Entropy measurement of nonequilibrium phonon heat transport

Entropy, as a fundamental concept in thermodynamics, provides a powerful tool for understanding the behavior of nonequilibrium systems. However, measuring time-dependent entropy in nonequilibrium, nonstationary processes remains largely unexplored. Here, we report the experimental measurement of entropy dynamics for nonequilibrium phonon heat transport in a multimode cavity optomechanical system. Taking the advantages of the capability for continuous monitoring of single trajectories and dynamic control over the system’s Hamiltonian, as well as the framework of entropy estimation in mesoscopic systems, we have directly measured the time evolution of system entropy change, entropy production rate, and entropy flux rate following a quench from the noninteracting to the interacting regime. The combination of Gaussian, exponential, and oscillatory relaxation dynamics of the system entropy change have been observed at different internal interaction strengths of the composite system. Moreover, we find that the system entropy change also strongly depends on the system-bath interactions, which is controlled to be either positive or negative by engineering the thermal reservoirs and relaxation rates. Our results enable entropy dynamics measurements for nonequilibrium transport in mesoscopic composite systems and offer opportunities for optimizing thermodynamic processes in cutting-edge disciplines. Entropy and its time-dependent measurement in nonstationary processes is important to understand non-equilibrium systems. Here, the authors report the experimental measurement of entropy dynamics for nonequilibrium phonon heat transport in a multimode cavity optomechanical system, revealing complex relaxation dynamics and dependencies on system-bath interactions.