  Matter-wave interferometers reveal some of the most fascinating phenomena of
the quantum world. Phase shifts due to rotation (the Sagnac effect) for
neutrons, free atoms and superfluid 3He reveal the connection of matter waves
to a non-rotating inertial frame. In addition, phase shifts in electron waves
due to magnetic vector potentials (the Aharonov-Bohm effect) show that physical
states can be modified in the absence of classical forces. We report here the
observation of interference induced by the Earth's rotation in superfluid 4He
at 2 K, a temperature 2000 times higher than previously achieved with 3He. This
interferometer, an analog of a dc-SQUID, employs a recently reported phenomenon
wherein superfluid 4He exhibits quantum oscillations in an array of sub-micron
apertures. We find that the interference pattern persists 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
100nm) and its potential resolution suggest that, when engineering challenges
such as vibration isolation are met, superfluid 4He interferometers could
become important scientific probes.
