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This Month in Physics History

Making Superatoms
June 5, 1995: First Bose Einstein Condensate

The density of the atomic cloud
The density of the atomic cloud is shown, with temperature decreasing from left to right. The high peak, the Bose-Einstein condensate, emerges above the other atoms. The picture is from the JILA laboratory.

The BEC phenomenon was first predicted by Satyendra Bose and Albert Einstein: when a given number of identical Bose particles approach each other sufficiently closely, and move sufficiently slowly, they will collectively convert to the lowest energy state: a BEC. This occurs when atoms are chilled to very low temperatures. The wavelike nature of atoms allows them to spread out and even overlap. If the density is high enough, and the temperature low enough (mere billionths of degrees above absolute zero), the atoms will behave like the photons in a laser: they will be in a coherent state and constitute a single "super atom."

JILA's Carl Wieman (University of Colorado, Boulder) and Eric Cornell (NIST) first started searching for a BEC around 1990 with a combination laser and magnetic cooling apparatus. Wieman pioneered the use of $200 diode lasers (the same type used in CD players) instead of the $150,000 lasers other groups were using. His approach was initially met with skepticism by his colleagues, but when he began to report real progress, several other groups joined the race to achieve the first BEC. Beginning with rubidium gas atoms at room temperature, the JILA team first slowed the rubidium and captured it in a trap created by laser light. This cooled the atoms to about 10 millionths of a degree above absolute zero—still far too hot to produce a BEC.

Once trapped, the lasers are turned off and the atoms are held in place by a magnetic field. The atoms are further cooled in the magnetic trap by selecting the hottest atoms and kicking them out of the trap. Then came the tricky part: trapping a sufficiently high density of atoms at temperatures that were cold enough to produce a BEC. To do this, Wieman and his colleagues had to devise a time- averaged orbiting potential trap (an improvement to the standard magnetic trap).

The world's first BEC was achieved at 10:54 AM on June 5, 1995 in a laboratory at JILA, a joint institute of University of Colorado, Boulder, and NIST. The BEC was formed inside a carrot sized glass cell, and made visible by a video camera; it measured only about 20 microns in diameter, or about one fifth the thickness of a sheet of paper. The result was a BEC of about 2,000 rubidium atoms that lasted for 15-20 seconds. Shortly thereafter, Wolfgang Ketterle also achieved a BEC in his laboratory at MIT.

Today, scientists can produce condensates of much greater numbers of atoms that can last as long as three full minutes, and they continue to glean intriguing new insights into this unusual form of matter. By September 2001, over three dozen other laboratories had replicated the discovery. In 1997, MIT researchers developed an atom laser based on BECs that was able to drip single atoms downward from a micro spout, and in February 1999, a team at Harvard University used a BEC to slow down light to just 38 MPH by shining a laser beam through the condensate. Two years later the team announced that it had briefly brought light to a complete stop.

In March 1999, scientists at the NIST facility in Gaithersburg, MD, nudged super cold atoms into a beam to create a device that shoots out streams of atoms in any direction.

The breakthrough could lead to a new technique for making very small computer chips, or to construct nanode-vices one atom at a time.

On June 18, 1999, JILA researchers used the technique to achieve the first Fermi degenerate gas of atoms. A group of German researchers demonstrated in 2001 that BECs can be created and manipulated using so-called atom chips, an achievement that could form the basis of integrated "atom circuits" based on the motion of atoms rather than electrons.

And in December 2002, physicists in Innsbruck created the first BEC out of cesium atoms, which are the basis of atomic clocks and also play a key role in certain metrological applications, including measurements of the electric dipole moment of the electron.

The Colorado group is now experimenting with this new form of matter by manipulating it in new and different ways. In July 2001, he and his colleagues were able to make a BEC shrink, which was followed by a tiny explosion similar in some ways to a microscopic supernova. So they dubbed it a "Bosenova."

About half of the original atoms appear to vanish in the process. They cooled the matter to 3 billionths of a degree above absolute zero—the lowest temperature ever achieved to date.

Cornell, Ketterle and Wieman shared the 2001 Nobel Prize in physics for their accomplishment. Their joint discovery of the BEC is "going to bring revolutionary applications in such fields as precision measurement and nanotechnology," the citation from the Royal Swedish Academy of Sciences said. The apparatus used by the JILA team is now part of the permanent collection of the Smithsonian Institute in Washington, DC.

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Editor: Alan Chodos
Associate Editor: Jennifer Ouellette