Can a localised quantum system see soft photons?

TL;DR

This paper investigates whether soft photons, characterized by asymptotic charges, influence local quantum processes, finding that they affect de-excitation rates but not excitation in a localized two-state system.

Contribution

It demonstrates that soft charges impact the de-excitation process of a localized quantum system coupled to a gauge field, revealing a new connection between asymptotic symmetries and local quantum dynamics.

Findings

De-excitation rate depends on soft charges.

Excitation rate remains zero regardless of soft charges.

Soft photons influence local quantum process outcomes.

Abstract

We ask whether soft photons, defined by asymptotic charges, can have consequences for the outcome of localised quantum processes. We consider a spatially localised two-state system, at rest in flat spacetime, coupled to a $U(1)$ gauge invariant charged scalar field. We find that the system's de-excitation rate does depend on the soft charges that correspond to the radial component of the electric field dressing at the asymptotic infinity; the excitation rate, by contrast, remains zero, regardless of the soft charges. Some implications are discussed.