Atoms head toward single quantum state

Georgia Institute of Technology physicists have demonstrated the first all-optical technique for cooling atoms to a fraction of a degree above absolute zero.

Georgia Institute of Technology physicists have demonstrated the first all-optical technique for producing Bose-Einstein condensates, a form of matter in which atoms cooled to a fraction of a degree above absolute zero stop their normal motion — and enter a single quantum state in which all atoms behave identically.

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 dispenses with 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.

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.

The Georgia Tech process is said to rely 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.

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 favourable 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 is said to have no such restrictions, 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.