Super-radiance and flux conservation

TL;DR

This paper clarifies the theoretical foundations of super-radiance, emphasizing flux conservation and distinguishing it from related phenomena, through analysis and a simple solvable model.

Contribution

It provides a clear conceptual framework for understanding super-radiance in quantum field theory, resolving common confusions and illustrating key points with an exactly solvable toy model.

Findings

Super-radiance is fundamentally linked to flux conservation.

Flux non-conservation occurs with dissipation.

A simple toy model demonstrates super-radiance and damping effects.

Abstract

The theoretical foundations of the phenomenon known as super-radiance still continues to attract considerable attention. Despite many valiant attempts at pedagogically clear presentations, the effect nevertheless still continues to generate some significant confusion. Part of the confusion arises from the fact that super-radiance in a quantum field theory [QFT] context is not the same as super-radiance (super-fluorescence) in some condensed matter contexts; part of the confusion arises from traditional but sometimes awkward normalization conventions, and part is due to sometimes unnecessary confusion between fluxes and probabilities. We shall argue that the key point underlying the effect is flux conservation, (and, in the presence of dissipation, a controlled amount of flux non-conservation), and that attempting to phrase things in terms of reflection and transmission probabilities only works in the absence of super-radiance. To help clarify the situation we present a simple exactly solvable toy model exhibiting both super-radiance and damping.