Sympathetic cooling and detection of a hot trapped ion by a cold one

Abstract

We investigate the dynamics of an ion sympathetically cooled by another\nlaser-cooled ion or small ion crystal. To this end, we develop simple models of\nthe cooling dynamics in the limit of weak Coulomb interactions. Experimentally,\nwe create a two-ion crystal of Ca$^+$ and Al$^+$ by photo-ionization of neutral\natoms produced by laser ablation. We characterize the velocity distribution of\nthe laser-ablated atoms crossing the trap by time-resolved fluorescence\nspectroscopy. We observe neutral atom velocities much higher than the ones of\nthermally heated samples and find as a consequence long sympathethic cooling\ntimes before crystallization occurs. Our key result is a new technique for\ndetecting the loading of an initially hot ion with energy in the eV range by\nmonitoring the motional state of a Doppler-cooled ion already present in the\ntrap. This technique not only detects the ion but also provides information\nabout dynamics of the sympathetic cooling process.\n