The Entropy Flow of a Laser Beam
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arXiv
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| Format: | Preprint |
| Published: |
2025
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| _version_ | 1866912895534628864 |
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| author | Wiseman, Howard M. |
| author_facet | Wiseman, Howard M. |
| contents | 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 $\dot{S} = k_B \sqrt{\dot{N}\ell}$, 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's. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2512_01230 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | The Entropy Flow of a Laser Beam Wiseman, Howard M. Quantum Physics 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 $\dot{S} = k_B \sqrt{\dot{N}\ell}$, 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's. |
| title | The Entropy Flow of a Laser Beam |
| topic | Quantum Physics |
| url | https://arxiv.org/abs/2512.01230 |