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For the past fifty years, atomic frequency standards based on the cesium ground-state hyperfine splitting have been the most accurate timepieces in the world. One of the most accurate, current-generation, cesium standards is the NIST-F1 fountain, which has recently been evaluated with an inaccuracy of about 4×10−16. However, it has long been recognized that a frequency standard based upon an atomic optical transition has the potential for both accuracy and stability substantially better than that of a cesium standard. We now report a comparison between NIST-F1 and our optical frequency standard based on an ultraviolet transition in a single, laser-cooled, trapped mercury ion. In this comparison, the fractional systematic frequency uncertainty of the mercury standard was below 1.2×10−16 and the absolute frequency of the transition was measured versus cesium to be 1 064 721 609 899 144.94 (97) Hz, with a statistically limited total fractional uncertainty of 9.1×10−16. This result is not only the most accurate absolute measurement of an optical frequency to date, but also demonstrates conclusively that frequency control in the mercury-ion system is possible at a level superior to that of the best present-day cesium frequency standards.
Physical Review Letters
atomic clock, cesium fountain, femtosecond laser frequency comb, mercury-ion frequency standard, optical frequency standard, single-ion frequency standard, trapped ions