  Engineered dynamical maps combining coherent and dissipative transformations
of quantum states with quantum measurements, have demonstrated a number of
technological applications, and promise to be a crucial tool in quantum
thermodynamic processes. Here, we exploit the control on the effective open
spin qutrit dynamics of an NV center, to experimentally realize an autonomous
feedback process (Maxwell demon) with tunable dissipative strength. The
feedback is enabled by random measurement events that condition the subsequent
dissipative evolution of the qutrit. The efficacy of the autonomous Maxwell
demon is quantified by experimentally characterizing the fluctuations of the
energy exchanged by the system with the environment by means of a generalized
Sagawa-Ueda-Tasaki relation for dissipative dynamics. This opens the way to the
implementation of a new class of Maxwell demons, which could be useful for
quantum sensing and quantum thermodynamic devices.
