  The characteristic function for the joint measurement of the changes of two
commuting observables upon an external forcing of a quantum system is derived.
In particular, the statistics of the internal energy, the exchanged heat and
the work of a quantum system that {\it weakly} couples to its environment is
determined in terms of the energy changes of the system and the environment due
to the action of a classical, external force on the system. If the system and
environment initially are in a canonical equilibrium, the work performed on the
system is shown to satisfy the Tasaki-Crooks theorem and the Jarzynski
equality.
