Reducing backaction when measuring temporal correlations in quantum systems

TL;DR

This paper advances measurement techniques for quantum systems by generalizing noninvasive measurement protocols to arbitrary observables, enabling more accurate dynamic correlation measurements with reduced backaction.

Contribution

It extends ancilla-based noninvasive measurement methods to arbitrary spin observables and introduces a framework for simultaneous extraction of correlation parts.

Findings

Generalized measurement protocols for arbitrary observables.

Identified observables with no backaction concern.

Framework for analyzing measurement dynamics using POVMs.

Abstract

Dynamic correlations of quantum observables are challenging to measure due to measurement backaction incurred at early times. Recent work [P. Uhrich et al., Phys. Rev. A, 96:022127 (2017)] has shown that ancilla-based noninvasive measurements are able to reduce this backaction, allowing for dynamic correlations of single-site spin observables to be measured. We generalise this result to correlations of arbitrary spin observables and extend the measurement protocol to simultaneous noninvasive measurements which allow for real and imaginary parts of correlations to be extracted from a single set of measurements. We use positive operator-valued measures to analyse the dynamics generated by the ancilla-based measurements. Using this framework we prove that special observables exist for which measurement backaction is of no concern, so that dynamic correlations of these can be obtained without making use of ancillas.