Bose–Einstein condensate

state of matter of a dilute gas of bosons cooled to temperatures very near absolute zero

A Bose-Einstein condensate (BEC) is a state of matter (named after Satyendra Nath Bose and Albert Einstein) which can be created by cooling a gas of bosons at low densities to temperature near absolute zero (–273.15 °C). Bose-Einstein condensates (BECs) are important in the study of superfluity and superconductivity.

QuotesEdit

  • In brief, the conditions for BEC in alkali gases are reached by combining two cooling methods. Laser cooling is used to precool the gas. The principle of laser cooling is that scattered photons are on average blue-shifted with respect to the incident laser beam. As a result, the scattered light carries away more energy than has been absorbed by the atoms, resulting in net cooling. Blue-shifts are caused by Doppler shifts or ac Stark shifts. The different laser cooling schemes are described in the 1997 Nobel lectures in physics ... After the precooling, the atoms are cold enough to be confined in a magnetic trap. Wall-free confinement is necessary, otherwise the atoms would stick to the surface of the container. It is noteworthy that similar magnetic confinement is also used for plasmas which are too hot for any material container. After magnetically trapping the atoms, forced evaporative cooling is applied as the second cooling stage ... In this scheme, the trap depth is reduced, allowing the most energetic atoms to escape while the remainder rethermalize at steadily lower temperatures. Most BEC experiments reach quantum degeneracy between 500 nK and 2 μK, at densities between 1014 and 1015 cm-3. The largest condensates are of 100 million atoms for sodium, and a billion for hydrogen; the smallest are just a few hundred atoms.
  • Evidence for a BEC has long been seen in superfluids and superconductors, but the constituents of those condensates have strong interactions with one another, so the pristine nature of the BECs is hard to predict and observe. The best hope for clear manifestation of BEC behavior, it seemed, lay with a gas of weakly interacting atoms. ... Possible applications are still on the far horizon. Those that people often mention are ones that exploit the unprecedented control and manipulation of atoms that BECs offer at the quantum level. BECs offer hope of enhanced precision for atomic interferometry, rotation measurements, and atomic clocks. They might find a use in nanofabrication and atom lithography. Or they might play a role in quantum computing.
    • Barbara Goss Levi: (2001). "Cornell, Ketterle, and Wieman Share Nobel Prize for Bose–Einstein Condensates". Physics Today 54 (12): 14–16. ISSN 0031-9228. DOI:10.1063/1.1445529.
  • Most Bose-Einstein condensates of atomic gases have internal degrees of freedom originating from the spin. ... When a Bose-Einstein condensate is trapped in a magnetic potential, the spin aligns along the direction of a local magnetic field, and the internal degrees of freedom are virtually frozen. The condensate is therefore described by a scalar order parameter. When it is trapped in an optical potential, the internal degrees of freedom are liberated because the optical potential exerts the same force on an atom irrespective of which magnetic sublevel it is in. Such a condensate is called a spinor Bose-Einstein condensate; the order parameter of this condensate has 2ƒ+1 components, where ƒ is the hyperfine spin for alkali atoms and the electronic spin for chromium.

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