Superfluid ⁴He interferometer operating near 2 K

We report the observation of quantum interference in superfluid ⁴He. The interferometer, an analog of a dc-superconducting quantum interference device (SQUID), employs a recently reported phenomenon wherein superfluid ⁴He exhibits Josephson frequency oscillations in an array of submicron apertures. An interference pattern is generated by reorienting the loop of the superfluid “SQUID” with respect to the Earth’s rotation vector, thereby varying the rotation flux in the loop. The experiment is performed at 2 K, a temperature 2000 times higher than previously achieved with superfluid ³He. We find that the interference exists not only when the aperture array current-phase relation is a sinusoidal function characteristic of the Josephson effect, but also at lower temperatures where it is linear and oscillations occur by phase slips. The modest requirements for the interferometer (2 K cryogenics and fabrication of apertures at the level of 100 nm) and its potential resolution suggest that, when engineering challenges such as vibration isolation are met, superfluid ⁴He interferometers could become important scientific probes.