Frozen reservoir fuels atom lasers

Laser beams made of matter rather than light are a step closer to reality now that researchers at the Massachusetts Institute of Technology have figured out how to continuously replenish a reservoir of ultra-cold atoms.

The technique merges tiny clouds of atoms that are so cold they behave as a single entity. These Bose-Einstein Condensates are the main components of atom lasers. Prototype atom lasers have, to date, worked only as pulsed rather than continuous beams.

Pulsed atom lasers are analogous to a dripping faucet, said Ananth Chikkatur, a researcher at MIT. "We have now implemented a bucket -- our reservoir trap -- where we collect these drips to have a continuous source of water. If we poke a hole in this bucket, we will have a steady stream of water," he said.

Atom lasers could be used to deposit matter on a surface atom by atom to, for instance, produce very fine wires on a computer chip. They could also make extremely sensitive movement sensors because atom waves, like light waves, can interfere with each other, and the interference patterns are affected by subtle changes in acceleration and gravity.

The project's leader, MIT physics professor Wolfgang Ketterle, was one of the recipients of the 2001 Nobel Prize in physics, which was awarded for the discovery of Bose-Einstein Condensates.

One of the strange properties of quantum particles like atoms and photons is that they also act like waves. In a Bose-Einstein Condensate, the crests and troughs of the atoms' waves are in sync, much like the photons in a laser beam.

Getting the atoms to snap into this quantum lockstep requires cooling them to a fraction of a degree above absolute zero. Forming Bose-Einstein Condensates is a two-step cooling process involving lasers and evaporation.

The hotter matter is, the faster its atoms move, though not all of the atoms move at the same speed. The researchers used a laser tuned to send a stream of photons into the paths of the fastest moving, and therefore hottest, atoms in a gas. The impact transfers energy from the atoms to the photons, slowing and thus cooling the atoms.

In the second step of the cooling process, the researchers held the atoms in a trap formed by a magnetic field, and then gradually diminished the strength of the trap to allow the hottest atoms to escape.

The researchers formed a Bose-Einstein Condensate consisting of about 2 million sodium atoms. They then formed another condensate and added it to the first, which lost half of its atoms in the time it took to produce the second condensate. The merged condensate totaled 2,300,000 atoms.

The researchers move the condensates using laser tweezers. When a laser beam shines through a small, transparent object the light bends, which transfers momentum to the object, much like wind moving the vanes of a windmill. This force can also be used to hold an object within a laser beam.

Bose-Einstein Condensates are very fragile and the challenge is being able to merge them without heating them up, said Chikkatur. The condensates held in the laser beams were elongated, and the researchers found that gently lowering one condensate lengthwise on top of the other did the trick, he said.

Researchers have already developed several techniques for draining Bose-Einstein Condensates to form atom lasers. "By combining [output] techniques using optical laser beams with our continuous source, we [will be able to] generate a continuous beam of coherent atoms," said Chikkatur.

The researchers' next step is to increase the number of atoms collected in the reservoir, said Chikkatur.

The researchers' work is a "tour de force and a major step forward in the technology of Bose-Einstein condensation," said Aephraim Steinberg, an associate professor of physics at the University of Toronto. "It is extremely promising for the development of real continuous-wave atom lasers," he said.

In the short-term, continuous-wave atom lasers will allow scientists to study how quantum particles change, or decohere, when they come into contact with their environment, said Steinberg.

Isolated atoms and subatomic particles are in the weird quantum mechanical condition of superposition, an unknowable mix of all possible orientations. When energy from the environment, like a stray magnetic field, hits a particle and knocks it out of superposition, it resumes one, definite orientation. "Decoherence [is] one of the processes defining the boundary between quantum and classical, and one of the important obstacles to overcome if we are to develop quantum computers," said Steinberg.

It is impossible to predict whether atom lasers will have direct technological applications, said Steinberg. "We're more or less in the situation of laser researchers in 1960 who could never have envisioned the applications lasers have today," he said.

It will take five to ten years for continuous-wave atom lasers to be used to deposit atoms on a surface in practical applications, said Chikkatur. "For atom lithography one needs to have a very high [flow rate] of atoms, which is not possible currently," he said.

Chikkatur's research colleagues were Yong-Il Shin, Aaron E. Leanhardt, David Kielpinski, Edem Tsikata, Todd L. Gustavson, David E. Pritchard and Wolfgang Ketterle. They published the research in the May 16, 2002 issue of the online issue of the journal Science. The research was funded by the National Science Foundation (NSF), the Office of Naval Research (ONR), the Army Research Office (ARO), the Packard Foundation and NASA.

Timeline:   5-10 years
Funding:   Government, Private
TRN Categories:   Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "A Continuous Source of Bose-Einstein Condensed Atoms," Sciencexpress, May 16, 2002