An Optical Clock Based on a Single Trapped ¹⁹⁹ Hg ⁺ Ion

TLDR

Microwave atomic clocks have dominated precision timekeeping for five decades, but optical clocks promise greater stability due to their higher operating frequency. The study demonstrates an all‑optical atomic clock based on the 1.064‑petahertz transition of a single trapped 199Hg⁺ ion. The clock uses a mode‑locked femtosecond laser to generate 1‑GHz pulses phase‑coherently locked to the optical frequency. Compared with a laser‑cooled calcium standard, the Hg⁺ clock achieved a fractional frequency instability below 7 × 10⁻¹⁵ in one second, surpassing the best microwave clocks.

Abstract

Microwave atomic clocks have been the de facto standards for precision time and frequency metrology over the past 50 years, finding widespread use in basic scientific studies, communications, and navigation. However, with its higher operating frequency, an atomic clock based on an optical transition can be much more stable. We demonstrate an all-optical atomic clock referenced to the 1.064-petahertz transition of a single trapped 199Hg+ ion. A clockwork based on a mode-locked femtosecond laser provides output pulses at a 1-gigahertz rate that are phase-coherently locked to the optical frequency. By comparison to a laser-cooled calcium optical standard, an upper limit for the fractional frequency instability of 7 x 10(-15) is measured in 1 second of averaging-a value substantially better than that of the world's best microwave atomic clocks.