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Physica Scripta

Spatial fluorescence distribution and laser cooling of Ca+in a Paul trap

Th. Gudjons, F. Arbes, M. Benzing, Fritz Kurth, Günther Werth

Published Jan 1, 1995• 6 citations
Publisher

Abstract

Experiments on Ca* ions, confined in a Paul trap, deal with the spatial distribution of the fluorescence intensity, from which information on the velocity distribution of an ion cloud can be derived.We obtained laser cooling of a large (about lo4) ion cloud.For a few or single trapped ions crystallization, "dark resonances" and quantum jumps have been observed.

DOI
10.1088/0031-8949/1995/t59/054

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Experiments on Ca* ions, confined in a Paul trap, deal with the spatial distribution of the fluorescence intensity, from which information on the velocity distribution of an ion cloud can be derived.We obtained laser cooling of a large (about lo4) ion cloud.For a few or single trapped ions crystallization, "dark resonances" and quantum jumps have been observed.
A laser cooled ion crystal containing several hundred Ca+ ions has been stored in a linear Paul trap. Cooling is provided by a red detund laser at the 4S1/2−4P1/2 resonance transition. A second laser serves for repumping of those ions which decay from the excited 4P1/2 level to the metastable 3D3/2 state. The ions can be additionally excited by a third laser to a long lived metastable 3D5/2 energy level which decouples them from the cooling laser radiation. The light pressure acting upon the laser cooled ions pushes them into the direction of the laser beam. The ions in the metastable 3D5/2 state, however, do not experience any light pressure force and diffuse to the crystal side which points towards the cooling laser. Part of the crystal then appears dark. Depending on the number of ions in the crystal and on the fraction of the excited ions, the spatial separation of the ions in the metastable state and the ions in the cooling cycle can be as large as several hundred microns.
This paper reports on trapping and laser cooling of singly ionized strontium ions in a linear Paul trap. We describe the loading technique based on two-photon absorption of femtosecond pulses at a wavelength of 431 nm and we compare this technique to electron-bombardment ionization. We perform Doppler cooling of the ions in the few-ions regime as well as in the case of large clouds. The analysis of the fluorescence spectra and direct imaging of the cloud allows us to identify the Coulomb-crystal regime. These experiments open the way to the use of a large cold trapped-ion cloud as a medium for quantum optics and quantum information experiments.
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.
A cloud of laser-cooled 40 Ca + is successfully trapped and manipulated under well control in our home-built linear ion trap, which is designed and constructed solely for studying quantum information processing. By exploring the variation of the ion cloud with respect to the trap parameters, we have optimized the trapping condition and obtained very good fluorescence spectra. We observe the dynamics of the ion cloud, and estimate the temperature of the ion cloud to be of the order of milli-Kelvin.
A cloud of three-level ions (Ba+{\mathrm{Ba}}^{+}Ba+) confined in a Paul trap was illuminated with two lasers and the resulting fluorescence was observed as a function of the laser detunings from the ionic resonance. The observed spectra show features due to the oscillatory motion of the trapped ions. In order to describe the observed line shapes, calculations are presented in which the ion ensemble is separated in classes of velocity amplitudes of the oscillatory motion rather than velocity classes.