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Important steps on the way to an atom laser

Could a beam of atoms as narrow and intense as a laser beam be created? A group at MIT has now achieved some of the elements required for such a device. In July 1995, a group at the Joint Institute for Laboratory Astrophysics of the University of Colorado demonstrated the phenomenon of Bose-Einstein condensation in an atomic vapor [Science, 269, 198, (1995)]. In Bose-Einstein condensation, ultracold atoms all become part of a single macroscopic quantum state. In a research article in the 31 January 1997 issue of Science Andrews et al. report that such a condensate exhibits quantum coherence, an important demonstration of the "laser-like" properties of the Bose-Einstein state. As shown on the cover of the issue, two separate condensates were allowed to interact, yielding striking interference fringes just like those seen from coherent light beams. In the same issue, Taubes [see below] provides the background to this achievement. His news story also describes a means of turning a condensate into a pulsed beam by coupling the atoms out of the atomic container--work reported by the MIT group in Physcal Review Letters.

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First Atom Laser Fires Pulses of Coherent Matter

by Gary Taubes

To the uninitiated, a laser is a pin-thin beam of brightly colored light that you'd be wise not to shine in your eyes. To connoisseurs, it is a coherent beam of photons locked in identical quantum states, meaning they all have exactly the same wavelength and travel precisely in step, crest to crest, trough to trough. Now the word laser has taken on yet another meaning: a beam of atoms marching in quantum lockstep, like the photons of a light laser.

Two years ago, physicists achieved the crucial starting point for an atom laser, which could aid everything from atomic clocks to chipmaking, when they created an exotic state of matter known as a Bose-Einstein condensate. Now, a group at the Massachusetts Institute of Technology (MIT) led by Wolfgang Ketterle reports on page 637 that they have shaped this novel material into pulses of atoms that have the hallmarks of a laser beam. "The experiments are gorgeous," says Oxford University physicist Keith Burnett, and the demonstration that this is really a laser is "the most beautiful clear evidence."

A Bose-Einstein condensate is a dense cloud of atoms cooled in a magnetic trap to within an iota of absolute zero, where their quantum-mechanical waves merge. The formerly disparate atoms take on the characteristics of a single particle, in which the microscopic laws of quantum physics are writ large. Simply making a condensate is "bloody difficult," says Burnett, let alone turning it into a laser. Since Eric Cornell and his colleagues at the National Institute of Standards and Technology in Boulder, Colorado, made the first one in 1995 (Science, 14 July 1995, p. 198), only Ketterle's group and a team at Rice University led by Randy Hulet have been able to follow suit. Now the MIT group has found a way to extract pulses of atoms from a condensate and has shown, by allowing two pulses to interfere with each other, that each constitutes the single coherent wave required of a laser.

The first step, creating what laser physicists call an output coupler to extract the atoms from the trap, was relatively easy--"peanuts," says Ketterle. In a conventional laser, the output coupler simply taps light from the lasing cavity, where it is bouncing back and forth between mirrors. "Laser light is like a big wave," Ketterle explains, sloshing back and forth between the mirrors. "You want to take a little bit out for the beam, and then the big wave is amplified again and regenerated." An ordinary laser relies on leaky mirrors to allow perhaps 10% of the light to escape and form a beam.

For the atom laser, says Ketterle, the MIT team opened a leak in the trap confining their sodium atoms. The trap, he says, "can be described loosely as like atoms bouncing back and forth between magnetic walls." The walls, however, only retain atoms whose spin axis is pointing up. Flip those spins, and "the restoring forces become expelling or repulsive forces." So the MIT researchers simply apply another magnetic field to the atoms, which tilts their spins to any desired angle. "We varied the angle between 0 and 180 degrees, and at 0 degrees the magnetic mirror was still reflective so nothing was coupled out, and at 180 degrees everything was coupled out." By controlling the angle, the researchers could then "pulse out" portions of the condensate, the way a laser pulses out dollops of coherent light.

That was the peanuts part, which Ketterle's group reported at a quantum-electronics conference in Sydney, Australia, in July and in the 27 January Physical Review Letters. What's reported in this issue of Science is the challenging part: showing that these dollops of condensate are coherent, which means the quantum-mechanical wave functions of the particles are all oscillating up and down in phase. To show that, says Ketterle, "you have to overlap matter waves from two different sources, [just as] you can prove light is a wave by passing it through two slits and looking at the interference pattern."

