Single-trapped-ion frequency standards based on a 282 nm transition in 199Hg+ and on a 267 nm transition in
27Al+ have been developed at NIST over the past several years. Their frequencies are measured relative to each
other and to the NIST primary frequency standard, the NIST-F1 cesium fountain, by means of a self-referenced
femtosecond laser frequency comb. Both ion standards have demonstrated instabilities and inaccuracies of less
than 1 × 10-16.
Using a laser that is frequency-locked to a Fabry-Perot etalon of high finesse and stability, we probed the 5d106s 2S1/2(F equals O, mF equals O) $ARLR 5d96s22D5/2 (F equals 2, mF equals O) electric-quadrupole transition of a single laser-cooled 199Hg+ ion stored in a cryogenic radio-frequency ion trap. We observed Fourier-transform limited linewidths as narrow as 6.7 Hz at 282 nm (1.06 X 1015 Hz). The functional form and estimated values of some of the frequency shifts of the 2S1/2 $ARLR 2D5/2 clock transition (including the quadrupole shift), which have been calculated using a combination of measured atomic parameters and ab initio calculations, are given.
We describe in detail an optical clockwork based on a 1 GHz repetition rate femtosecond laser and silica microstructure optical fiber. This system has recently been used for the absolute frequency measurements of the Ca and Hg+ optical standards at the National Institute of Standards and Technology (NIST). The simplicity of the system makes it an ideal clockwork for dividing down high optical frequencies to the radio frequency domain where they can readily be counted and compared to the existing cesium frequency standard.
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 standard grating-tuned extended-cavity diode laser is used for injection seeding of a tapered semiconductor laser/amplifier. With sufficient injection power the output of the amplifier takes on the spectral characteristics of the master laser. We have constructed master-oscillator power-amplifier systems that operator near 657 nm, 675 nm, 795 nm, and 850 nm. Although the characteristics vary from system to system, we have demonstrated output powers of greater than 700 mW in a single spatial mode, linewidths less than 1 kHz, coarse tuning greater than 20 nm, and continuous single-frequency scanning greater than 150 GHz. We discuss the spectroscopic applications of these high power, highly coherent, tunable diode lasers as applied to Ca, Hg+, I2, and two-photon transitions in Cs.
Spectroscopy of ions in electromagnetic traps using laser-cooling and detection has reached a sensitivity where it is now possible to unambiguously monitor state changes in a single ion. While these techniques may not be generally applicable, the sensitivity and precision that is obtained for laser-cooled ions give broad opportunities for experiments in many areas of fundamental physics and-high resolution spectroscopy. In this paper, the authors describe two experiments with a single laser-cooled Hg+ ion. In one they achieve the highest fractional resolution (highest Q) in atomic or molecular spectroscopy, and in the second they cool the ion to its zero-point energy.