Real-time geopotentiometry with synchronously linked optical lattice clocks

TL;DR

This paper demonstrates high-precision real-time geopotentiometry using synchronized optical lattice clocks over 15 km, achieving 5 cm height difference measurement accuracy by rejecting laser noise and enhancing stability.

Contribution

It introduces a novel protocol for synchronously linking optical clocks via telecom fibre, enabling precise gravitational red shift measurements and potential for a scalable network of quantum height references.

Findings

Achieved 5 cm height difference measurement accuracy.

Demonstrated stable fractional frequency difference of 1,652.9(5.9)×10^{-18}.

Validated measurements with independent levelling and gravimetry.

Abstract

According to the Einstein's theory of relativity, the passage of time changes in a gravitational field. On earth, raising a clock by one centimetre increases its tick rate by 1.1 parts in 10$^{18}$, enabling optical clocks to perform precision geodesy. Here, we demonstrate geopotentiometry by determining the height difference of master and slave clocks separated by 15 km with uncertainty of 5 cm. The subharmonic of the master clock is delivered through a telecom fibre to phase-lock and synchronously interrogate the slave clock. This protocol rejects laser noise in the comparison of two clocks, which improves the stability of measuring the gravitational red shift. Such phase-coherently operated clocks facilitate proposals for linking clocks and interferometers. Over half a year, 11 measurements determine the fractional frequency difference between the two clocks to be $1,652.9(5.9)\times 10^{-18}$, or a height difference of 1,516(5) cm, consistent with an independent measurement by levelling and gravimetry. Our system is as a building block of an internet of clocks, consisting of a master and a number of slave clocks, which will provide "quantum benchmarks" that are height references with dynamic response.