Dissipative flow of superfluid⁴He through a small orifice by quantum and thermal nucleation of vortices

Flow measurements in ultra-pure⁴He through a micron-size orifice at millikelvin temperatures show the transition from thermal to quantum nucleation of nanometer-size vortices below a crossover temperature of 0.147 K. Detailed analysis, similar to that done for macroscopic quantum tunnelling in superconducting junctions, yields parameters which accurately predict the mean value and the distribution width of the critical velocities at all temperatures. These observations establish the close relationship between this type of critical flow and negative ion motion in superfluid⁴He and strongly suggest that the underlying mechanism is identical.