  The principle of microscopic reversibility is a fundamental element in the
formulation of fluctuation relations and the Onsager reciprocal relations. As
such, a clear description of whether and how this principle is adapted to the
quantum mechanical scenario might be essential to a better understanding of
nonequilibrium quantum processes. Here, we propose a quantum generalization of
this principle, which highlights the role played by coherence in the symmetry
relations involving the probability of observing a quantum transition and that
of the corresponding time reversed process. We study the implications of our
findings in the framework of a qubit system interacting with a thermal
reservoir, and implement an optical experiment that simulates the dynamics. Our
theoretical and experimental results show that the influence of coherence is
more decisive at low temperatures and that the maximum departure from the
classical case does not take place for maximally coherent states. Classical
predictions are recovered in the appropriate limits.
