Synchronous comparison of two thulium optical clocks

The experimental comparison of two thulium optical lattice clocks in a time interval of up to one hour has been carried out. The synchronous comparison of a clock transition in two independent atomic ensembles using a single ultrastable laser has allowed us to eliminate fluctuations of the laser frequency from the measured frequency difference and to reach a relative measurement error of $10^{-16}$ after $500$-s averaging, which corresponds to a relative instability of $2 \times {{10}^{{ - 15}}}/\sqrt \tau $. The successful demonstration of the long-term operation of two systems using the synchronous comparison of clock transitions opens the possibility of studying systematic shifts in thulium optical clocks with an uncertainty of $10^{-17}$.