Fast quantum limited read-out of a superconducting qubit using a slow oscillator

TL;DR

This paper presents a method for fast, quantum-limited read-out of a superconducting qubit using a slow oscillator and homodyne detection, enabling rapid single-shot measurements with minimal backaction.

Contribution

The authors introduce a quantum-limited read-out technique using a low-Q resonator and homodyne detection, allowing fast (~50 ns) single-shot qubit measurements in superconducting circuits.

Findings

Detection is quantum limited even with low-Q resonators

Fast (~50 ns) single-shot read-out is achievable

Formulas for backaction and optimal parameters are provided

Abstract

We describe how to perform fast quantum limited read-out of a solid state qubit biased at its degeneracy point. The method is based on homodyne detection of the phase of a microwave signal reflected by a slow oscillator coupled to the qubit. Analyzing the whole quantum read-out process, we find that the detection is indeed quantum limited and that this limit may be reached even using a resonance circuit with a low quality factor, thus enabling the use of short measurement pulses. As an example, we discuss in detail the read-out of a Cooper-pair box capacitively coupled to a lumped element LC-oscillator. Furthermore, we give formulas for the backaction while not measuring, and discuss optimal parameters for a realistic design capable of fast (~50 ns) single-shot read-out.