It took months of hard work to do this. "It only looked easy after it was finished," says Ketterle. First, he and his colleagues created two condensates by beaming a laser up through the middle of their magnetic trap. The laser light repelled the atoms and split the condensate into two distinct halves. For this test, there was no need to pulse the condensates out of the trap; instead, the group just turned off the trap and let them "free fall"--expand into the surrounding vacuum. The condensates swelled until they overlapped and interfered, demonstrating the atomic version of the bright and dark fringes in an interference pattern.

"The density of the overlapping region is modulated," says Ketterle. "Every 15 microns, we have matter, no matter, matter, no matter. Now we just shine some light onto the pattern and see this shadow with black-and-white stripes." Says Burnett, "It's not just a little crappy demonstration but a big, juicy interference pattern."

Having proved the condensate is coherent, Ketterle and his colleagues can use the output coupler to extract the condensate in pulses, which makes the setup effectively the first primitive atom laser and raises the question of where they go next. So far, they have been able to get eight pulses out of a condensate before they have to reload, which takes 20 to 30 seconds. One of their first goals is to figure out a way to restock the condensate as they go along to create the atomic version of a continuous wave laser. "Remember, these things are a few weeks old," Ketterle says, "and we need a major improvement in output power, a major reduction in complexity, and also improvement in shaping the pulses."

At that point, any field that relies on beams of atoms might benefit from the brighter and better controlled beams of an atom laser. Atomic clocks, which are based on the vibrations of atoms drifting through a cavity, are one candidate. Another is nanolithography, the technique by which circuit designers lay out minuscule features. It now depends on a mask or stencil to control where atoms or light land on a surface, but an atom laser--which could be focused and directed like a light laser--might provide a way of writing the patterns directly, says University of Texas physicist Dan Heinzen.

The technology does seem to come with a handicap: Unlike light, an atomic laser beam can't propagate freely through the atmosphere. But Burnett says it's too early to focus on limits. After all, at the birth of the light laser, "people talking about applications really didn't imagine them being in every supermarket check-out counter."

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Line of fire. A rudimentary atom laser emits pulses of atoms, all propagating as a single wave, at intervals of 5 milliseconds.

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A New Recipe for Atom Condensates

by Gary Taubes

While researchers at the Massachusetts Institute of Technology have concentrated on turning a Bose-Einstein condensate into an atom laser (see main text), a group at the University of Colorado and the National Institute of Standards and Technology (NIST) in Boulder has found a new recipe for these exotic assemblages of atoms. They serendipitously created two of them co-existing in the same magnetic trap, "a little bit like a big blob of oil next to a big blob of vinegar," says NIST's Eric Cornell.

In a paper in the 27 January Physical Review Letters, Cornell, Carl Wieman, and their colleagues--who created the first Bose-Einstein condensate in 1995--report that they cooled a single cloud containing rubidium atoms in two subtly different quantum states, distinguished by whether the electrons and nucleus of each atom have spins that are oriented in the same or opposite directions. Because the mechanism used to cool the atoms into a Bose-Einstein condensate works on only a single state, the researchers normally "take care to put all the atoms in the same internal state," says Cornell. "But on this particular day, the apparatus was not working very well, and almost by accident we got two internal states" in the trap. The system promptly cooled the atoms in one of the states, but the others cooled "sympathetically," says Cornell, by losing heat to the adjacent, already-cooled cloud.

The end result was two distinct clouds, says Cornell: "They're all exactly the same isotope of rubidium, but you have one rather distinct cloud in one internal state and another distinct cloud in another internal state. They do overlap a little bit, but they find each other repulsive."

"Personally, I think that is the most remarkable thing about the experiment," says Wieman, who adds that the interaction holds "a lot of interesting physics." The sympathetic cooling process should also expand the repertory of condensates, he says. "There are all kinds of other atoms one can now stick in the trap and turn into Bose-Einstein condensate by sympathetic cooling. It's like a refrigerator where you only know how to cool Freon directly, but you can get anything cold by sticking it in thermal contact with the Freon." The condensate chefs now have a new tool.