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(2,3), which established a coherent link between optical and radio frequencies, optical clocks(4) have attracted increasing interest as regards future atomic clocks with superior precision. To date, single trapped ions(4-6) and ultracold neutral atoms in free fall(7,8) have shown record high performance that is approaching that of the best Cs fountain clocks(1). Here we report a different approach, in which atoms trapped in an optical lattice serve as quantum references. The 'optical lattice clock'(9,10) demonstrates a linewidth one order of magnitude narrower than that observed for neutral-atom optical clocks(7,8,11), and its stability is better than that of single-ion clocks(4,5). 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.