Two-path quantum interference in a liquid

Interference experiments have unequivocally shown the quantum wave nature of photons, electrons, neutrons, atoms, molecules, and even Bose–Einstein condensates. Now, a team of physicists at the University of California, Berkeley, has demonstrated quantum interference for superfluid helium-3. They built an interferometer analogous to a dc SQUID (superconducting quantum interference device) consisting of a loop of hollow tube filled with superfluid ³He and interrupted by two superfluid Josephson “weak links.” Each weak link consists of an array of thousands of submicron apertures in a 50-nm thick silicon nitride window on a Si chip. The group used a membrane to monitor both the pressure head across the interferometer and the mass current through it. Theory predicts—again similar to SQUIDS—that the total mass current depends on the superfluid phase winding. To adjust the quantum phase in the uncharged superfluid ³He, the team varied the loop’s orientation with respect to Earth’s rotation vector. The physicists found precisely the quantum interference pattern they expected for the current. Because the interferometer responds to rotation rather than magnetic field, potential applications include a superfluid quantum interference gyroscope (R. W. Simmonds et al., Nature 412 , 55, 2001 http://dx.doi.org/10.1038/35083518 .)