Laser cooling of trapped ytterbium ions using a four-level optical-excitation scheme.

Abstract

We demonstrate laser cooling of $^{172}\mathrm{Yb}^{+}$ ions in a rf quadrupole trap using an alternative optical-excitation scheme. The metastable 4${\mathit{f}}^{14}$5${\mathit{d}}^{2}$${\mathit{D}}_{3/2}$ state of ${\mathrm{Yb}}^{+}$, to which ions decay if the resonance (cooling) transition is driven, is depleted by driving an optical transition at a wavelength of 935 nm, which is much more attractive than the 2.438 \ensuremath{\mu}m radiation used previously. This couples the metastable state to a fourth energy level from which the ions decay rapidly to the ground state. Fluorescence line shapes of the cooling transition indicate that small numbers of ${\mathrm{Yb}}^{+}$ ions were cooled in this way to kinetic temperatures below 1 K.