Stopping Atoms (Extremely) Cold: Researchers Develop All-Optical Technique To Produce Bose-Einstein Condensates

Operating inside a vacuum chamber, the technique uses powerful carbon dioxide lasers to confine gaseous rubidium-87 atoms and produce the final cooling step needed to form the condensate.

The Georgia Tech method is simpler, faster and more flexible than the magnetic confinement technique used to produce the condensates since 1995. Dispensing with magnetic confinement should allow the new technique to be used on a wider variety of atoms, atomic mixtures and even molecules.

"This is the first time we've been able to make a condensate using a completely different technique," explained Michael Chapman, an assistant professor in the Georgia Tech School of Physics. "The simplicity of the technique and its speed are somewhat remarkable given that people have been trying to get all-optical Bose-Einstein condensation for so long."

Chapman and colleagues Murray Barrett and Jacob Sauer describe their work in the July 2 issue of Physical Review Letters.

Physicists create Bose-Einstein condensates through a multi-step process that uses both magnetic and optical techniques to confine and cool the gaseous atoms. First, a magneto-optical trap is used to confine the cloud of atoms. In a technique known as Doppler cooling, carefully-tuned lasers then remove energy from the atoms, dropping their temperature to a few millionths of a degree above absolute zero (-273.15 Celsius).

The final step uses evaporative cooling to remove the hottest atoms from the top of the confinement, dropping the temperature of the atom cloud to 100 billionths of a degree above absolute zero -- cold enough to form the condensate.

In the technique used since 1995, powerful magnetic fields confine -- or "trap" -- the cloud of atoms during evaporative cooling. Applying a field of radio frequency energy causes the most energetic - and hottest -- atoms to be ejected from the confined cloud, producing the final temperature drop needed produce condensation.

The Georgia Tech process relies on an all-optical technique -- two crossed laser beams -- to confine the cloud of atoms during evaporative cooling. To bring about the final cooling step, researchers rapidly reduce the laser power, lowering the depth of the confinement. That forces the hottest atoms to evaporate, forming the Bose-Einstein condensate in just two seconds -- several times faster than the magnetic process.

Physicists have attempted to produce condensates through optical means for years. Chapman doesn't yet know why his team succeeded where others failed, but he speculates that the carbon dioxide lasers or the rubidium-87 isotope may have provided an edge. Carbon dioxide lasers can be precisely controlled to avoid transferring energy to the atom cloud, and the rubidium-87 isotope has properties more favorable than the rubidium-85 studied by other researchers.

Because it relies on interaction with the magnetic dipole of atoms, magnetic confinement techniques work only with certain atoms in some of their energy states. That limits the elements from which physicists can make Bose-Einstein condensates.

The Georgia Tech optical technique has no such restriction, allowing physicists to use any atom that can be sufficiently cooled, including alkali rare earth elements such as magnesium and strontium. It could even produce condensates from atomic mixtures and molecules.

"That's quite exciting from a physics standpoint, because it is a whole new aspect of Bose-Einstein condensation that wasn't considered by Bose or Einstein," Chapman said. "Our technique is amenable to trapping a mixture of atoms, which opens up the possibility of condensing two different species of atoms at the same time."

Other advantages: the optical technique uses less sophisticated traps, lower vacuum levels -- and doesn't need bulky, power-hungry magnetic coils.

Chapman and his team have produced condensates containing up to 35,000 atoms, far less than the millions of atoms captured by magnetic means. But Chapman sees no fundamental reason why the optical process can't be scaled up to match those numbers.

Though potential applications remain far in the future, the Bose-Einstein phenomenon has attracted intense interest because it could do for atoms what lasers have done for photons. Lasers produce streams of photons with identical wavelengths and energy levels, all moving in the same direction. This coherence powers a broad range of applications from high-speed communications to metal cutting.

"A lot of the excitement about atomic Bose-Einstein condensates is that this sort of coherence -- getting all the atoms to be in one state and do everything at the same time -- could eventually lead to some interesting developments," Chapman noted. "Where this will lead is hard to predict, but historically whenever we've been able to get more control over physical systems, that has led to dramatic new directions in science and technology."

Atomic lithography, coherent matter wave optics and coherent atomic interferometry are among the applications proposed.

Chapman and his team began studying Bose-Einstein condensation because of their interest in controlling atoms for investigating quantum entanglements, which have applications for quantum logic -- a potential new area of computing. To accomplish quantum computing, however, scientists will have to control thousands of atoms individually.

The phenomenon is named after Albert Einstein and Satyendra Bose, who proposed its possibility in the 1920s.

An image produced by the research team is the first figure to be published on the newly-redesigned cover of Physical Review Letters, which is published by the American Physical Society.

Chapman's research was supported by the U.S. National Security Agency, the U.S. Army Research Office and the Advanced Research and Development Activity (ARDA).