Height difference measurement through comparison between optical lattice clocks:
New application of clocks as a probe for crustal activities

As part of the JST Targeted Basic Research Program, a team of researchers at the University of Tokyo (UTokyo) and RIKEN have measured the general-relativistic time dilation between distant optical lattice clocks, located at UTokyo and RIKEN separated by 15 km, determining the height difference of the clocks with an uncertainty of 5 cm. This research is conducted jointly with Geospatial Information Authority of Japan and Photon Frontier Network, Japan.

Optical lattice clocks, originally proposed by Professor Katori in 2001, are currently being developed all over the world as one of the promising candidates for the new definition of the second. To guarantee the reproducibility of the clocks, which is necessary for the redefinition of the second, it is important to transfer the clock frequency to a distant laboratory and verify the agreement of the clock frequencies. On the other hand, according to Einstein’s theory of relativity, for two clocks at different heights, the lower clock runs slower due to the gravity of the Earth. Therefore, the remote frequency comparison of highly-precise clocks opens up a new application of clocks, i.e., “relativistic geodesy” or “chronometric levelling”, which determines the height difference between two remote clocks by measuring the relativistic time dilation.

Researchers have demonstrated the frequency comparison of three “cryogenic Sr optical lattice clocks” at UTokyo and RIKEN, connected by a 30-km-long telecom fiber. The two clocks at RIKEN, which are at the same height, have agreed within an statistical uncertainty of 1×10^-18, demonstrating the reproducibility of these clocks. On the other hand, the clock at UTokyo runs slower by 1,652.9×10^-18, which translates to a height difference of 1,516 cm (Fig. 1). This result has agreed with the levelling measurement by Geospatial Information Authority of Japan within the clocks’ uncertainty of 5 cm. This is the first demonstration of cm-level relativistic geodesy.

Because the spirit-levelling and gravimetry determines the height difference by summing up the subdivided measurement section, their measurement uncertainty accumulates over a long distance. On the other hand, the relativistic geodesy is free from cumulative errors. Researchers have proposed an “Internet of Clocks, that consists of clocks connected by fibers and operated synchronously (Fig. 2). These clocks may serve as “quantum benchmarks”, which offer an alternative to conventional benchmarks. The dynamic response of “quantum benchmarks” enable us to monitor the crustal activities due to active volcanoes or build a highly-precise height-monitoring-system that complements the Global Navigation Satellite System. The optical lattice clocks are expected to be incorporated into the social infrastructure for future secure society.