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Japanese Journal of Applied Physics

Laser Cooling of Single174Yb+Ions Stored in a RF Trap

Kazuhiko Sugiyama

Published Apr 1, 1999• 9 citations

Abstract

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.

DOI: 10.1143/jjap.38.2141

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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.

We demonstrate laser cooling of Yb/sup +/, produced by ionization of a natural isotope mixture, stored in an RF trap. One of the three major even isotopes was directly laser cooled and other isotopes were simultaneously trapped as a result of sympathetic cooling. It was possible to carry out laser cooling to the temperature at which the cloud-to-crystal phase transition occurs for /sup 174/Yb/sup +/, as observed in the case where enriched isotopes are used. We observed sudden drops of fluorescence owing to pumping to the /sup 2/F/sub 7/2/ state in the case of a small number of trapped Yb/sup +/ ions.>

The limitations to the achievement of low kinetic energies for laser cooling of single ions confined in electromagnetic traps are discussed. Sideband cooling of an ion in an rf (Paul) trap is reexamined including the effects of finite laser bandwidth and the energy of the rf micromotion. The micromotion is the oscillatory motion of the ion at the same frequency as the rf voltage applied to the trap electrodes. Sideband cooling of ions in a Penning trap is examined for the first time. In both cases, cooling to the zero-point energy of the ion in the trap should be possible and a method for verifying this condition is suggested. The implications for high-resolution, high-accuracy spectroscopy are investigated. Under certain conditions, the uncertainty in the second-order Doppler shift may be significantly less than 1 part in

{10}^{18}

. .AE

Singly ionized mercury ions have been stored in a miniature rf (Paul) quadruple ion trap and observed by means of resonance fluorescence. The trap electrode spacings (~0.5 mm) and rf voltage (~500 V at 20 MHz) were chosen1 so that the Lamb-Dicke criterion could be satisfied for optical transitions on a single-laser-cooled ion. The presence of a single ion was determined by a statistical loading method2; that is, the loading conditions were scaled down to a point where only occasionally would any fluorescence be observed after a loading cycle. With high probability, this fluorescence was due to a single ion. The fluorescence observed was due to spontaneous scattering from a 194-nm cw source (~3-μW power focused to a beam waist of ~20 μm) directed at the ion and tuned near the strongly allowed first resonance line.

A single Sr+ atom was confined in a radio-frequency quadrupole trap and laser cooled with resonant laser light. Examination of the transition Doppler profile indicated an ion kinetic temperature of less than 500 mK. Quantum jumps into and out of the metastable 4d(2)D(5/2) level were observed by laser excitation on the electric-quadrupole-allowed transition from the ground state at 674 nm. The dark and bright periods in the strong fluorescence showed variations owing to the applied driving field on the electric-quadrupole transition. These results provide an improved determination of tau = 372 +/- 25 msec for the natural metastable-state lifetime.