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Optics Letters

Laser cooling of calcium ions by use of ultraviolet laser diodes: significant induction of electron-shelving transitions

Kenji Toyoda, A. Miura, S. Urabe, K. Hayasaka, Masayoshi Watanabe

Published Dec 1, 2001• 31 citations
Publisher

Abstract

Calcium ions are laser cooled in a Paul trap by use of a grating-stabilized UV laser diode. Unusual quantum jumps or fluctuations in the fluorescence signal are observed without application of any shelving lasers. It has become clear that such phenomena can be ascribed to shelving transitions induced by background 393-nm spontaneous photons emitted from the laser medium. Shelving transitions are suppressed by use of an interference filter, and the ions are successfully cooled to crystallization temperature with sufficient reproducibility.

DOI
10.1364/ol.26.001897

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Calcium ions are laser cooled in a Paul trap by use of a grating-stabilized UV laser diode. Unusual quantum jumps or fluctuations in the fluorescence signal are observed without application of any shelving lasers. It has become clear that such phenomena can be ascribed to shelving transitions induced by background 393-nm spontaneous photons emitted from the laser medium. Shelving transitions are suppressed by use of an interference filter, and the ions are successfully cooled to crystallization temperature with sufficient reproducibility.
An absolutely stabilized laser diode system for high-resolution spectroscopy of the 4 2 S 1/2 –3 2 D 5/2 electric-quadrupole-allowed transition in calcium ions at 729 nm is developed. Both its short-term and long-term stabilities are examined. A system for laser cooling of single calcium ions using an ultraviolet laser diode for excitation of the 4 2 S 1/2 –4 2 P 1/2 transition is also constructed, and by using it we have observed quantum jumps of single calcium ions with a high stability and a high signal-to-background ratio.
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 have built an external-cavity ultraviolet diode laser at 397 nm for Ca+ spectroscopy. The laser consists of a commercial ultraviolet diode laser and a holographic grating in the Littrow configuration. Stable single-mode operation with a power of 3 mW, a linewidth no larger than 2 MHz, and a mode-hop-free frequency scanning range of 15 GHz was achieved. This laser was used to cool a small number of Ca+ in a rf trap. The ions were cooled to a low temperature to form a crystallized state. The use of the laser will simplify experimental setups for diverse applications of Ca+, such as frequency standards, quantum computers and one-atom lasers.
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.