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arXiv:0704.2082 · Physical Review Letters

Direct Measurement of Quantum Phase Gradients in Superfluid He4 Flow

Yuki Sato, Aditya Joshi, R. E. Packard

Published May 9, 2007• 14 citations
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Abstract

We report a new kind of experiment in which we generate a known superfluid velocity in a straight tube and directly determine the phase difference across the tube's ends using a superfluid matter wave interferometer. By so doing, we quantitatively verify the relation between the superfluid velocity and the phase gradient of the condensate macroscopic wave function. Within the systematic error of the measurement (approximately 10%) we find nu(s)=Planck's/m4InvertedDeltaphi.

DOI
10.1103/physrevlett.98.195302

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We report a new kind of experiment in which we generate a known superfluid velocity in a straight tube and directly determine the phase difference across the tube's ends using a superfluid matter wave interferometer. By so doing, we quantitatively verify the relation between the superfluid velocity and the phase gradient of the condensate macroscopic wave function. Within the systematic error of the measurement (approximately 10%) we find nu(s)=Planck's/m4InvertedDeltaphi.
We report the first observation of quantum interference from a grating structure consisting of four weak link junctions in superfluid 4He. We find that an interference grating can be implemented successfully in a superfluid matter wave interferometer to enhance its sensitivity while trading away some of its dynamic range. We also show that this type of device can be used to measure absolute quantum mechanical phase differences. The results demonstrate the robust nature of superfluid phase coherence arising from quantum mechanics on a macroscopic scale.
Actually, the fundamental feature of Bose condensation is the macroscopic occupation of a single quantum state, whatever that state may be. As long as such a macroscopically-occupied state exists, the system will exhibit superfluid behavior. Superfluid flow thus appears as a "collective" phenomenon, in which the particles move together in order to preserve the macroscopic occupation of a single state. The equilibrium is metastable, because obviously there exists a state of lower energy, that of the fluid at rest; the essential point is that no transitions to the latter state will take place. Metastable superfluid flow is only possible because it involves condensate motion. The superfluid velocity is thus proportional to the gradient of the phase of the condensate wave-function. The superfluid motion is thus a uniform translation, in contrast with ordinary viscous flow, for which the fluid velocity varies from the walls to the center of the tube.
We report an experiment that uses a superfluid helium quantum interference device to probe the initial onset of the motion of a single vortex line driven by axial flow in a macroscopic channel. When the superfluid velocity reaches a temperature independent critical value $({v}_{c}\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{mm}∕\mathrm{s})$ periodic $2\ensuremath{\pi}$ phase slippage occurs with a frequency of the order of a few Hz. As the axial flow velocity increases, the frequency increases, possibly stepwise.