QND measurement of a superconducting qubit in the weakly projective regime

TL;DR

This paper demonstrates a quantum non-demolition measurement of a superconducting qubit using a Josephson junction detector, achieving high speed and fidelity while minimizing back-action.

Contribution

It introduces a method to reduce back-action in Josephson junction-based qubit measurements, enabling fast, high-fidelity QND readouts within 25 ns.

Findings

Single readout completed within 25 ns at 1 MHz repetition rate

QND fidelity of 75% achieved with the method

Back-action significantly reduced compared to traditional detectors

Abstract

Quantum state detectors based on switching of hysteretic Josephson junctions biased close to their critical current are simple to use but have strong back-action. We show that the back-action of a DC-switching detector can be considerably reduced by limiting the switching voltage and using a fast cryogenic amplifier, such that a single readout can be completed within 25 ns at a repetition rate of 1 MHz without loss of contrast. Based on a sequence of two successive readouts we show that the measurement has a clear quantum non-demolition character, with a QND fidelity of 75 %.