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Physics World

Superfluid quantum 'tunnelling'

Eric Varoquaux

Published Nov 1, 1988• 3 citations
Publisher

Abstract

Striking observations of quantised events in superfluid flow through a microscopic orifice have been made recently at Orsay, France. Sudden, stepwise reductions of the flow velocity at the critical velocity threshold, seen both in superfluid 3 He and 4 He, are interpreted as slips by 2π of the phase of the quantum mechanical wavefunction that describes the condensate of the superfluid. Moreover, in superfluid 3 He slightly below the superfluid transition, at low pressure and at a temperature of 0.9 mK, the mass flow through the orifice becomes fully dominated by quantum mechanics.

DOI
10.1088/2058-7058/1/11/21

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Striking observations of quantised events in superfluid flow through a microscopic orifice have been made recently at Orsay, France. Sudden, stepwise reductions of the flow velocity at the critical velocity threshold, seen both in superfluid 3 He and 4 He, are interpreted as slips by 2π of the phase of the quantum mechanical wavefunction that describes the condensate of the superfluid. Moreover, in superfluid 3 He slightly below the superfluid transition, at low pressure and at a temperature of 0.9 mK, the mass flow through the orifice becomes fully dominated by quantum mechanics.
We report on numerical solutions of the Gross-Pitaevskii equation for two-dimensional flow of a superfluid condensate through a small orifice. Above a critical velocity of about 30%30\%30% of the speed of sound, cavitation occurs in the throat of the orifice. The cavitating bubbles form the cores of singly quantized vortices which detach from the boundary and are convected downstream.
Superfluidity and superconductivity are striking examples of how quantum mechanics can affect the collective behaviour of large numbers of identical particles. When the temperature of a many-particle fluid is lowered below a critical threshold, thermal fluctuations are no longer able to prevent it from collapsing into a purely quantum state. If the particles are electrically neutral, like helium–3 and helium–4 atoms, such a 'coherent' state can flow without friction and is known as a superfluid. If, on the other hand, the particles are charged, the coherent state loses its resistance to electrical current and becomes a superconductor.
We describe the operation of a hydromechanical resonator immersed in 4He^{4}\mathrm{He}4He and 3B^{3}\mathit{B}3B superfluids and opened on the main bath by two holes, a micro-orifice acting as a weak link and a long parallel channel. Transitions between adjacent quantum states differing by one hydrodynamic circulation quantum can be induced coherently, yielding a succession of steps arranged in a staircase pattern. In 3He^{3}\mathrm{He}3He at about 0.8 mK and 0 bar, a nearly ideal, nondissipative hydrodynamic Josephson effect can be observed.
We have produced and observed quantized vortex rings emitted from orifices when superfluid helium is forced through channels at critical velocity. The vortices are identified by measuring their velocity and size, and are found to be in the n=1n=1n=1 quantum state.
Quantum fluids are able to flow past obstacles without creating drag and through channels without any resistance. This superfluidity is a consequence of Bose–Einstein condensation, a process in which all the particles gather in the same, lowest-energy quantum state. At low flow velocities, energy exchanges between the obstacle and the fluid are forbidden because of the twin requirements of energy and momentum conservation. As the flow speed is increased, however, the superfluidity breaks down at some critical velocity above which excitations in the fluid can occur. These excitations can be phonons (i.e. quantized vibrations), vortices or, in the case of superfluid liquid helium, rotons (see Rotons: a low-temperature puzzle by Russell Donnelly Physics World February 1997 p25)