Nonequilibrium thermodynamics and power generation in open quantum optomechanical systems

Cavity optomechanical systems are a paradigmatic setting for the conversion of electromagnetic energy into mechanical work. Experiments with atoms coupled to cavity modes are realized in nonequilibrium conditions, described by phenomenological models encoding nonthermal dissipative dynamics and falling outside the framework of weak system-bath couplings. This fact makes their interpretation as quantum engines, e.g., the derivation of a well-defined efficiency, quite challenging. Here, we present a consistent thermodynamic description of open quantum cavity-atom systems. Our approach takes advantage of their nonequilibrium nature and arrives at an energetic balance which is fully interpretable in terms of persistent dissipated heat currents. The interaction between atoms and cavity modes can further give rise to nonequilibrium phase transitions and emergent behavior and allows us to assess the impact of collective many-body phenomena on the engine operation. To enable this, we define two thermodynamic limits, one related to a weak optomechanical coupling and one related to a strong optomechanical coupling. We illustrate our ideas by focusing on a time-crystal engine and discuss power generation, energy-conversion efficiency, and the emergence of metastable behavior in these limits. © 2023 American Physical Society.