  A fluctuation relation for the heat exchange of an open quantum system under
a thermalizing Markovian dynamics is derived. We show that the probability of
that the system absorbs an amount of heat from its bath, at a given time
interval, divided by the probability of the reverse process (releasing the same
amount of heat to the bath) is given by an exponential factor which depends on
the amount of heat and the difference between the temperatures of the system
and the bath. We also argue that the probability of the violation of the second
law of thermodynamics (here in the form of net heat transfer from a cold system
to its hot bath) drops exponentially with both the amount of heat and the
temperature differences.
