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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Monday, December 10, 2007

First Observation of ‘Persistent Flow’ in a Gas

In a paper to be published in a forthcoming issue of Physical Review Letters, a team of scientists from the National Institute of Standards and Technology (NIST) and the Joint Quantum Institute (NIST/University of Maryland) have reported the first observation of “persistent” current in an ultracold atomic gas —a frictionless flow of particles.

The researchers first created a Bose-Einstein condensate (BEC), a gas of atoms cooled to such low temperatures that it transforms into matter with unusual properties. One of these properties is superfluidity, the fluid version of superconductivity (whereby electrical currents can flow essentially forever in a loop of wire). Although BECs in principle could support everlasting flows of gas, traditional setups for creating and observing BECs have not provided the most stable environments for the generally unstable superfluid flows, which have tended to break up after short periods of time.

To address this issue, the team used laser light and magnetic fields on a gas of sodium atoms to create a donut-shaped BEC—one with a hole in the center—as opposed to the usual ball- or cigar-shaped BEC. This configuration ends up stabilizing circular superfluid flows because it would take too much energy for the hole—containing no atoms—to disturb matters by moving into the donut—which contains lots of atoms.

(a) In a donut-shaped, or “toroidal” trap, atoms mostly exist in a red ring and do not reside in the center (blue region), which represents an energy hill they cannot climb. (b) Image of a Bose-Einstein condensate (BEC) in the donut trap. (c) When there is no fluid flow around the donut and the trap is turned off, atoms (red) rush to the center. (d) When fluid flows around the donut and the trap is turned off, the current around the donut persists and does not rush to fill the hole [Image courtesy: National Institute of Standard and Technology]

To stir the superfluid, the researchers zap the gas with laser light that has a property known as orbital angular momentum. Acting like a boat paddle sweeping water in a circle, the orbital angular momentum creates a fluid flow around the donut. After the stirring, the researchers have observed the gas flowing around the donut for up to 10 seconds. Even more striking, this persistent flow exists even when only 20% of the gas atoms were in the special BEC state.

This relatively long-lived flow, a hallmark of a special property known as “superfluidity,” might help bring to the surface some deep physics insights by providing ways to study the fundamental connection between BECs and superfluids.

This may also enable super-sensitive rotation sensors that could someday make navigation more precise. A BEC superfluid is very sensitive to rotation; its flow would change in fixed steps in response to small changes in rotation [Note that some research groups around the world already have taken the first step in this direction by demonstrating BECs on a chip].

Reference
"Observation of persistent flow of a Bose-Einstein condensate in a toroidal trap"
C. Ryu, M. F. Andersen, P. Cladé, Vasant Natarajan, K. Helmerson, and W. D. Phillips,
Physical Review Letters, 99, 260401 (2007)

Abstract (link updated on Dec 29, 2007 after the paper is published)

[We thank Media Relations, NIST for materials used in this posting]

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