Atom Laser Emits Well-collimated Beam

Raman output coupling allows researchers to direct the coherent matter beam and control the velocity of the atoms in the beam.

By: Yvonne Carts-Powell

An atom laser with a unique output coupler produces a directional, well-collimated quasi-continuous beam of coherent sodium atoms (see Figure 1). Researchers at the National Institute of Standards and Technology (Gaithersburg, MD), Georgia State University (Statesboro, GA), and University of Tokyo (Japan) recently extracted atoms from a trapped Bose-Einstein condensate using a coherent stimulated Raman process.¹

The researchers magnetically trapped and cooled a cloud of sodium atoms to temperatures on the order of nano-Kelvins. At such temperatures, the millions of atoms condense into a state of matter predicted by Bose-Einstein statistics, in which the wave functions of the individual atoms merge—in effect, creating coherent matter waves. This Bose-Einstein condensate can be considered roughly analogous to coherent light in a laser cavity.²

History
Coupling the beam out of the "cavity" is tricky. The first atom laser, developed by Wolfgang Ketterle's group at the Massachusetts Institute of Technology (MIT; Cambridge, MA) in 1997, used radio frequency (RF) pulses and gravity to generate pulses of atoms. These pulses were found to be coherent, because they showed interference effects, but because they were accelerated from rest, they spent enough time within the condensate to interact replusively with the still-trapped atoms, resulting in pulses that spread significantly.

A laser recently developed by Tilman Esslinger and other researchers at Max Planck Institute for Quantum Optics (Garching, Germany) and the University of Munich (Germany) couples out coherent atoms in the same way as the MIT device. The system was engineered to produce continuous RF energy, however. Instead of short pulses, the system produces a continuous atom beam over periods as 0.1 s. The condensate, similar to the MIT experiment, is a cloud shaped like a cigar on its side. As a result of the cloud form, the output is shaped like fan: flat like a beam in one dimension, spreading like the MIT atom laser in another.

A third atom laser, developed by Mark Kasevich's group at Yale University (New Haven, CT) produces well-defined pulses of atoms, but like the previous two atom lasers mentioned, uses gravity.

A swift kick
The output coupler developed by the NIST group, however, uses a two-photon Raman process to produce a quasi-continuous beam of atoms with little spreading, in a controllable direction, with a continuously tunable energy. "The atoms we're holding in a magnetic trap can be in three magnetic sublevels: one trapped, one that feels no potential, and one that's repelled," explains Kristian Helmerson, of the NIST group. "We want to transfer atoms from the trapped to the antitrapped levels."

To do this, the researchers use two counterpropagating laser beams, frequency-tuned so that the condensate absorbs a photon from one beam and emits a photon into the second beam, which both change the internal magnetic state (i.e., flip the spin) and impart some momentum and kinetic energy to the atoms. The atoms thus affected are kicked in the direction of one of the beams, in this case, at a speed of 6 cm/s. Because these atoms exit the condensate more quickly than those in atom lasers outcoupled by gravity, the atoms spend less time interacting with still-trapped atoms in the condensate, and are thus more collimated when they emerge in a beam about 60 µm wide. The output beam has a deBroglie wavelength of 295 nm.

The researchers use an argon-ion-pumped dye laser to generate the yellow sodium D line at 589 nm. Two acousto-optic modulators (AOMs) generate the frequency-shifted beams needed for the Raman output coupling. The AOMs are also used as fast switches to pulse the optical laser beams. Helmerson says, "For the quasi-continuous atom laser output, we used 1 µs-long pulses at a repetition rate of 20 kHz for a total duration of 7 ms (140 pulses)."

Because the magnetic trap uses a rotating magnetic field, the researchers had to pulse the optical lasers, resulting in pulsed output from the atom laser. The atomic pulses, overlapped, however, resulting in nearly continuous stream of atoms. The researchers add, "The duration of this stream is limited only by the finite number in the condensate."³

Various aspects of the atom laser can be controlled by the optical lasers. By changing the frequencies of the optical lasers, the atom beam's speed can be altered. Not surprisingly, optical intensity and direction controls the atomic intensity and direction.

Applications
The atom laser is still very much a laboratory device, but potential applications include atom interferometers and gyroscopes, and atom lithography, a building device for micro- and nano-technology.

References
1. E. W. Hagley et. al., Science 283, p. 1706, 12 Mar 1999.
2. Nature 397., p 594, 18 Feb 1999.
3. For more information about the NIST atom laser, see http://physics.nist.gov/lab.html

About the author…
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.