Observation of the superfluid shapiro effect in a ³He weak link

We have studied the mass currents through a superfluid ³He Josephson weak link in the presence of an externally applied ac pressure modulation. Characteristic changes in the dc mass currents are observed whenever the superfluid Josephson frequency ωJ is an integer multiple of the ac modulation frequency ω. The measured dependencies of these current changes on ac pressure amplitude are in excellent agreement with theory describing quantum phase dynamics of superfluid ³He weak links. These results establish the superfluid analog of the superconducting Shapiro effect.