  We consider a quantum system strongly driven by forces that are periodic in
time. The theorem concerns the probability $P(e)$ of observing a given energy
change $e$ after a number of cycles. If the system is thermostated by a
(quantum) thermal bath, $e$ is the total amount of energy transferred to the
bath, while for an isolated system
  $e$ is the increase in energy of the system itself. Then, we show that
$P(e)/P(-e)=e^{\beta e}$, a parameter-free, model-independent relation.
