Continuous quantum nondemolition measurement of the transverse component of a qubit

TL;DR

This paper demonstrates the first continuous quantum nondemolition measurement of a qubit's transverse component, enabling observation of quantum jumps between superposition states, which is valuable for quantum simulation and error correction.

Contribution

It introduces a novel method to measure the transverse component of a qubit continuously without destroying its quantum state.

Findings

Successful implementation of a transverse quantum nondemolition measurement

Observation of quantum jumps between superposition states

Potential applications in quantum error correction

Abstract

Quantum jumps of a qubit are usually observed between its energy eigenstates, also known as its longitudinal pseudo-spin component. Is it possible, instead, to observe quantum jumps between the transverse superpositions of these eigenstates? We answer positively by presenting the first continuous quantum nondemolition measurement of the transverse component of an individual qubit. In a circuit QED system irradiated by two pump tones, we engineer an effective Hamiltonian whose eigenstates are the transverse qubit states, and a dispersive measurement of the corresponding operator. Such transverse component measurements are a useful tool in the driven-dissipative operation engineering toolbox, which is central to quantum simulation and quantum error correction.