Laser Cooling of Ca+ with an External-Cavity Ultraviolet Diode Laser

Abstract
We have built an external-cavity ultraviolet diode laser at 397 nm for Ca+ spectroscopy. The laser consists of a commercial ultraviolet diode laser and a holographic grating in the Littrow configuration. Stable single-mode operation with a power of 3 mW, a linewidth no larger than 2 MHz, and a mode-hop-free frequency scanning range of 15 GHz was achieved. This laser was used to cool a small number of Ca+ in a rf trap. The ions were cooled to a low temperature to form a crystallized state. The use of the laser will simplify experimental setups for diverse applications of Ca+, such as frequency standards, quantum computers and one-atom lasers.