  Motivated by the recent interest in thermodynamics of micro- and mesoscopic
quantum systems we study the maximal amount of work that can be reversibly
extracted from a quantum system used to store temporarily energy. Guided by the
notion of passivity of a quantum state we show that entangling unitary controls
extract in general more work than independent ones. In the limit of large
number of copies one can reach the thermodynamical bound given by the
variational principle for free energy.
