Quantum non-demolition measurement of a superconducting two-level system

TL;DR

This paper demonstrates a quantum non-demolition measurement technique for superconducting flux qubits, showing high correlation between consecutive measurements and advancing their use in quantum information processing.

Contribution

It provides the first experimental realization of QND measurements on superconducting qubits, confirming their suitability as fundamental quantum systems.

Findings

High correlation between consecutive measurements confirms QND nature

QND measurement preserves eigenstates of the qubit

Results support use of superconducting qubits in quantum info protocols

Abstract

In quantum mechanics, the process of measurement is a subtle interplay between extraction of information and disturbance of the state of the quantum system. A quantum non-demolition (QND) measurement minimizes this disturbance by using a particular system - detector interaction which preserves the eigenstates of a suitable operator of the quantum system. This leads to an ideal projective measurement. We present experiments in which we perform two consecutive measurements on a quantum two -level system, a superconducting flux qubit, by probing the hysteretic behaviour of a coupled nonlinear resonator. The large correlation between the results of the two measurements demonstrates the QND nature of the readout method. The fact that a QND measurement is possible for superconducting qubits strengthens the notion that these fabricated mesoscopic systems are to be regarded as fundamental quantum objects. Our results are also relevant for quantum information processing, where projective measurements are used for protocols like state preparation and error correction.