  With the progress of nano-technology, thermodynamics also has to be scaled
down, calling for specific protocols to extract and measure work. Usually, such
protocols involve the action of an external, classical field (the battery) of
infinite energy, that controls the energy levels of a small quantum system (the
calorific fluid). Here we suggest a realistic device to reversibly extract work
in a battery of finite energy : a hybrid optomechanical system. Such devices
consist in an optically active two-level quantum system interacting strongly
with a nano-mechanical oscillator that provides and stores mechanical work,
playing the role of the battery. We identify protocols where the battery
exchanges large, measurable amounts of work with the quantum emitter without
getting entangled with it. When the quantum emitter is coupled to a thermal
bath, we show that thermodynamic reversibility is attainable with
state-of-the-art devices, paving the road towards the realization of a full
cycle of information-to-energy conversion at the single bit level.
