  Simple feedback loops, inspired from extremum-seeking, are proposed to lock a
probe-frequency to the transition frequency of a single quantum system
following quantum Monte-Carlo trajectories. Two specific quantum systems are
addressed, a 2-level one and a 3-level one that appears in coherence population
trapping and optical pumping. For both systems, the feedback algorithm is shown
to be convergent in the following sense: the probe frequency converges in
average towards the system-transition one and its standard deviation can be
made arbitrarily small. Closed-loop simulations illustrate robustness versus
jump-detection efficiency and modeling errors.
