Macroscopic Quantum Measurements of noncommuting observables

TL;DR

This paper demonstrates that macroscopic quantum measurements can simultaneously achieve high information gain and low disturbance, exemplified by measuring collective spin directions with an optimal coupling regime.

Contribution

It introduces a formalism showing that large quantum systems can be measured with minimal disturbance while maintaining high informativeness, challenging traditional notions of measurement disturbance.

Findings

Optimal intermediate coupling regime identified

Guessing fidelity peaks with minimal disturbance

Measurement formalism applicable to spin ensembles

Abstract

Assuming a well-behaving quantum-to-classical transition, measuring large quantum systems should be highly informative with low measurement-induced disturbance, while the coupling between system and measurement apparatus is "fairly simple" and weak. Here, we show that this is indeed possible within the formalism of quantum mechanics. We discuss an example of estimating the collective magnetization of a spin ensemble by simultaneous measuring three orthogonal spin directions. For the task of estimating the direction of a spin-coherent state, we find that the average guessing fidelity and the system disturbance are nonmonotonic functions of the coupling strength. Strikingly, we discover an intermediate regime for the coupling strength where the guessing fidelity is quasi-optimal, while the measured state is almost not disturbed.