Quantum nondemolition-like, fast measurement scheme for a superconducting qubit

TL;DR

This paper proposes a two-step measurement protocol for superconducting flux qubits coupled to a SQUID, enabling rapid qubit state imprinting followed by a long, non-invasive measurement phase, akin to quantum nondemolition techniques.

Contribution

It introduces a novel measurement scheme that quickly imprints qubit states onto a SQUID and then allows for a long, error-free measurement period, enhancing quantum measurement fidelity.

Findings

Qubit state is rapidly imprinted onto the SQUID with minimal disturbance.

The measurement process can be extended without inducing additional qubit errors.

Complete dephasing of the qubit occurs after the first step, enabling effective state readout.

Abstract

We present a measurement protocol for a flux qubit coupled to a dc-Superconducting QUantum Interference Device (SQUID), representative of any two-state system with a controllable coupling to an harmonic oscillator quadrature, which consists of two steps. First, the qubit state is imprinted onto the SQUID via a very short and strong interaction. We show that at the end of this step the qubit dephases completely, although the perturbation of the measured qubit observable during this step is weak. In the second step, information about the qubit is extracted by measuring the SQUID. This step can have arbitrarily long duration, since it no longer induces qubit errors.