NIST’s experimental next-generation atomic clocks, based on ytterbium, strontium, aluminum, and mercury atoms, have reached a level of precision and stability that may allow them to contribute to official U.S. civilian timekeeping. These optical clocks operate at much higher frequencies than the microwave frequencies of current cesium clocks, but they are complex and typically run intermittently.
NIST simulations suggest that to achieve the same performance as a cesium-fountain-calibrated time scale, an optical clock would need to run for 12 minutes every 12 hours, or 1 hour per day, or 4 hours every 2 and 1/3 days, or 12 hours per week. The simulations indicate that optical clocks could provide good support for official timekeeping even with their modest availability.
NIST is already using optical clocks to monitor masers in the time scale. The plan is to build a system to use optical clock results to create a “paper time scale” and accumulate data on how it compares to the real thing. This could reduce the error in official time to just 2 nanoseconds (ns), better than current offsets in NIST official time. The study suggests running an optical clock 4 hours at a time at least 3 times a week to achieve the same performance as a cesium-fountain-calibrated time scale.
The international scientific community is expected to redefine the second, selecting a new atom as the basis for standard atomic clocks and official timekeeping in the coming years. This could lead to further improvements in the accuracy of official timekeeping.
Keywords: optical clocks, atomic clocks, frequency reference, calibrations, official time
