Experimentally probing entropy reduction via iterative quantum information transfer

In the last few decades, multiple experimental demonstrations have illuminated the interplay between thermodynamics and information. However, experimental verification in the quantum regime has so far been limited. In particular, the role of dynamical quantum information flow in thermodynamics remains unexplored, despite its significance in quantum feedback control for creating and stabilizing desired quantum states.

In this work, we experimentally verify the fundamental laws of quantum thermodynamics out of equilibrium, the second law and the fluctuation theorem, which incorporate measures of quantum information flow caused by iterative quantum measurement and feedback. Our experiments are conducted with a Silicon-Vacancy center coupled to a diamond nanocavity, where feedback protocols are implemented to reduce the system's entropy.

We further evaluate the reducible entropy based on the causal structure of feedback, and quantitatively demonstrate the thermodynamic advantage of non-Markovian feedback over Markovian feedback. To enable such an experimental analysis, we develop the quantum thermodynamics theoretical framework, which elaborates on the concept of quantum information flow to reflect its causal structure.

Our work provides a foundation for investigating the entropic and energetic costs of real-time quantum control in various quantum systems.