  The theory of quantum thermodynamics predicts fundamental bounds on work
extraction from quantum states. As these bounds are derived in a very general
and abstract setting, it is unclear how relevant they are in an experimental
context, where control is typically limited. Here we address this question by
showing that optimal work extraction is possible for a realistic engine. The
latter consists of a superconducting circuit, where a LC-resonator is coupled
to a Josephson junction. The oscillator state fuels the engine, providing
energy absorbed by Cooper pairs, thus producing work in the form of an
electrical current against an external voltage bias. We show that this machine
can extract the maximal amount of work from all Gaussian and Fock states.
Furthermore, we consider work extraction from a continuously stabilized
oscillator state. In both scenarios, coherence between energy eigenstates is
beneficial, increasing the power output of the machine. This is possible
because the phase difference across the Josephson junction provides a phase
reference.
