Exploiting nonequilibrium phase transitions and strong symmetries for continuous measurement of collective observables

TL;DR

This paper presents a method to measure collective observables in dissipative quantum systems by leveraging nonequilibrium phase transitions and strong symmetries, enabling direct inference from continuous monitoring.

Contribution

It introduces a protocol that uses strong symmetries and phase transitions to measure collective observables without repeated projective measurements.

Findings

Total angular momentum can be inferred from emission signals.

Continuous monitoring replaces the need for reinitializations.

Applicable to quantum tomography and metrology.

Abstract

Dissipative many-body quantum dynamics can feature strong symmetries which give rise to conserved quantities. We discuss here how a strong symmetry in conjunction with a nonequilibrium phase transition allows to devise a protocol for measuring collective many-body observables. To demonstrate this idea we consider a collective spin system whose constituents are governed by a dissipative dynamics that conserves the total angular momentum. We show that by continuously monitoring the system output the value of the total angular momentum can be inferred directly from the time-integrated emission signal, without the need of repeated projective measurements or reinitializations of the spins. This may offer a route towards the measurement of collective properties in qubit ensembles, with applications in quantum tomography, quantum computation and quantum metrology.