Laser Entropy

TL;DR

This paper calculates the entropy of photon and atom lasers, revealing nonzero entropy near threshold and challenging traditional views of pure states and zero entropy in Bose-Einstein condensates.

Contribution

It provides the first calculations of entropy for both photon and atom lasers, contrasting with prior assumptions of zero entropy in these systems.

Findings

Nonzero entropy of single mode laser near threshold

Entropy of Bose-Einstein condensate ground state calculated for the first time

Challenges the idea that laser radiation and condensates are pure zero-entropy states

Abstract

The entropy of an ordinary (photon) laser and an atom laser (Bose condensate) is calculated. In particular, the nonzero entropy of a single mode laser or maser operating near threshold is obtained. This result is to be compared with the statement frequently made in the study of the maser heat engine to the effect that: "because maser radiation is in a pure state, its entropy is zero." Similarly, the entropy of the ground state of a Bose-Einstein condensate (a.k.a. the atom laser) is also calculated for the first time. This is to be compared with the textbook wisdom which holds that: "The condensed particles ... are condensed in momentum space, a set of stationary particles ... having zero energy and zero entropy."