“Superfluid helium is a Bose condensed “entangled” state of matter in which the individual atoms relinquish their identity to share in a macroscopic wavefunction whose quantum phase depends on external macroscopic fields. The key element required to measure the phase information is an array of nanometer sized apertures that exhibits properties described by Josephson’s equations. We have studied these so-called superfluid weak links in both superfluid 3He and 4He. When we try to force the superfluid through the apertures the fluid oscillates at a quantum frequency rather than flowing down the pressure gradient. This is the characteristic signature of a quantum two level system. By exploiting the weak link properties we have developed Superfluid Helium Quantum Interference Devices (SHeQUIDs) which are analogs of superconducting SQUIDs. With high resolution these new devices reveal the rotation state of the walls confining the liquid and may find uses in geodesy and inertial navigation. Other SHeQUIDs may detect Aharonov-Bohm phase shifts in neutral nonpolar matter and may set upper limits to gravito-magnetic fields present near rotating masses.”
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