Laser Technique Cools Broader Range of Elements

A versatile laser cooling technique proposed 15 years ago now has partial experimental verification.

In principle the technique could be used to cool a broader range of elements than is possible with current methods.

Researchers at the University of Southampton used matter-wave interferometry to lower a sample of already-cold rubidium from 21 to 3 μK — nearly to the fundamental temperature limit of laser cooling.

"There is a great push to extend ultracold physics to the rest of the periodic table to explore a greater wealth of fundamental processes and develop new technologies, and we hope that our demonstration will help," said Dr. Alex Dunning, who led the project.

The laser-cooling experiment was carried out in this vacuum chamber, which features an ion pump (left) and a photomultiplier tube and light-collection lenses (right). Courtesy of University of Southampton.

Research involving ultracold atoms has yielded atomic clocks, which enable global positioning systems, and could form the basis of quantum computing.

The current technique of cooling atoms down from room temperature to the ultracold regime is sometimes referred to as "optical molasses." It involves the preferential scattering of laser photons from a particle in motion, slowing it down.

This approach is limited to atoms with a favorable electronic structure. As a result, only a small fraction of atomic elements, along with a select few diatomic molecules, have been cooled in this manner.

The new approach is based on a proposal made by Martin Weitz and Nobel laureate Ted Hänsch in 2000. Matter-wave interferometry involves using a laser pulse to place an atom (the matter wave) into a superposition of states. The atom travels simultaneously along two paths that interfere at a later time.

The impulse imparted to the atom depends upon how the difference in energy along the two paths compares with the energy of the laser photons, where the atom's energy is formed of potential (internal electron configuration) and kinetic (external motion) parts.

In Weitz and Hänsch's scheme, the laser interacts with the atom in such a manner as to remove the dependence on the potential energy, leaving the interference based solely on the atom's kinetic energy.

"Our technique, should we succeed in extending it to Weitz and Hänsch's complete scheme, would be sort of a catch-all," Dunning said. "Progress so far in cooling molecules tends to use the details of specific molecules, rather than being something general. That's why this is exciting, even though our actual experiment just uses atoms."

"These beautiful results have demonstrated that the method is feasible and can result in colder atoms than conventional Doppler cooling," said group leader Dr. Tim Freegarde. "To move on to other atoms and molecules will require more powerful lasers with shorter pulses, of the type used in coherent control chemistry, so the future of this method is very promising."

The research was published in Physical Review Letters (doi: 10.1103/PhysRevLett.115.073004).

For more information, visit www.southampton.ac.uk.