Entropy Flow of a Laser Beam
arXiv:2512.01230 · doi:10.1364/OL.599036
The paper analyzes the entropy of an ideal laser beam whose phase undergoes diffusion, deriving a simple expression for the entropy flow in terms of the photon flux and the phase diffusion rate.
Abstract
A laser beam is often modelled by a pure coherent state. In fact its state is mixed, even if it has coherent-state photon-number statistics (Poissonian), because the phase must vary. We consider such an ideal laser beam, with phase diffusion rate , equal to its (Lorentzian) spectral width. We show that the beam entropy is extensive, with an entropy flow of $\dot{S} = \kB\sqrt{\dot{N}\ell}$, where is the number flow. We give an intuitive explanation for this remarkably simple result, and compare it to the entropy flow of a unidirectional thermal beam.
3 pages