ParisDiderot-Paris 7 -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.

This paper reports on photoionisation loading based on ultrafast pulses of singly-ionised strontium ions in a linear Paul trap. We take advantage of an autoionising resonance of Sr neutral atoms to form Sr+ by two-photon absorption of femtosecond pulses at a wavelength of 431nm. We compare this technique to electron-bombardment ionisation and observe several advantages of photoionisation. It actually allows the loading of a pure Sr+ ion cloud in a low radio-frequency voltage amplitude regime. In these conditions up to 4x10^4 laser-cooled Sr+ ions were trapped

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