Observation of the Josephson plasma mode for a superfluid weak link

The quantum phase difference across a superfluid Josephson-like weak link may exhibit periodic motion describable by a nonlinear equation identical to that of a rigid pendulum. We have directly observed this mode and find that the phase oscillation frequency decreases toward zero as the phase amplitude increases toward π. We also find that the low amplitude frequency is a direct measure of the weak link’s critical current in quantitative agreement with theory.