Local and nonlocal observables in quantum optics

TL;DR

This paper clarifies the distinction between local and nonlocal observables in quantum optics, emphasizing the importance of careful measurement approaches due to their coexistence and the nonlocal nature of certain observables.

Contribution

It introduces an unambiguous definition of locality in quantum optics and discusses the measurement challenges of local and nonlocal observables.

Findings

Nonlocal observables depend on spatial position and cannot be measured locally without disturbance.

Local and nonlocal observables coexist in quantum optics.

Careful measurement strategies are required for nonlocal observables.

Abstract

It is pointed out that there exists an unambiguous definition of locality that enables one to distinguish local and nonlocal quantities. Observables of both types coexist in quantum optics but one must be very careful when attempting to measure them. A nonlocal observable which formally depends on the spatial position $\bi r$ cannot be {\em locally} measured without disturbing the measurements of this observable at all other positions.