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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

High-accuracy frequency standards using laser-cooled Hg+ ions

Dana J. Berkeland, J.D. Miller, Brent C. Young, James C. Bergquist, Wayne M. Itano, David J. Wineland

Published May 15, 1998• 1 citation
Publisher

Abstract

We discuss frequency standards based on laser-cooled 199Hg+ ions confined in cryogenic rf traps. In one experiment, the frequency of a microwave source is served to the ions' ground-state hyperfine transition at 40.5 GHz. For seven ions and a Ramsey free precession time of 100 s, the fractional frequency stability is 3.3 (2) X 10-13 (tau) -1/2 for measurement times (tau) < 2 h. The ground-state hyperfine interval is measured to be 40 507 347 996.841 59 (14) (41) Hz, where the first number in parentheses is the uncertainty due to statistics and systematic errors, and the second is the uncertainty in the frequency of the time scale to which the standard is compared. In a second experiment under development, a strong-binding cryogenic trap will confine a single ion used for an optical frequency standard based on a narrow electric quadrupole transition at 282 nm. The bandwidth of the laser used to drive this transition is less than 10 Hz at 563 nm.

DOI
10.1117/12.308372

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We discuss frequency standards based on laser-cooled 199Hg+ ions confined in cryogenic rf traps. In one experiment, the frequency of a microwave source is served to the ions' ground-state hyperfine transition at 40.5 GHz. For seven ions and a Ramsey free precession time of 100 s, the fractional frequency stability is 3.3 (2) X 10-13 (tau) -1/2 for measurement times (tau) < 2 h. The ground-state hyperfine interval is measured to be 40 507 347 996.841 59 (14) (41) Hz, where the first number in parentheses is the uncertainty due to statistics and systematic errors, and the second is the uncertainty in the frequency of the time scale to which the standard is compared. In a second experiment under development, a strong-binding cryogenic trap will confine a single ion used for an optical frequency standard based on a narrow electric quadrupole transition at 282 nm. The bandwidth of the laser used to drive this transition is less than 10 Hz at 563 nm.
A stable and accurate frequency standard based on the 40.5 GHz ground-state hyperfine transition in 199Hg+{}^{199}{\mathrm{Hg}}^{+}199Hg+ ions is described. The ions are confined in a cryogenic linear Paul (rf) trap and laser cooled to form a linear crystal. With seven ions and a Ramsey interrogation time of 100 s, the fractional frequency stability is 3.3 $(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{\ensuremath{\tau}}^{\ensuremath{-}1/2}$ for measurement times $\ensuremath{\tau}<2\phantom{\rule{0ex}{0ex}}\mathrm{h}$. The ground-state hyperfine interval is measured to be 40 507 347 996.841 59 (14) (41) Hz, where the first number in the parentheses is the uncertainty due to statistical and systematic effects, and the second is the uncertainty in the frequency of the time scale to which our standard is compared.
A cryogenic linear rf trap has been constructed to study the accuracy of a clock based on laser cooled /sup 199/Hg/sup +/. Crystals of tens of ions have been observed. The residual micromotion of the ions has been minimized and Ramsey interrogation periods of 25 s have been demonstrated for the 40.5 GHz transition. This system should eventually provide a frequency standard of high precision and accuracy. We report on the development of compact, efficient, solid-state sources at 194 nm and 282 nm for laser cooling of /sup 199/Hg/sup +/ and investigation of the /sup 2/S/sub 1/2//spl rarr//sup 2/D/sub 5/2/ quadrupole transition of interest for an optical frequency standard.
We have previously reported experiments using a linear rf trap to store laser-cooled {sup 199}Hg{sup +} ions. Trapping multiple ions increases the signal-to-noise ratio; yet all ions have equally low second-order Doppler shifts if they are confined along the nodal center line of a linear trap. In such an environment, the 40.5 GHz ground-state hyperfine transition in {sup 199}Hg{sup +} is a candidate for an extremely accurate and stable microwave frequency standard. Our redesigned apparatus will operate at cryogenic temperatures to provide very low background gas pressure (resulting in longer ion storage times and immunity from background pressure shifts) and superconductive shielding against stray magnetic fields. We shall discuss progress on this frequency standard and applications of our apparatus to new cooling and squeezing techniques.
Experimental research toward the realization of an optical frequency standard of high accuracy is briefly reviewed. The studies concentrated on single, laser-cooled ions since they offer several attractive features for the achievement of high accuracy. These features include long storage time which eliminates transit-time broadening, confinement that is nearly nonperturbative to the internal level structure of the ion, laser-cooling that reduces motional shifts to small values, and electron-shelving whereby transitions to long-lived states can be detected with unit probability. The authors studied spectroscopically the electric-quadrupole allowed transition at 282 nm in a single, laser-cooled /sup 199/Hg/sup +/ ion stored in an RF Paul trap with extremely high resolution. The measured linewidth is limited by the spectral purity of the laser to about 80 Hz. Possible improvements and future directions are discussed.>