Abstract
Optical clocks improved the accuracy of state-of-the-art cesium clocks by more than two orders of magnitude and enabled frequency comparison with a fractional uncertainty of one part in 1018. Gravitational redshift of two such clocks allows determining their height difference with an uncertainty of 1 cm. In Europe, chronometric leveling has been extensively conducted for unifying the height reference systems. Temporal response of the leveling, which affords monitoring a cm height variation within hours of averaging time, may offer new opportunities to explore seismology and volcanology. Superb stability of optical lattice clocks will be best used for such applications. This article outlines the prospects of chronometric leveling in Japan. Combining optical lattice clocks with an existing observation network of GNSS, crustal deformations may be monitored with unprecedented accuracy in the future.
Highlights
Atomic clocks are used in various applications, which are indispensable to support modern society, such as Global Navigation Satellite Systems (GNSSs), high-speed communications, and basic science
Ellipsoidal height observed by GNSS and potential change obtained by chronometric leveling are fundamentally different physical quantities since the latter reflects geometric changes and mass changes
The uncertainty caused by neglecting temporally varying tidal effects can reach 2 cm in terms of equivalent height difference, even when chronometric leveling is performed over a Japanese Island Arc scale (= 1000 km) (Kurosihi, 2017), which can be detected by optical lattice clocks (Fig. 2C)
Summary
Atomic clocks are used in various applications, which are indispensable to support modern society, such as Global Navigation Satellite Systems (GNSSs), high-speed communications, and basic science. Recent progress of optical atomic clocks has improved this uncertainty by more than 100 times and achieved fractional uncertainties of low 1 0–18 (Nicholson et al 2015; Ushijima et al 2015; Brewer et al 2019; McGrew et al 2018). Such unprecedented accuracy of optical clocks allows testing fundamental laws of physics (Safronova et al, 2018), and finds useful applications, including chronometric leveling (Delva et al 2019) and the redefinition of the SI second (Riehle et al 2018).
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