Observation of the Josephson plasma mode for a superfluid ³He 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 $\pi$. We also find that the low amplitude frequency is a direct measure of the weak link's critical current in quantitative agreement with theory.