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MIT physicists have accomplished two long-standing goals in the manipulation of Bose-Einstein condensates, a recently discovered form of ultracold matter. They are now able to trap condensates with light, and "tune" a condensate's behavior with magnetic fields.
Both advances open new possibilities for scientific study and for the manipulation of ultracold atoms. They can also be used for further development of the atom laser, a device that is analogous to the well-known optical laser but emits atoms instead of light. A prototype of the atom laser was demonstrated by the same MIT team last year.
In the current work, the researchers first developed a new trap to confine Bose-Einstein condensates that holds the atoms by purely optical forces exerted by an optical laser beam. "This realizes 'optical tweezers' for condensates. We can now move a condensate around simply by steering a weak optical laser beam," said Professor Wolfgang Ketterle, leader of the team, who holds appointments in the Department of Physics and the Research Laboratory of Electronics (RLE).
The new trap allows the researchers to apply arbitrary magnetic fields and study the behavior of Bose-Einstein condensates. That led to the first observation of a long-predicted phenomenon associated with ultracold atoms: a Feshbach resonance. This phenomenon, essentially the temporary formation of molecules, dramatically changes the properties of a condensate and can be used to study and manipulate ultracold atoms.
Feshbach resonances have another MIT connection: they are named after Institute Professor Emeritus Herman Feshbach, who predicted this effect decades ago. Feshbach resonances have been seen in other systems, but this is the first time they have been observed in ultracold atoms. "This is a beautiful experiment and that team of people has really done a splendid job," said Professor Feshbach regarding the Ketterle work.
The all-optical trap and the observation of Feshbach resonances were reported in the March 9 issue of Physical Review Letters and the March 12 issue of Nature, respectively. Professor Ketterle's collaborators were Dan M. Stamper-Kurn, Shin Inouye, Michael R. Andrews and Ananth P. Chikkatur, all graduate students in physics, and Drs. Hans-Joachim Miesner and Joern Stenger, RLE visiting scientists.
Bose-Einstein condensates form when atoms are cooled to around one millionth of a degree Kelvin (that's more than a million times colder than interstellar space). At such low temperatures the atomic matter waves overlap, and the atoms lose their individual identities. They essentially "march in lockstep" as a single giant matter wave. Bose-Einstein condensation was first observed in 1995 by a team at the University of Colorado at Boulder and shortly afterwards by an MIT group led by Professor Ketterle. The discovery led to many efforts worldwide to study and control this new form of matter.
About a year ago the MIT team was able to extract a coherent beam of atoms from a Bose-Einstein condensate, demonstrating the first atom laser (see MIT Tech Talk, January 29, 1997). This laser could have a variety of applications in fundamental and applied research. For example, it could allow the precise manipulation of atomic matter, including extremely accurate deposition of atoms on surfaces.
A major impediment to precision measurements and manipulation, however, was the use of magnetic fields to insulate the condensate from the room-temperature walls of the apparatus it was formed in. "Such magnetic fields affect the motion of atoms and interfere with ultra-precise manipulations," Professor Ketterle said.
A NOVEL TRAP
That limitation has now been eliminated. The Ketterle team succeeded in confining a Bose-Einstein condensate with light rather than a magnetic field. Key to the new setup is an infrared laser beam that "sucks" the condensate into its focus, similar to how a toy magnet attracts a piece of iron.
"The laser field polarizes the atoms by separating the positive and negative charges a tiny bit, thus creating an electric dipole, which is trapped in the alternating electric field of the laser beam," Professor Ketterle said.
These results came as a surprise. "We expected the laser beam to heat up the ultracold atoms and destroy the condensate, but nothing happened. The condensate survived," said Mr. Stamper-Kurn. This was in contrast to earlier work on optical confinement of ultracold atoms, which showed strong heating due to unavoidable laser beam jitter, laser power fluctuations and spontaneous scattering of photons. However, the condensate was so cold that extremely small laser powers of only a few milliwatts were sufficient to trap it, minimizing the heating.
The new trap realizes "optical tweezers" for Bose-Einstein condensates, which can now be moved around simply by steering a laser beam that is comparable in power to a common laser pointer. This opens many new possibilities for future research, such as studying the interactions of condensates with surfaces or with electromagnetic radiation in cavities.
The optical trap also opened the door for experimenting with magnetic fields and their effects on condensates. "Trapping the atoms magnetically limited our exploration of magnetic fields," said Mr. Andrews. "Now we can play with magnetic fields at will." The researchers were doing just that when they saw the first evidence for a Feshbach resonance on November 19, 1997. More specifically, they exposed a Bose-Einstein condensate of sodium atoms in the new optical trap to a variable magnetic field.
When two atoms collide, they usually just "touch" each other and separate immediately. However, at a certain value of the magnetic field, the colliding atoms "stick" together, form a molecule for a while and then separate again. This effect, the Feshbach resonance, was predicted for ultracold atoms by Boudewijn Verhaar and collaborators in the Netherlands. It was named after Professor Feshbach, who discussed a similar effect in nuclear reactions in 1962.
According to Professor Ketterle, atomic physicists have eagerly searched for this effect because it profoundly changes the properties of a Bose-Einstein condensate and can be used to "design" its properties. The interactions between atoms did indeed dramatically change when the magnetic field was swept across the resonance.
"The forces between the atoms below the resonance were 10 times stronger than above," said Mr. Inouye. The MIT team recently learned that Professor Dan Heinzen and collaborators at the University of Texas at Austin have observed a Feshbach resonance in a cold thermal cloud of rubidium using spectroscopic techniques.
The researchers' conclusion? "We should be able to make the forces between atoms strong or weak, repulsive or attractive, merely by minute changes of the magnetic field," said Dr. Stenger. "This is ultimate quantum control of a macroscopic sample of atoms."
The work was supported by the Office of Naval Research, the National Science Foundation, the Army Research Office and the Packard Foundation.
A version of this article appeared in MIT Tech Talk on March 18, 1998.