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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" )

Tuesday, September 06, 2005

Bose-Einstein condensate

About 80 years ago, based on previous work by the Indian physicist
Satyendra Nath Bose, Einstein proposed that if a gas of neutral atoms is
cooled to a low enough temperature, all atoms of the gas would fall into the
same quantum state. In other words, all of the million or billion atoms in
the gas would end up in the same place at the same time, a weird quantum
state dubbed a Bose-Einstein condensate.

The supercold atoms are created from a hot gas of neutral atoms that is
laser cooled, collected in a magneto-optic trap, cooled further by evaporation,
and then spun off into a magnetic trap for a few seconds of study before it
warms up and dissipates.

A team of physicists at UC Berkeley has created a Bose-Einstein condensate
of rubidium atoms and nudged it into a circular racetrack 2 millimeters
across, creating a particle storage ring analogous to the accelerator storage
rings of high energy physics. This ring, the first to contain a Bose-Einstein gas,
is full of cold particles at a temperature of only one-millionth of a degree
above absolute zero (which is -273 degree centigrade), traveling with
energies a billion trillion times less than the particles in a high-energy storage
ring [The atoms circled the racetrack at a speed of about 50 to 150 millimeters
per second, which is equal to an energy of about one nano-electron volt (eV)
per atom, or one billionth of an electron volt. High-energy particle accelerators
routinely bump particles to energies of a few tera-electron volts, or a trillion
eV - a billion trillion times more energetic than the cold rubidium atoms].

Though such slow-moving rubidium atoms would be useless for producing
the exotic collision particles that are the bread and butter of high-energy
accelerators, cold collisions of such atoms might reveal new quantum physics,
said Dan Stamper-Kurn, assistant professor of physics at UC Berkeley and
leader of the study. Their paper was accepted for publication by Physical
Review Letters last week.

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