BoseEinstein condensate
From Academic Kids

A BoseEinstein condensate is a gaseous superfluid phase formed by atoms cooled to temperatures very near to absolute zero. The first such condensate was produced by Eric Cornell and Carl Wieman in 1995 at the University of Colorado at Boulder, using a gas of rubidium atoms cooled to 170 nanokelvins (nK). Under such conditions, a large fraction of the atoms collapse into the lowest quantum state, producing a superfluid.
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Contents 
Theory
The collapse of the atoms into a single quantum state is known as Bose condensation or BoseEinstein condensation. This phenomenon was predicted in the 1920s by Satyendra Nath Bose and Albert Einstein, based on Bose's work on the statistical mechanics of photons, which was then formalized and generalized by Einstein. The result of their efforts is the concept of a Bose gas, governed by the BoseEinstein statistics, which describes the statistical distribution of certain types of identical particles now known as bosons. Bosonic particles, which include the photon as well as atoms such as helium4, are allowed to share quantum states with each other. Einstein speculated that cooling bosonic atoms to a very low temperature would cause them to fall (or "condense") into the lowest accessible quantum state, resulting in a new form of matter.
The critical temperature (in a uniform threedimensional gas with no or uniform external potential) at which this happens can be derived to be:
 <math>T_c=\left(\frac{n}{\zeta(\frac{3}{2})}\right)^{2/3}\frac{h^2}{2\pi m k_B}<math>
Where:
 <math>T_c<math> = the critical temperature
 <math>n<math> = particle density
 <math>m<math> = mass per boson
 <math>h<math> = Planck's constant,
 <math>k_B<math> = Boltzmann constant
 <math>\zeta<math> = the Riemann zeta function.
Discovery
In 1938, Pyotr Kapitsa, John Allen and Don Misener discovered that helium4 became a new kind of fluid, now known as a superfluid, at temperatures below 2.2 kelvins (K). Superfluid helium has many unusual properties, including the ability to flow without dissipating energy (i.e. zero viscosity) and the existence of quantized vortices. It was quickly realized that the superfluidity was due to BoseEinstein condensation of the helium4 atoms, which are bosons. In fact, many of the properties of superfluid helium also appear in the gaseous BoseEinstein condensates created by Cornell, Wieman and Ketterle (see below). However, superfluid helium4 is not commonly referred to as a "BoseEinstein condensate" because it is a liquid rather than a gas, which means that the interactions between the atoms are relatively strong. The original BoseEinstein theory has to be heavily modified in order to describe it.
The first "true" BoseEinstein condensate was created by Cornell, Wieman, and coworkers at JILA on June 5, 1995. They did this by cooling a dilute vapor consisting of approximately 2000 rubidium87 atoms to 170 nK using a combination of laser cooling (a technique that won its inventors Steven Chu, Claude CohenTannoudji, and William D. Phillips the 1997 Nobel Prize in Physics) and magnetic evaporative cooling. About four months later, an independent effort led by Wolfgang Ketterle at MIT created a condensate made of sodium23. Ketterle's condensate had about a hundred times more atoms, allowing him to obtain several important results such as the observation of quantum mechanical interference between two different condensates. Cornell, Wieman and Ketterle won the 2001 Nobel Prize for their achievement.
The initial results by the JILA and MIT groups have led to an explosion of experimental activity. For instance, the first molecular BoseEinstein condensates were created in November 2003 by teams surrounding Rudolf Grimm at the University of Innsbruck, Deborah S. Jin at the University of Colorado at Boulder and Wolfgang Ketterle at MIT.
BoseEinstein condensates are extremely fragile, compared to other states of matter more commonly encountered. The slightest interaction with the outside world can be enough to warm them past the condensation threshold, causing them to break back down into individual atoms again; it will likely be some time before any practical applications are developed for them.
Slowing light
Despite our inability to fully understand these new states of matter, several interesting properties have already been observed in experiments. BoseEinstein condensates can be made to have an extremely high gradient in optical density. Normally, condensates do not have a particularly special refractive index, due to having an atomic density far less than normal solid materials. However, additional pump lasers can be used at frequencies designed to alter the state of atoms in the BoseEinstein condensate, increasing drastically the index for a beam of a precise target frequency recorded at a probe point. This results in extremely low measured speed of light within it; some condensates have slowed beams of light down to mere meters per second, speeds which can be exceeded by a human on a bicycle. A rotating BoseEinstein condensate could be used as a model black hole, allowing light to enter but not to escape. Condensates could also be used to "freeze" pulses of light, to be released again when the condensate breaks down. This is done by shutting off the pumping lasers with pulses still in transit and allowing the photons to be absorbed. Reapplying the pump lasers can then release the pulses of light, and due to the coherence of the BoseEinstein condensate, there may be very little degradation. Research in this field is still young and ongoing.
See also
 Electromagnetically induced transparency
 Slow glass
 Gravastar
 Superfluid
 Supersolid
 Superheavy atom
 TonksGirardeau gas
 Gas in a box
 Bose gas
External links
 BoseEinstein Condensates at JILA (http://jilawww.colorado.edu/bec/)
 Atom Optics at UQ (http://www.physics.uq.edu.au/atomoptics/)
 Europhysics News Report on Slowed Light (http://www.europhysicsnews.com/full/26/article1/article1.html)
References
 S. N. Bose, Z. Phys. 26, 178 (1924)
 A. Einstein, Sitz. Ber. Preuss. Akad. Wiss. (Berlin) 22, 261 (1924)
 L.D. Landau, J. Phys. USSR 5, 71 (1941)
 L.D. Landau, Phys. Rev. 60, 356 (1941)
 M.H. Anderson, J.R. Ensher, M.R. Matthews, C.E. Wieman, and E.A. Cornell, Science 269, 198 (1995).
 D.S. Jin, J.R. Ensher, M.R. Matthews, C.E. Wieman, and E.A. Cornell, Phys. Rev. Lett. 77, 420 (1996).
 M.R. Matthews, B.P. Anderson,P.C. Haljan, D.S. Hall, C.E.Wieman, E.A. Cornell, Phys. Rev. Lett. 83, pp. 2498 (1999)
 S. Jochim, M. Bartenstein, A. Altmeyer, G. Hendl, S. Riedl, C. Chin, J. Hecker Denschlag, and R. Grimm, Science 302, 2101 (2003)
 M. Greiner, C.A. Regal, and D.S. Jin, Nature 426, 537 (2003)
 M.W. Zwierlein, C.A. Stan, C.H. Schunck, S.M.F. Raupach, S. Gupta, Z. Hadzibabic, and W. Ketterle, Phys. Rev. Lett. 91, 250401 (2003).
 C. J. Pethick and H. Smith, "BoseEinstein Condensation in Dilute Gases", Cambridge University Press, Cambridge, 2004.da:BoseEinstein kondensat
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