Super Atoms from Bose-Einstein Condensation

by Imogen Colton and Jeanette Fyffe

IN 1995, a research group at JILA, Colorado, produced a new form of matter by super-cooling 87Rb atoms to near absolute zero. This was the first observation of a Bose-Einstein condensate, a phenomenon predicted by Bose and Einstein more than seventy years ago.

They predicted that for an ideal (non-interacting) gas of bosons at very low temperatures, all the particles would go into the lowest possible energy state. Since the atoms are all in the same state, they have the same momentum and therefore the same de Broglie wavelength. As the temperature decreases, so does the momentum. Conversely, the wavelength increases until it becomes so large that the particle wavefunctions overlap. When this happens, the atoms become indistinguishable from each other and form a giant "super atom" called a Bose-Einstein condensate (BEC).

Before the JILA experiment, BEC was known to occur in strongly interacting systems. It is used to explain the superfluid phase of liquid 4He, and the superconducting properties of some materials. However, interactions between particles in a liquid or solid change the theoretical description considerably. Therefore the observation of BEC in a weakly interacting gas (which more closely resembles an ideal gas) was a remarkable breakthrough.

To make a BEC, the atoms are trapped in a magneto-optical trap which is made using a magnetic field and laser beams at room temperature. They are then laser-cooled to around 20 micro-K (just above absolute zero temperature) and then evaporatively cooled to below the temperature where BEC occurs. For 87Rb, this is at 170 nano-K. A typical condensate density is around 1014 atoms per cubic centimetre, i.e., about 10-5 times the density of air. The condensate is detected by a sharp peak in the velocity distribution near the zero point. Since BEC was seen in 87Rb, it has also been observed in 23Na, 7Li and most recently in 1H.

The condensate is unlike any other form of matter. It behaves as a single quantum object despite its size (up to 1 mm across). Because of this behaviour, it is hoped that fundamental tests of Quantum Mechanics can be made. An example of this Quantum Mechanical behaviour is the interference between two condensates. Just as water ripples or light waves interfere (as in Young's double slit experiment), two separate condensates can be released from their traps and interfere as they overlap. This was first observed in 1996 at MIT, demonstrating the wave-like behaviour of the condensate. Another important area is using BEC to make an atom laser. This is a coherent atom beam, which has many potential uses including building higher precision atomic clocks and making computer chips.

Following the observation of BEC, there has been a vast amount of theoretical work. In particular, we look at possible models for the interactions between atoms in and the dynamics of a condensate. By using various computational techniques such as Monte Carlo methods, and numerical solutions to the condensate's Schroedinger equation, we hope to improve our understanding of BEC and the underlying Quantum Mechanics.

You may find more information on BEC at the BEC Homepage.

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Created: December 24, 1998
Last Modified: February 3, 1999
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