Phase control of a path-entangled photon state by a deformable membrane mirror

We demonstrate the potentialities of a deformable mirror for closed-loop control of a two-photon path-entangled state subject to phase fluctuations. A custom-made membrane mirror is used to set a relative phase shift between the arms of an interferometric apparatus. The control algorithm estimates the phase of the quantum state by measurements of the coincidence events at the output ports of the interferometer and uses the measurement results to provide a feedback signal to the deformable mirror. Stabilization of the coincidence rate to within 1.5 standard deviation of the Poissonian noise is demonstrated over 2000 s.