1. Engineering Research Institute, The University of Tokyo and
2. PRESTO, Japan Science and Technology Agency, Bunkyo-ku, Tokyo 113-8656, Japan
3. National Metrology Institute of Japan/National Institute of Advanced Industrial Science and Technology (NMIJ/AIST), Tsukuba, Ibaraki 305-8563, Japan

Correspondence to: Hidetoshi Katori^1,2 Correspondence and requests for materials should be addressed to H.K. (Email: katori@amo.t.u-tokyo.ac.jp).

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 clocks⁴ have attracted increasing interest as regards future atomic clocks with superior precision. To date, single trapped ions^{4, 5, 6} and ultracold neutral atoms in free fall^{7, 8} have shown record high performance that is approaching that of the best Cs fountain clocks¹. 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.

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