coupling. Instead we start from an approximation where the qubit follows the oscillator adiabatically, and show that non-adiabatic corrections are small. We find analytic expressions for the measurement time, as well as for the back-action, both while measuring and in the off-state. The quantum efficiency is found to be unity within our approximation, both for charge and flux qubit readout.