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Chinese Physics Letters

Background-Free Doppler Cooling of Trapped Ions Using Quadrupole Transition

Fei Zhou, Yi Xie, Liang Chen, Wei Wan, Haoyu Wu, Mang Feng

Published Mar 1, 2013• 4 citations
Publisher

Abstract

We demonstrate experimentally the Doppler cooling of 40 Ca + ions using the S 1/2 → D 3/2 electric quadrupole transition, in which the scattering light that is much different from the exciting light makes it possible to collect background-free fluorescence. The idea excluding 397 nm excitation has the advantage of high-sensitivity detection and only employs lasers in red and infrared wavelengths, which should be practical for trapped-ion quantum information processing.

DOI
10.1088/0256-307x/30/3/033701

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•Dec 9, 2006•0 citations
We demonstrate experimentally the Doppler cooling of 40 Ca + ions using the S 1/2 → D 3/2 electric quadrupole transition, in which the scattering light that is much different from the exciting light makes it possible to collect background-free fluorescence. The idea excluding 397 nm excitation has the advantage of high-sensitivity detection and only employs lasers in red and infrared wavelengths, which should be practical for trapped-ion quantum information processing.
We demonstrate the ability to load, cool and detect singly charged calcium ions in a surface electrode trap using only visible and infrared lasers for the trapped-ion control. As opposed to the standard methods of cooling using dipole-allowed transitions, we combine power broadening of a quadrupole transition at 729 nm with quenching of the upper level using a dipole allowed transition at 854 nm. By observing the resulting 393 nm fluorescence we are able to perform background-free detection of the ion. We show that this system can be used to smoothly transition between the Doppler cooling and sideband cooling regimes, and verify theoretical predictions throughout this range. We achieve scattering rates which reliably allow recooling after collision events and allow ions to be loaded from a thermal atomic beam. This work is compatible with recent advances in optical waveguides, and thus opens a path in current technologies for large-scale quantum information processing. In situations where dielectric materials are placed close to trapped ions, it carries the additional advantage of using wavelengths which do not lead to significant charging, which should facilitate high rate optical interfaces between remotely held ions.
Doppler cooling of calcium ions has been experimentally demonstrated using the ${S}_{1∕2}\ensuremath{\rightarrow}{D}_{5∕2}$ dipole-forbidden transition. Scattering forces and fluorescence levels a factor of 5 smaller than for the usual Doppler cooling on the dipole-allowed ${S}_{1∕2}\ensuremath{\rightarrow}{P}_{1∕2}$ transition have been achieved. Since the light scattered from the ions can be monitored at (violet) wavelengths that are very different from the excitation wavelengths, single ions can be detected with an essentially zero background level. This, as well as other features of the cooling scheme, can be extremely valuable for ion-trap-based quantum-information processing.
The electric quadrupole transition of confined Ca/sup +/ ions has been proposed as a frequency standard in the visible domain. To access metrologic conditions, we have built a cylindrical miniature trap, allowing to obtain high frequencies of motion and preparing the access to the Lamb-Dicke regime where the first order Doppler broadening is suppressed. Laser cooling and interrogation of the confined ions require lasers at 397 nm and at 729 nm. We propose the generation of these wavelengths using multimode laser diodes and a frequency-doubling set-up.
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.
The National Institute of Information and Communications Technology in Japan is proceeding with development of an optical frequency standard using an electric quadrupole transition of Ca+ ions in a small radio-frequency trap. The single, trapped 40Ca+ ions are laser cooled to a temperature of <10 mK using extended-cavity diode lasers at 397 nm and 866 nm. In order to measure the electric quadrupole transition, the linewidth of the clock laser diode at 729 nm is reduced to ~66 Hz using an ultra-low-expansion (ULE) cavity. We observed the electric quadrupole transition with a linewidth of ~600 kHz, together with the ionic motional sidebands. We are also developing optical frequency combs without photonic fibers broadening the spectrum. The experimental is now improved in order to observe the narrower linewidth of the quadrupole transition.