  The problem of extracting the work from a quantum-thermodynamic system driven
by slowly varying external parameters is discussed. It is shown that there
naturally emerges a gauge-theoretic structure. The field strength identically
vanishes if the system is in an equilibrium state, i.e., the nonvanishing field
strength implies that the system is in a nonequilibrium quasi-stationary state.
The work done through a cyclic process in the parameter space is given in terms
of the flux of the field. This general formalism is applied to an example of a
single spin in a varying magnetic field, and the maximum power output is
discussed in a given finite-time cyclic process.
