  Dephasing is a main noise mechanism that afflicts quantum information, it
reduces visibility, and destroys coherence and entanglement. Therefore, it must
be reduced, mitigated, and if possible corrected, to allow for the
demonstration of quantum advantage in any application of quantum technology,
from computing to sensing and communications. Here we discuss a hardware scheme
of error filtration to mitigate the effects of dephasing in optical quantum
metrology. The scheme uses only passive linear optics and ancillary vacuum
modes, and we do not need single-photon sources or entanglement. It exploits
constructive and destructive interference to partially cancel the detrimental
effects of statistically independent sources of dephasing. We apply this scheme
to preserve coherent states and to phase stabilize stellar interferometry, and
show that a significant improvement can be obtained by using only a few
ancillary modes.
