IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 46, NO. 2, APRIL 1997 133 Clearly Resolved Secular Sidebands on the S D 674-nm Clock Transition in a Single Trapped Sr Ion Geoffrey Patrick Barwood, P. Gill, Hugh A. Klein, and W. R. C. Rowley Abstract— A prestabilized 674-nm diode laser has been nar- rowed by locking it to a high-finesse ultra-low-expansion cavity and kilohertz error signals have been observed. Lamb–Dicke confinement of single laser cooled Sr ions in a miniature rf trap has been demonstrated and micromotion reduction achieved. RF trap sidebands have been observed on the Sr S D 674 nm clock transition. The transition frequency has been measured to be (444 779 043.98 0.12) MHz (2 ). I. INTRODUCTION I N RECENT YEARS, significant progress toward the goal of future frequency standards using trapped ions has been made, both in the microwave and optical/infrared regions. Suitable microwave transitions have been observed and as- sessed in a number of ion species, including Be , Hg , and Yb in various trap and cooling configurations [1], [2]. In the optical/infrared region, the cold, single-ion linewidth of 80 Hz observed on the Hg 282 nm clock transition [3] remains a formidable benchmark. Other results where cold single-ion clock-transition lineshapes have been observed include Ba (1.76 m and 12.48 m), Sr (674 nm), Yb (435 nm) and In (236 nm), with work also underway on Ca (729 nm) [1], [2], [4]. At NPL, two ion species, strontium and ytterbium are under investigation as possible optical standards. In Sr the 674 nm S D optical “clock” transition has a lifetime of (347 33) ms [5]–[7], and a corresponding natural linewidth of 0.4 Hz. This paper reports Lamb–Dicke confinement of single Sr ions in a miniature rf trap, and the probing of the 674 nm Sr clock transition in the single ion, using a prestabilized 674 nm diode laser locked to a very-high- finesse ultra-low-expansion Fabry–Perot cavity. The profile of this 674 nm transition is constructed from the observation of quantum jumps in fluorescence at the 422 nm cooling wavelength, as a function of 674 nm probe laser frequency. Clearly resolved rf trap secular sidebands have been observed on this S D 674 nm transition. Finally, an improved frequency measurement of the transition center frequency is presented. II. CONFINEMENT AND LASER COOLING OF SINGLE Sr IONS A partial term scheme for Sr is shown in Fig. 1. The 422 nm radiation needed for laser cooling of the Sr ions Manuscript received June 20, 1996; revised October 1, 1996. The authors are with the National Physical Laboratory, Teddington, Mid- dlesex TW11 OLW, U.K. Publisher Item Identifier S 0018-9456(97)01795-6. Fig. 1. Partial term scheme of Sr . is generated by frequency doubling of 844 nm diode laser light in KNbO in a resonant enhancement cavity. The 844 nm diode laser has an anti-reflection coated front facet, and is placed within an extended cavity arrangement, and delivers 30 mW to the doubler enhancement cavity. Doubled outputs at 422 nm of up to 1 mW are obtained with linewidths of 2 MHz. Subsidiary light at 1092 nm, necessary to prevent the loss of the ion from the cooling cycle by branching decay to the D metastable level, is generated by means of a Nd - doped silica fiber laser pumped by an 826 nm 120 mW single mode diode laser. The prestabilized 674 nm AlGaInP laser for probing the D D clock transition is described in detail below. The miniature rf trap comprises two end caps and a 1 mm diameter ring, constructed from 0.5 mm diameter Ta wire. The trap is operated at a ultra-high vacuum base pressure of 10 Pa. Sr ions are created by ionization of a Sr atomic beam within the trap electrode structure, and confined by means of a 14 MHz rf drive frequency between the ring and end-caps, with amplitudes up to 450 V peak-to- peak. Using this arrangement, single Sr ions can be loaded, confined, laser cooled, and stored within the trap for periods 0018–9456/97$10.00  1997 British Crown Copyright