Trapped-ion frequency standards

Abstract

Experimental research at NIST toward the realization of frequency standards of high accuracy is briefly reviewed. Our studies have concentrated on laser-cooled, stored ions since they offer several attractive features toward the achievement of high accuracy (better than one part in 1015). These features include long storage times which eliminate transit-time broadening, gentle confinement which is nearly nonperturbative to the internal level structure of the ion, and laser-cooling which can reduce motional shifts to small values. We have spectroscopically studied ions confined in Penning traps at low density and single ions in rf traps. An rf oscillator at a frequency near 303 MHz has been locked to a hyperfine transition in 9Be+ ions that are stored in a Penning trap and sympathetically laser-cooled. Stability was better than 3 × 10–12 is τ–½ and accuracy was better than one part in 1014. We have also probed an electric-quadrupole allowed transition (frequency ~1 × 1015 Hz) in a single, laser-cooled 199Hg+ ion stored in an rf Paul trap with extremely high resolution. The measured linewidth of this metastable transition is limited presently by the spectral purity of the laser to about 75 Hz. Finally, improvement of signal-to-noise ration by linear trap geometries will also be discussed.