Entropy flow of a laser beam.

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 ℓ, equal to its (Lorentzian) spectral width. We show that the beam entropy is extensive, with an entropy flow of S˙=kBN˙ℓ, where N˙ 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.