Researchers have synchronized two optical clocks to a record-breaking level of accuracy.
Jean-Daniel Deschênes and his colleagues at the US National Institute of Standards and Technology in Boulder, Colorado, created optical clocks that keep accurate time with pulses of light. They used lasers to synchronize two such devices placed on a rooftop in the open air, and used mirrors to create a 4-kilometre path for the light. Despite atmospheric turbulence and even light snow, the clocks never deviated from each other by more than 40 femtoseconds over 50 hours. On average, they remained within 1 femtosecond of each other for more than 108 minutes.
The scheme could allow optical clocks to be used in gravitational and relativity experiments, for measuring quantum systems, and in ultra-precise global positioning networks, say the authors.