Friday, February 20, 2009

What is a BOSE Einstein Condensate ?

A Bose Einstein Condensate (BEC) is a special phase of matter when a dilute bosonic gas is cooled to a temperature below the critical temperature (Tc). The critical temperature is in the range of some micro Kelvins to nano Kelvins. The Bose Einstein Condensation was predicted by Albert Einstein in 1924 by applying the Bose Statistics to massive particles - atoms. Satyendra Nath Bose , an Indian physicist, developed a statistics for the photons (light particles), which are massless.

Because of the lack of cooling technology, the world had to wait for the next 70 years to realize a BEC in a laboratory. In 1995, a BEC was realized in JILA by cooling down a dilute atomic gas of Rubidium-87 to a temperature of about 170 nano Kelvins. In the same year, BECs were observed in MIT by cooling down a dilute atomic gas of Sodium-23 and in Rice University by cooling down a dilute atomic gas of Lithium-7. Three physicists- Carl Wieman, Eric Cornell and Wolfgang Ketterle were awarded the 2001 Nobel Prize in Physics for this great achievement.

A BEC is made by cooling a dilute gas in the following sequence. A dilute atomic gas obtained from oven is cooled down by Helium to its temperature which is about 4 Kelvin. Then it is loaded in a trap called a Magneto-Optical-Trap (MOT), where the atoms are trapped and cooled by six polarized lasers directed orthogonally towards the center of the trap. Lasers cool the atoms to a lower temperature but not to the critical temperature. Beyond that the trapped gas is cooled by evaporative cooling, where the cooling is done by varying the frequency of the radio-frequency waves.

A nice analogy to evaporative cooling is the cooling of the coffee in a coffee mug. The most energetic coffee molecules close to the surface of the coffee in a coffee mug escape from the surface taking some energy and so the rest of the coffee in the mug will have less energy. As a result of which the coffee in the mug cools down. Similar is the case of evaporative cooling where the most energetic gas atoms from the trap escape taking the energy away, cooling down the remaining atoms. Finally, when the critical temperature is reached, all the atoms in the trap come to occupy the ground state, forming a blob of matter. This is neither solid, liquid, nor a gas. It is a condensate and is called a Bose-Einstein Condensate (BEC).

A BEC is a very nice mesoscopic quantum system and so is a nice candidate to test the laws of quantum mechanics and perform atom interferometric experiments.

Daniel Kleppner, an MIT professor, gives a really illuminating picture of the Bose-Einstein condensate in the following YOUTUBE video.

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