The Entropy Flow of a Laser Beam

• URL: http://arxiv.org/abs/2512.01230v1
• Date: Mon, 01 Dec 2025 03:16:43 GMT
• Title: The Entropy Flow of a Laser Beam
• Abstract summary: We consider an ideal laser beam, with phase diffusion rate $ell$, equal to its (Lorentzian) spectral width.<n>We show that the beam entropy is extensive, with an entropy flow of $dotS = sqrtdotNell$, where $dotN$ is the number flow.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• 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 $\ell$, equal to its (Lorentzian) spectral width. We show that the beam entropy is extensive, with an entropy flow of $\dot{S} = \sqrt{\dot{N}\ell}$, where $\dot{N}$ is the number flow. We give an intuitive explanation for this remarkably simple result, and compare it to a unidirectional thermal beam.

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