NIST researchers have made significant advancements in comparing three of the world’s leading atomic clocks with record accuracy over both air and optical fiber links. The study, published in Nature, is the first to compare three clocks based on different atoms and the first to link the most advanced atomic clocks in different locations over the air. The comparisons place the scientific community one step closer to meeting the guidelines for redefinition of the second.
The study compared the aluminum-ion clock and ytterbium lattice clock, located in different laboratories at NIST Boulder, with the strontium lattice clock located 1.5 kilometers away at JILA, a joint institute of NIST and the University of Colorado Boulder. The team’s measurements were so accurate that uncertainties were only 6 to 8 parts in 10^18 for both fiber and wireless links.
The key to the air link was the use of optical frequency combs, which enable accurate comparisons of widely different frequencies. NIST researchers developed two-way transfer methods to precisely compare optical clocks over the air, even in conditions of atmospheric turbulence and laboratory vibrations. The comb-based signal transfer technique had been demonstrated previously, but the latest work was the first to compare state-of-the-art atomic clocks.
The new NIST results set other important records, including the three most accurate measurements ever made of natural constants. Frequency ratios offer an important advantage as a metric for evaluating optical atomic clocks. A direct measurement of an optical clock frequency in the usual units of hertz is limited by the accuracy of the current international standard, the cesium microwave clock. Frequency ratios overcome this limitation because they are not expressed in any units.
The study was supported in part by the Defense Advanced Research Projects Agency, the Air Force Office for Scientific Research, the National Science Foundation, the Office of Naval Research, NASA Fundamental Physics, and the Department of Energy.
Keywords: optical clocks, frequency ratios, atomic clocks, time standards
