Trapping and cooling of Sr+ ions: strings and large clouds

Large and cold ion clouds are very promising media for quantum information processing and quantum memories [1,2]. We report on the trapping and laser cooling of large clouds of singly ionized strontium ions in a linear Paul trap. We describe our loading technique based on two-photon absorption of femtosecond pulses and compare it to electron-bombardment ionization. The sample are Doppler-cooled to form Coulomb crystals both in the few-ion regime and in the case of large clouds. Our setup is designed, in particular, to obtain optically dense clouds in the perspective of an ion-based quantum memory for continuous variables [2]. We present absorption measurements, that are consistent with the estimated number of ions present in the trap. These experiments open the way to the use of a large cold trapped-ion cloud for the realization of a long-lived quantum memory. \begin{thebibliography}{99} \vspace{-.4cm} \bibitem{Drewsen} Herskind P et al., Appl. Phys. B 93 373 (2008) \vspace{-.2cm} \bibitem{our2009} Coudreau T et al., J. Phys. B: At. Mol. Opt. Phys. 40 413 (2007) \end{thebibliography}