On Realization of Back-Action-Evading Measurements and Quantum Non-Demolition Variables via Linear Systems Engineering

TL;DR

This paper develops a framework for implementing back-action-evading and quantum non-demolition measurements in linear quantum systems, using Hamiltonian conditions and feedback control.

Contribution

It introduces a unified approach to realize BAE and QND measurements through Hamiltonian design and feedback in linear quantum systems.

Findings

Purely imaginary Hamiltonian enables BAE measurements of conjugate variables.

Symmetric coupling leads to QND variables.

Coherent feedback can engineer BAE measurements in non-compliant systems.

Abstract

We establish a framework for realizing back-action-evading (BAE) measurements and quantum non-demolition (QND) variables in linear quantum systems. The key condition, a purely imaginary Hamiltonian with a real or imaginary coupling operator, enables BAE measurements of conjugate observables. Symmetric coupling further yields QND variables. For non-compliant systems, coherent feedback is designed to engineer BAE measurements. Crucially, the QND interaction condition simultaneously ensures BAE measurements and promotes the coupling operator to a QND observable.