In a significant advance toward the future redefinition of the international unit of time, the second, a research team led by the National Institute of Standards and Technology (NIST) has compared three of the world's leading atomic clocks with record accuracy over both air and optical fiber links.

Described in the March 25 issue of Nature, the NIST-led work 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. These atomic clock comparisons place the scientific community one step closer to meeting the guidelines for redefinition of the second.

"These comparisons are really defining the state of the art for both fiber-based and free-space measurements—they are all close to 10 times more accurate than any clock comparisons using different atoms performed so far," NIST physicist David Hume said.

The new measurements were challenging because the three types of atoms involved "tick" at vastly different frequencies, because all the many network components had to operate with extreme accuracy, and because the wireless link required cutting-edge laser technology and design.

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 1018—that is, errors never exceeded 0.000000000000000008—for both fiber and wireless links.