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Time and frequency are the most precisely measurable physical quantities. Almost all technological processes require precise timing or refe‑rence frequencies, and improvements in the rea‑lization and dissemination of time and frequency are expected to have widespread impact on innovation, science, and daily life, in particular in the areas of communication and navigation. Hundreds of atomic frequency standards are in operation in telecom networks, military and sci‑ence centers, and metrological institutes. To fully exploit this potential, novel techniques for time and frequency transfer are required. National metrology institutes have signifi‑cant capabilities in atomic clocks, time scale ge‑neration, time dissemination, space technology, and network synchronization. In a worldwide network, time laboratories contribute with pre‑sently approximately 250 atomic clocks to the international atomic time scale TAI. Caesium fountain clocks have demonstrated a relative accuracy of better than 10–15 with the potential to reach the low 10–16 range in the next five years [1]. In contrast to conventional atomic clocks which use atomic reference transitions in the microwave range, an emerging new generation of atomic clocks is based on laser excitation of reference transitions in the optical frequency range. These “optical” clocks have the potential to reach a relative accuracy of better than 10–17 together with a short–term stability (Allan devia‑tion) in the range of σy= 10–15 (τ/s)–1/2 [2]. The wide range of different atomic clocks with diverse operational characteristics requires dedicated techniques for their comparison. On the one hand, routinely operational …