Recent studies have shown that light propagating in a nonlinear, highly multimode system can thermalize in a manner totally analogous to that encountered in traditional statistical mechanics. At thermal equilibrium, the system’s entropy is at a maximum, in full accord with the second law of thermodynamics. In such arrangements, the entropy is extremized once the statistical power allocation among modes associated with this photon gas attains a Rayleigh-Jeans distribution that is fully characterized by an optical temperature $T$ and a chemical potential $\mu$. However, it has been theoretically argued that the variables $T$ and $\mu$ represent actual thermodynamic forces that control the exchange of the respective conjugate quantities between two subsystems. In this work, we report, for the first time, optical calorimetric measurements in nonlinear multimode fibers, which unambiguously demonstrate that both the temperature $T$ and the chemical potential $\mu$ dictate the flow of their associated extensive quantities, i.e., the energy and the optical power. Specifically, we study the process of light thermalization associated with two orthogonally polarized laser beams. Our observations are enabled by recently developed techniques that allow one to judiciously multiplex/demultiplex the optical power within various mode groups. Our results indicate that because of photon-photon collisions, “heat” only flows from a hot to a cold photon gas subsystem—thus providing an unequivocal demonstration of the second law in such all-optical thermodynamic arrangements. In addition to being fundamental, our findings provide a new approach to manipulate laser beams using thermodynamic principles.

中文翻译：

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非线性多模光纤中光子气体的量热法

最近的研究表明，在非线性、高度多模系统中传播的光可以以完全类似于传统统计力学中遇到的方式热化。在热平衡时，系统的熵达到最大值，完全符合热力学第二定律。在这种布置中，一旦与该光子气体相关的模式之间的统计功率分配达到完全由光学温度表征的瑞利-詹斯分布，则熵被极端化$\mathrm{时间}$和化学势$\mu$。然而，理论上有人认为变量$\mathrm{时间}$和$\mu$代表控制两个子系统之间各自共轭量交换的实际热力学力。在这项工作中，我们首次报告了非线性多模光纤中的光学量热测量，这明确地证明了温度$\mathrm{时间}$和化学势$\mu$决定了它们相关的广延量的流动，即能量和光功率。具体来说，我们研究与两束正交偏振激光束相关的光热化过程。我们的观察是通过最近开发的技术实现的，这些技术允许人们在各种模式组内明智地复用/解复用光功率。我们的结果表明，由于光子与光子的碰撞，“热量”仅从热光子气体子系统流向冷光子气体子系统，从而明确证明了这种全光学热力学排列中的第二定律。除了具有基础性之外，我们的发现还提供了一种利用热力学原理操纵激光束的新方法。