Atomic clocks have been instrumental in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Timekeeping precision at 1 part in 10(18) enables new timing applications in relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests of physics beyond the standard model. Here, we describe the development and operation of two optical lattice clocks, both using spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of 1.6 × 10(-18) after only 7 hours of averaging.

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Atomic clocks based on optical transitions are the most stable, and therefore precise, timekeepers available. These clocks operate by alternating intervals of atomic interrogation with 'dead' time required for quantum state preparation and readout. This non-continuous interrogation of the atom system results in the Dick effect, an aliasing of frequency noise of the laser interrogating the atomic transition 1,2 . Despite recent advances in optical clock stability achieved by improving laser coherence, the Dick effect has continually limited optical clock performance. Here we implement a robust solution to overcome this limitation: a zero-dead-time optical clock based on the interleaved interrogation of two cold-atom ensembles 3 . This clock exhibits vanishingly small Dick noise, thereby achieving an unprecedented fractional frequency instability of  6 10 17 /  for an averaging time  in seconds. We also consider alternate dual-atom-ensemble schemes to extend laser coherence and reduce 2 the standard quantum limit of clock stability, achieving a spectroscopy line quality factor  Q  4 10 15 .An optical atomic clock operates by tuning the frequency of a laser (optical local oscillator, OLO) into resonance with a narrowband, electronic transition in an atomic