Phase measurements of barrier crossings in a periodically modulated double-well potential.

We report on the experimental observation of the phase angle of a particle escaping over a periodically modulated potential barrier. Optical tweezers and back-focal plane position detection were used to record particle trajectories in the entire double-well potential. These measurements provide a sensitive test of theories proposed in the past decade of escape driven by random thermal noise from a periodically modulated potential. The observed phase shifts as a function of modulation frequency are consistent with those calculated using existing theories.