Synchronous frequency comparison of optical lattice clocks to approach the quantum limit

We demonstrate a synchronous frequency comparison of two optical lattice clocks operated using ⁸⁷Sr and ⁸⁸Sr atoms, whose Allan standard deviation achieved 1 × 10^-17 in an averaging time of 1,600 s by cancelling out the Dick effect to approach the QPN limit. The scheme manifests an advantage of using a large number N ≈ 2,000 of atoms in optical clocks and paves the way to investigate clocks´ inherent uncertainties and relativistic geodesy on a time scale of tens of minutes.