  Maxwell's demon explores the role of information in physical processes.
Employing information about microscopic degrees of freedom, this "intelligent
observer" is capable of compensating entropy production (or extracting work),
apparently challenging the second law of thermodynamics. In a modern
standpoint, it is regarded as a feedback control mechanism and the limits of
thermodynamics are recast incorporating information-to-energy conversion. We
derive a trade-off relation between information-theoretic quantities empowering
the design of an efficient Maxwell's demon in a quantum system. The demon is
experimentally implemented as a spin-1/2 quantum memory that acquires
information, and employs it to control the dynamics of another spin-1/2 system,
through a natural interaction. Noise and imperfections in this protocol are
investigated by the assessment of its effectiveness. This realization provides
experimental evidence that the irreversibility on a non-equilibrium dynamics
can be mitigated by assessing microscopic information and applying a
feed-forward strategy at the quantum scale.
