Frequency comparison of optical lattice clocks beyond the Dick limit

The supreme accuracy of atomic clocks(1) relies on the universality of atomic transition frequencies. The stability of a clock, meanwhile, measures how quickly the clock's statistical uncertainties are reduced. The ultimate measure of stability is provided by the quantum projection noise(2), which improves as 1/root N by measuring N uncorrelated atoms. Quantum projection noise limited stabilities have been demonstrated in caesium clocks(3) and in single-ion optical clocks(4,5), where the quantum noise overwhelms the Dick effect(6) attributed to local oscillator noise. Here, we demonstrate a synchronous frequency comparison of two optical lattice clocks using Sr-87 and Sr-88 atoms(7), respectively, for which the Allan standard deviation reached 1x10(-17) in an averaging time of 1,600 s by cancelling out the Dick effect to approach the quantum projection noise limit. The scheme demonstrates the advantage of using a large number ( N approximate to 1,000) of atoms in optical clocks and paves the way to investigating the inherent uncertainties of clocks and relativistic geodesy(8) on a timescale of tens of minutes.