  A quantum flywheel is studied with the purpose of storing useful work in
quantum levels, while additional power is extracted continuously from the
device. The flywheel gains its energy form a quantum heat engine. Generally,
when a work repository is quantized the work exchange with the engine is
accompanied with heat exchange, which may degrade the charging efficiency. In
the particular realization of a quantum harmonic oscillator work repository,
quantum and thermal fluctuations dominates the dynamics. Quantum monitoring and
feedback control are applied to the flywheel, as it is shown to be an essential
part of stabilizing and regulating its state of operation, and bringing the
system to a steady state. A particular balance between information gained by
measuring the system and the information fed back to the system is found to
maximize the charging efficiency. The dynamics of the flywheel are described by
a stochastic master equation that accounts for the engine, the external
driving, the measurement, and the feedback operations.
