SuperfluidHe4interferometer operating near2K

We report the observation of quantum interference in superfluid $^{4}\mathrm{He}$. The interferometer, an analog of a dc-superconducting quantum interference device (SQUID), employs a recently reported phenomenon wherein superfluid $^{4}\mathrm{He}$ 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 $2\phantom{\rule{0.2em}{0ex}}\mathrm{K}$, a temperature 2000 times higher than previously achieved with superfluid $^{3}\mathrm{He}$. 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 ($2\phantom{\rule{0.2em}{0ex}}\mathrm{K}$ cryogenics and fabrication of apertures at the level of $100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) and its potential resolution suggest that, when engineering challenges such as vibration isolation are met, superfluid $^{4}\mathrm{He}$ interferometers could become important scientific probes.