Frequency ratios of Sr, Yb and Hg based optical lattice clocks and their applications

摘要

This article describes the recent progress of optical lattice clocks withneutral strontium ($^{87}$Sr), ytterbium ($^{171}$Yb) and mercury ($^{199}$Hg)atoms. In particular, we present frequency comparison between the clockslocally via an optical frequency comb and between two Sr clocks at remote sitesusing a phase-stabilized fibre link. We first review cryogenic Sr opticallattice clocks that reduce the room-temperature blackbody radiation shift bytwo orders of magnitude and serve as a reference in the following clockcomparisons. Similar physical properties of Sr and Yb atoms, such as transitionwavelengths and vapour pressure, have allowed our development of a compatibleclock for both species. A cryogenic Yb clock is evaluated by referencing a Srclock. We also report on a Hg clock, which shows one order of magnitude lesssensitivity to blackbody radiation, while its large nuclear charge makes theclock sensitive to the variation of fine-structure constant. Connecting allthree types of clocks by an optical frequency comb, the ratios of the clockfrequencies are determined with uncertainties smaller than possible throughabsolute frequency measurements. Finally, we describe a synchronous frequencycomparison between two Sr-based remote clocks over a distance of 15 km betweenRIKEN and the University of Tokyo, as a step towards relativistic geodesy.