Atomic Clocks to sync wirelessly for more accurate GPS satellite systems

NIST researchers transferred ultra-precise time signals over the air between a laboratory on NIST's campus in Boulder, Colo., and nearby Kohler Mesa. Signals were sent in both directions, reflected off a mirror on the mesa, and returned to the lab, a total distance of approximately 2 km indicated study co-author Nathan Newbury of NIST's Quantum Electronics and Photonics Division. "The actual link is a loop." The experiment used an infrared laser to generate ultra-short pulses at a very precise rate of 1 picosecond every 10 nanoseconds, where 10 ns corresponds to a set number of "ticks" of an optical atomic clock.  The two-way technique overcomes timing distortions on the signals from turbulence in the atmosphere, and shows how next-generation atomic clocks at different locations could be linked wirelessly to improve distribution of time and frequency information and other applications.

The stability of the transferred infrared signal matched that of NIST's best experimental atomic clock, which operates at optical frequencies. Infrared light is very close to the frequencies used by these clocks, and both are much higher than the microwave frequencies in conventional atomic clocks currently used as national time standards. Operating frequency is one of the most important factors in the precision of optical atomic clocks, which have the potential to provide a 100-fold improvement in the accuracy of future time standards. But the signals need to be distributed with minimal loss of precision and accuracy. 

The test was done across land, but eventually, the researchers hope, it should be possible to transfer the pulses via satellites. 

In the future, optical atomic clocks could be used for satellite-based experiments to create more precise GPS satellite navigation systems, which "could be improved in the sense that you could put better optical clocks in satellites and cross-link them optically," Newbury said. 

For GPS systems, an error of just one nanosecond, or a billionth of a second, would mean the location is about 12 inches (30 centimeters) off.