Real-time geopotentiometry with synchronously linked optical lattice clocks

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.

25 pages, 4 figures, published online 15 August 2016 in Nature Photonics

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• Uncertainty evaluation of an ${}^{171}$Yb optical lattice clock at NMIJ
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• Unification of global height system at centimeter level using precise frequency signal links
• Determining geopotential difference via relativistic precise point positioning time comparison: A case study using simulated observations
• Preliminary experimental results of determining the geopotential difference between two synchronized portable hydrogen clocks at different locations
• Gravity field modeling using space frequency signal transfer technique between satellites