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Optics Letters

Laser cooling of trapped Ca^+ and measurement of the 3 ^2D_5/2 state lifetime

Shinji Urabe, Masayoshi Watanabe, Hidetsuka Imajo, Kazuhiro Hayasaka

Published Aug 15, 1992• 28 citations
Publisher

Abstract

We describe the laser cooling of Ca(+) ions in a rf ion trap and the measurement of its 3(2)D(5/2) state lifetime. The ions are cooled to below 1 K by radiation from a frequency-doubled Ti:sapphire laser and a laser diode. The radiative lifetime is measured by observing the change in fluorescence when the ions, which are optically pumped to the metastable state, return to the cooling cycle. The lifetime of the 3(2)D(5/2) state is measured to be 0.77 +/- 0.07 s.

DOI
10.1364/ol.17.001140

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Japanese Journal of Applied Physics•Apr 1, 1999•9 citations
We describe the laser cooling of Ca(+) ions in a rf ion trap and the measurement of its 3(2)D(5/2) state lifetime. The ions are cooled to below 1 K by radiation from a frequency-doubled Ti:sapphire laser and a laser diode. The radiative lifetime is measured by observing the change in fluorescence when the ions, which are optically pumped to the metastable state, return to the cooling cycle. The lifetime of the 3(2)D(5/2) state is measured to be 0.77 +/- 0.07 s.
This paper describes our recent experimental results on trapping and laser cooling of Ca+ ions. The measurement of the characteristics of laser-cooled Ca+ ions in an rf quadrupole ion trap, such as the fluorescence line shape and phase transition, is described. Observation of two types of collision-induced “quantum jumps” of a Ca+ ion in a background gas of H2 or air is also described. Since the rate of the collision-induced jumps is very high for Ca+, it is necessary to achieve an extremely high vacuum condition for frequency standard applications.
Ca + ions confined as a cloud in a Paul trap were excited and cooled by radiation from two lasers, tuned to the 4s 2 S 1/2 − 4p 2 P 1/2 and 4p 2 P 1/2 − 3d 2 D 3/2 transitions. The strong fluorescence of these lines disappears when we excite the ions simultaneously by a third laser on the 3d 2 D 3/2 − 4p 2 P 3/2 line because of population trapping in the metastable 3d 2 D 5/2 state, into which the 4p 2 P 3/2 level decays. From the time dependence of the fluorescence after blocking the third laser we determined the lifetime of the 3d 2 D 5/2 level. Our result of τ = 1064(17) ms improves earlier results and agrees with the value recently computed by Liaw using the Brueckner approximation.
We have fabricated an rf ion trap and laser-cooling light sources to observe shelving transition in a single calcium ion. All the radiation required for laser cooling and observation of the transition can be prepared with diode lasers in this system. Continuous-wave radiation at 397 nm for laser cooling was generated by frequency doubling of a GaAlAs laser in an external enhancement cavity. As a preliminary experiment for observing the shelving transition, laser cooling of one hundred calcium ions was demonstrated using this radiation and another diode laser.
We demonstrate laser cooling of single174Yb+ions stored in a rf trap. Single174Yb+ions were loaded using a natural isotope mixture as source. To locate a single ion at the center of the trap, and consequently, to minimize the micromotion caused by the rf driving field, we introduced two compensation electrodes and applied dc voltages to them and to one of the endcap electrodes. We estimated the temperature of a trapped single174Yb+ion to be 150 mK from the residual Doppler width of the spectra observed by a frequency sweep of the cooling laser. In the estimation, the saturation broadening in the spectra was substituted by measurement of the power dependence of the spectral width. To count the number of trapped174Yb+ions, we detected sudden drops in the fluorescence intensity (quantum jumps) of individual ions caused by pumping to and deexcitation of the2F7/2state. We found that deexcitation of the2F7/2state is possible by driving the2F7/2–1D[3/2]3/2transition at 760 nm and the2F7/2–3D[9/2]7/2transition at 828 nm, in addition to the former methods of driving the2F7/2–1D[5/2]5/2transition at 639 nm and the2F7/2–2D5/2transition at 3.43 µm.