Optical clock based on a sympathetically-cooled indium ion
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We report on the progress of an optical clock based on an indium ion (115In+) sympathetically cooled with a calcium ion (40Ca+) in a linear trap. In our previous work, we have measured the clock transition frequency with an uncertainty of 5× 10− 15, prompting an update of the Comité International des Poids et Mesures (CIPM) recommended value. The uncertainty was mainly limited by the evaluation of the Zeeman shift which was complicated by unresolved sublevel components. In contrast to the previous measurement, Zeeman sublevels are now separated by application of a magnetic field. Combined with optical pumping to specific Zeeman substates, the magnetic field application method successfully reduces the observed linewidth of the clock transition spectrum down to about 80 Hz full width at half maximum (FWHM). Frequency locking of the clock laser to the transition is demonstrated for the first time. The clock reaches a relative instability of 1.5× 10− 15 for an integration time of 4000 seconds.
KeywordsOptical clock Ion trap Sympathetic cooling Indium ion
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