LASER COOLING: Laser-cooled mercury may become time standard

A new mercury-based atomic clock under development at the National Institute of Standards and Technology (NIST, Boulder, CO) is expected to offer accuracy at least an order of magnitude better than the best atomic cesium standard whose ground-state hyperfine transition currently defines the second. Using input at 257 nm from a frequency-doubled argon-ion laser and at 792 nm from a master-oscillator/power-amplifier diode laser (SDL; San Jose, CA), the NIST group generates coherent output at 194 nm using a sum-frequency process in b-barium borate. Radiation pressure cooling with the 194-nm output brings the mercury atoms nearly to rest. Because they are tightly confined electromagnetically in a linear Paul trap, the cooled atoms crystallize into a string of individual atoms. Mercury has a ground-state hyperfine transition at 40 GHz, compared to the cesium hyperfine transition at 10 GHz. Led by James Bergquist, the NIST group is now exploring the stability and accuracy of a standard based on this transition, working toward a future time standard.

The cooled mercury atoms fluoresce strongly when irradiated at 194 nm. A fast lens focuses scattered light from the atoms onto an ultraviolet imaging tube; the images can be displayed in real time or captured by computer, as was the image above.