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Chinese Physics Letters

Frequency Measurement of the Electric Quadrupole Transition in a Single Laser-Cooled 40 Ca +

Qu Liu, Yao Huang, Jian Zhong Cao, Bao-Quan Ou, Bin Guo, Hua Guan, Xue-Ren Huang, Kelin Gao

Published Jan 1, 2011• 13 citations
Publisher

Abstract

The optical frequency of the 4s 2 S 1/2 -3d 2 D 5/2 transition in a single trapped and laser-cooled 40 Ca + ion is measured with an optical frequency comb system referenced to a hydrogen maser. A 729-nm laser can be locked to the clock transition about ten hours and the Allan deviation is better than 2 × 10 −14 /1000 s.
DOI
10.1088/0256-307x/28/1/013201

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Research on the development of the optical frequency standard based on trapped and cold 40 Ca + with the 4s 2 S 1/2 -3d 2 D 5/2 clock transition at 729 nm is reported. A single calcium ion was trapped and laser cooled in the Paul trap and stay in trap for more than 15 days. The linewidth of a 729 nm laser was reduced to less than 10 Hz by locking to a cavity for longer than 50 hours uninterruptedly. The overall systematic uncertainty of the clock transition has been characterized to be better than 6.5×10 −16 . The absolute frequency of the clock transition was measured at 10 −15 level using an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the global positioning system (GPS). The frequency value was 411042129776393.0(1.6) Hz after the correction of the systematic shifts.
The National Institute of Information and Communications Technology in Japan is proceeding with development of an optical frequency standard using an electric quadrupole transition of Ca+ ions in a small radio-frequency trap. The single, trapped 40Ca+ ions are laser cooled to a temperature of <10 mK using extended-cavity diode lasers at 397 nm and 866 nm. In order to measure the electric quadrupole transition, the linewidth of the clock laser diode at 729 nm is reduced to ~66 Hz using an ultra-low-expansion (ULE) cavity. We observed the electric quadrupole transition with a linewidth of ~600 kHz, together with the ionic motional sidebands. We are also developing optical frequency combs without photonic fibers broadening the spectrum. The experimental is now improved in order to observe the narrower linewidth of the quadrupole transition.
The frequency of the 5s 2S(1/2)-4d 2D(5/2) electric quadrupole clock transition in a single, trapped, laser-cooled 88Sr+ ion has been measured by using an optical frequency comb referenced to a cesium fountain primary frequency standard. The frequency of the transition is measured as 444,779,044,095,484.6 (1.5) hertz, with a fractional uncertainty within a factor of 3 of that of the cesium standard. Improvements required to obtain a cesium-limited frequency measurement are described and are expected to lead to a 88Sr+ optical clock with stability and reproducibility exceeding that of the primary cesium standard.
We have measured the absolute frequency of a clock transition at the 10${}^{\ensuremath{-}15}$ level with a single, trapped, laser-cooled 40{}^{40}40Ca+{}^{+}+ ion in a miniature Paul trap. In our measurement, an optical frequency comb is referenced to a hydrogen maser, which is calibrated to the SI second through the Global Positioning System (GPS). By analyzing the experimental data obtained within 32 days of a total averaging time $>$2 $\ifmmode\times\else\texttimes\fi{}$ 106{}^{6}6 s, the absolute frequency of the 40{}^{40}40Ca+{}^{+}+ 4sss 2S1/2{}^{2}\phantom{\rule{-0.16em}{0ex}}{S}_{1/2}2S1/2​--3ddd 2D5/2{}^{2}\phantom{\rule{-0.16em}{0ex}}{D}_{5/2}2D5/2​ clock transition is measured as 411 042 129 776 393.0 (1.6) Hz with a fractional uncertainty of 3.9 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{\ensuremath{-}15}$.