A New Trap for Atoms
One millionth of a degree above absolute zero on the temperature scale, everything becomes different for atoms. So far, physicists have only been able to cool matter to this extent using strong magnetic fields. Now this is also possible with laser light - a prerequisite for new experiments that give us an insight into the world of ultra-cold matter. When atoms are cooled to about one-millionth Kelvin, a Bose–Einstein condensate forms. In this state, the wave functions of the atoms overlap, causing them to lose their individuality. The atoms behave like a single gigantic wave of matter, they "march in step, so to speak." In 1995, a Bose-Einstein condensate was formed at the University of Colorado at Boulder for the first time, followed by worldwide efforts to study and control this exotic new state of matter.
About a year ago, scientists at the Massachusetts Institute of Technology (MIT) succeeded in generating a coherent atomic beam from a Bose-Einstein condensate. There is a great need for such atomic lasers in basic and applied research. With them, for example, individual atoms could be precisely influenced and positioned on surfaces.
A major obstacle to accurate measurements and manipulation of the condensate was the magnetic fields used to isolate the matter from the walls of its container. "Such magnetic fields influence the movements of the atoms and interfere with ultra-precise manipulation attempts," explains Wolfgang Ketterle from MIT.
But this hurdle has now been overcome. Ketterle's team has succeeded in making a Bose-Einstein condensate using light instead of magnetic fields (Physical Review Letters of March 9, 1998). The key to success was an infrared laser that "sucked" the condensate into its focal point. "The [electromagnetic] field from the laser polarizes the atoms, separating their positive and negative charges a tiny bit, creating an electric dipole that gets trapped in the changing electric field of the laser beam," says Ketterle.
The event came as a surprise to the researchers themselves. "We expected that the laser beam would heat up the ultracold atoms and destroy the condensate state - but nothing happened. The condensate survived the procedure," says physicist Stamper-Kurn. This contrasted with previous work on optical trapping of ultracold atoms. At that time, the sample was strongly heated by unavoidable fluctuations in the laser beam, fluctuations in light intensity, and spontaneous scattering of photons. This time, however, the condensate was so cold that the tiny laser intensity of a few milliwatts was enough to trap it and heat it only minimally.
This new "nuclear trap" realizes the idea of optical tweezers for the Bose-Einstein condensate, with which the sample can be moved back and forth at will. And it allows the use of magnetic fields to study, not capture, the condensate.
With the new experimental freedom, in November 1997, MIT scientists achieved the first detection of the Feshbach resonance (Nature 12 March 1998).
When two atoms collide, they usually "touch" briefly and then immediately separate. At a certain magnetic field strength, however, the atoms "stick" to each other - they temporarily form a molecule. This effect was similarly discussed for nuclear reactions by Professor Feshbach in 1962, and a Dutch group predicted it for ultracold atoms.
According to Ketterle, atomic physicists have tried hard to prove this effect, since it drastically changes the properties of a Bose-Einstein condensate. Thus, the forces between the atoms increased tenfold when the resonance conditions were met. This gives the physicists courage for new experiments: "We should be able to make the forces between the atoms strong or weak, attractive or repulsive, simply by making tiny changes in the magnetic field. This is absolute quantum control of a macroscopic sample of atoms."
