An optical lattice clock.

The precision measurement of time and frequency is a prerequisite not only for fundamental science but also for technologies that support broadband communication networks and navigation with global positioning systems (GPS). The SI second is currently realized by the microwave transition of Cs atoms with a fractional uncertainty of 10(-15) (ref. 1). Thanks to the optical frequency comb technique, which established a coherent link between optical and radio frequencies, optical clocks have attracted increasing interest as regards future atomic clocks with superior precision. To date, single trapped ions and ultracold neutral atoms in free fall have shown record high performance that is approaching that of the best Cs fountain clocks. Here we report a different approach, in which atoms trapped in an optical lattice serve as quantum references. The 'optical lattice clock' demonstrates a linewidth one order of magnitude narrower than that observed for neutral-atom optical clocks, and its stability is better than that of single-ion clocks. The transition frequency for the Sr lattice clock is 429,228,004,229,952(15) Hz, as determined by an optical frequency comb referenced to the SI second.

Frequency measurement of a Sr lattice clock using an SI-second-referenced optical frequency comb linked by a global positioning system (GPS).

We have established a transportable frequency measurement system using an optical frequency comb linked to a commercial Cs atomic clock, which is in turn linked to international atomic time (TAI) through global positioning system (GPS) time. An iodine-stabilized Nd:YAG laser is used as a flywheel in the frequency measurement system. This system is used to measure the absolute frequency of the clock transition of (87)Sr in an optical lattice. We obtained a fractional uncertainty of 2x10(-14) in the frequency measurement with a total averaging time of ~ 10(5) s over 9 days.