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プラズマ・核融合学会誌

Laser-Cooled Ions and Their Applications

Shinji Urabe

Published Jan 1, 2005• 1 citation
Publisher

Abstract

In an iontrap,laser-cooled ions can be confined in a small space in an ultra-high vacuum so that collisions rarely happen and the ions are well isolated from the environment. Since these conditions enable high-resolution spectroscopy of single ions, laser-cooled ions are suitable for use in developing frequency standards. The ions can be cooled to the motional ground state to give a well-defined initial quantum state for quantum state manipulation and quantum computing. In this review, principles of ion traps and laser cooling, and recent progress in optical frequency standards and quantum computing with trapped ions are described.

DOI
10.1585/jspf.81.755

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In an iontrap,laser-cooled ions can be confined in a small space in an ultra-high vacuum so that collisions rarely happen and the ions are well isolated from the environment. Since these conditions enable high-resolution spectroscopy of single ions, laser-cooled ions are suitable for use in developing frequency standards. The ions can be cooled to the motional ground state to give a well-defined initial quantum state for quantum state manipulation and quantum computing. In this review, principles of ion traps and laser cooling, and recent progress in optical frequency standards and quantum computing with trapped ions are described.
Single ions trapped and laser cooled provide ideal objects for high resolution spectroscopy and frequency standards. For the latter purpose we are investigating the 5s2 1S0 → 5s5p 3P0 transition of In+. With a natural linewidth of only 1.1 Hz this resonance offers very high resolution and is highly immune to frequency shifts due to external electromagnetic fields, because it connects two levels with vanishing electronic magnetic momenta [1]. The wavelength of this clock transition is 236.5 nm and is technically very convenient, since it coincides with the fourth harmonic of the 946 nm Nd:YAG laser line. So this intrinsically frequency stable solid-state laser can be used to excite the transition [2]. For laser cooling and fluorescence detection of the indium ion the 5s2 1So -> 5s5p 3P1 transition at 230.6 nm can be employed. The use of the relatively narrow intercombination line for laser cooling allows us to study optical sideband cooling in the strong binding regime, where the oscillation frequencies of the ion in the trap (around 1 MHz) are larger than the optical linewidth of 360 kHz. In this parameter range laser cooling is possible to the quantum ground state of the vibrational motion in the trap corresponding to a temperature of roughly 100 μK. The same low temperature was obtained with sideband cooling of a small Coulomb crystal consisting of two ions, creating an interesting new quantum few-particle system.
Trapped and laser-cooled ions are increasingly used for a variety of modern high-precision experiments, for frequency standard applications, and for quantum information processing. Therefore laser cooling of trapped ions is reviewed, the current state of the art is reported, and several new cooling techniques are outlined. The principles of ion trapping and the basic concepts of laser cooling for trapped atoms are introduced. The underlying physical mechanisms are presented, and basic experiments are briefly sketched. Particular attention is paid to recent progress by elucidating several milestone experiments. In addition, a number of special cooling techniques pertaining to trapped ions are reviewed; open questions and future research lines are indicated.
Single trapped ions represent elementary quantum systems that are well isolated from the environment. They can be brought nearly to rest by laser cooling, and both their internal electronic states and external motion can be coupled to and manipulated by light fields. This makes them ideally suited for quantum-optical and quantum-dynamical studies under well-controlled conditions. Theoretical and experimental work on these topics is reviewed in the paper, with a focus on ions trapped in radio-frequency (Paul) traps.
Experimental research toward the realization of an optical frequency standard of high accuracy is briefly reviewed. The studies concentrated on single, laser-cooled ions since they offer several attractive features for the achievement of high accuracy. These features include long storage time which eliminates transit-time broadening, confinement that is nearly nonperturbative to the internal level structure of the ion, laser-cooling that reduces motional shifts to small values, and electron-shelving whereby transitions to long-lived states can be detected with unit probability. The authors studied spectroscopically the electric-quadrupole allowed transition at 282 nm in a single, laser-cooled /sup 199/Hg/sup +/ ion stored in an RF Paul trap with extremely high resolution. The measured linewidth is limited by the spectral purity of the laser to about 80 Hz. Possible improvements and future directions are discussed.>
Transitions between discrete energy levels in well-isolated atomic systems are highly reproducible and therefore make ideal references for highly accurate frequency standards or clocks. In particular, a single laser-cooled ion confined in a radiofrequency ion trap closely approximates the spectroscopic ideal of a particle at rest in a perturbation-free environment. Narrow optical transitions in such systems are therefore very promising for the realization of optical clocks with accuracy around 2 orders of magnitude higher than current microwave primary standards. In this paper the fundamental aspects, current performance and applications of trapped ion optical clocks are reviewed and the potential for future improvements is considered.
New developments in spectroscopy and quantum optics with stored ions are summarized and discussed. Currently used ion trap devices are briefly described, and newly proposed cooling techniques are indicated. Recent results in precision spectroscopy are reviewed with particular emphasis on time and frequency standards. New applications of stored ions in the field of quantum optics are proposed such as the preparation of nonclassical states of motion in an ion trap. These can be detected by observation of the quantum collapse and revival phenomenon.