To keep atoms spinning together, don’t let them relax

Hank Hogan, hank.hogan@photonics.com

There’s a new world record, according to researchers who were able to preserve the spin-polarization of atoms in an alkali vapor for about 60 seconds at room temperature, roughly 100 times the previous best. During that time, atoms within the metallic cloud in their demonstration magnetometer underwent about a million collisions with the cell walls. This record-breaking performance could pave the way for more sensitive atomic magnetometers, more capable atomic clocks and improved quantum memories.

The long lifetime, or slow relaxation back to a ground state, was the result of using a new type of coating based on an alkene paraffin material on the inside of the vapor chamber walls. Implementing this innovation wasn’t easy, said researcher Micah P. Ledbetter.

By using a new coating material, researchers (from left to right) Mikhail Balabas, Todor Karaulanov, Micah Ledbetter and Dmitry Budker achieved record-breaking spin coherence performance from the alkali-vapor cell pictured. The result could be better atomic magnetometers and clocks, along with improved quantum memories. Courtesy of Micah Ledbetter, University of California, Berkeley.

“Discovering the antirelaxation properties of this material was primarily a matter of trial and error, requiring a great deal of patience and systematic study,” he said.

Ledbetter is a postdoctoral fellow at the University of California, Berkeley. Others in the group were from the US Department of Energy’s Lawrence Berkeley National Laboratory and the S.I. Vavilov State Optical Institute in St. Petersburg, Russia.

An alkali-vapor magnetometer exploits the interaction of atoms and light. In this work, the researchers used commercially available distributed-feedback lasers operating near the D1 transition of the alkali metal rubidium, or about 794.8 nm. They pumped a vaporized cloud of atoms into a spin state by using a circularly polarized beam.

They then probed the cloud with another laser, again operating near the transition wavelength. The spin vector of the atoms altered the polarization of the beam, which the investigators measured to assess the atomic spin.

In a magnetometer, the atoms lose their spin coherence by bumping into the walls of the glass chamber and into each other. One method that reduces this effect is the use of a buffer gas, but this approach introduces other relaxation processes and broadens the optical transition. The preferred solution is to use an alkane paraffin coating. These materials are composed of saturated long-chain hydrocarbons, with no double bonds present.

On a whim, the researchers decided to try coating the chamber walls with an alkene paraffin, a hydrocarbon with a carbon double bond. This material proved to have amazing antirelaxation properties, Ledbetter said.

He added, though, that the use of a new coating by itself was not enough to achieve the record. The researchers also employed a special lock arrangement to shield the atoms in the cloud from the alkali reservoir.

This approach had been tried before, he said, but the effect was too small to be important. In a Physical Review Letters paper, published Aug. 12, 2010, though, the group reported that the use of the lock upped the spin coherence time from 3 to 60 seconds when an alkene coating was used.

The researchers eliminated a final source robbing the atoms of spin coherence by reducing the magnetic field to about 0.1 microgauss, a millionth of the strength of Earth’s field. This minimized spin-exchange-relaxation, which arises when atoms bump into each other. This effect, like that of the lock, was only noticeable when an alkene coating was used, Ledbetter said.

In the future, the group will try to produce the same results in a magnetic field strength closer to that of the Earth, which would make the devices more useful. One likely application will be atomic magnetometers, although Ledbetter said achieving high sensitivity may require more extensive laser stabilization measures.

There is also ongoing work on new wall covering materials, he said. “Who knows? Maybe we will find a coating that enables another factor-of-10 improvement in [our] lifetime. That would really be amazing.”