Measuring the frequency of a Sr optical lattice clock using a 120-km coherent optical transfer

TL;DR

This paper reports a highly precise measurement of the 87Sr optical lattice clock frequency over a 120-km fiber link, demonstrating the potential of optical clocks for redefining the second.

Contribution

It introduces a method for measuring optical clock frequency over long-distance fiber links with high precision, matching the best existing standards.

Findings

Frequency measurement accuracy of 2.4 Hz at 429228004229874.1 Hz

Agreement with other international standards at a fractional level of 6×10^-16

Successful demonstration of long-distance optical frequency transfer

Abstract

We demonstrate a precision frequency measurement using a phase-stabilized 120-km optical fiber link over a physical distance of 50 km. The transition frequency of the 87Sr optical lattice clock at the University of Tokyo is measured to be 429228004229874.1(2.4) Hz referenced to international atomic time (TAI). The measured frequency agrees with results obtained in Boulder and Paris at a 6*10^-16 fractional level, which matches the current best evaluations of Cs primary frequency standards. The results demonstrate the excellent functions of the intercity optical fibre link, and the great potential of optical lattice clocks for use in the redefinition of the second.