We report on the experimental realization of an autonomous Maxwell’s demon in a cavity QED system composed of a high-quality microwave resonator and circular Rydberg atoms [1]. The modelled system is a qubit Q exchanging heat with a cavity C under the control of a demon D. Both Q and D are coded with a single three-level atom. The demon feedback is implemented by an auxiliary microwave source transferring the atomic population into the dark state, thus preventing the following QC interaction. The experimental ability to access independently the states of all system components allows us to fully characterize the energy, entropy and information flow in the system. While the classical expression for the Second Law seems to be violated, its generalized expression which includes the information acquired by D on the QD sub-system is respected.
We propose also several different ways to access the entropy production in a QDC protocol. By recording and counting the evolution of individual trajectories, we independently measure EP with trajectory probabilities, with heat and information transfer, with relative entropy and mutual information between the system’s components, etc [2]. We present and discuss the obtained experimental and theoretical results.

References
[1] B.-L. Najera-Santos et al., Phys. Rev. Research 2, 032025(R) (2020)
[2] P. A. Camati et al., in preparation