dark period in the fluorescence caused by the cooling laser. The $^2P_{1/2}$ excited state has a finite probability of decaying to the metastable $^2D_{3/2}$ state. This requires a repumping laser tuned to the $^2D_{3/2} - \,^2P_{1/2}$ transition that returns the ion to the cooling cycle.

A high scattering rate is crucial for both Doppler cooling and fluorescence detection of the trapped ion. It is compromised by the formation of dark states, which can be angular momentum eigenstates or superpositions of Zeeman sublevels of the $^2S_{1/2}$ and/or $^2D_{3/2}$ states (coherent population trapping, CPT [@Alzetta1976a; @Arimondo1996a]). CPT resonances between the $^2S_{1/2}$ and the $^2D_{3/2}$ states can be experimentally observed by sweeping the detuning of one of the laser beams [@Janik1985a; @Klein1990a; @Siemers1992a; @Kurth1995a; @Barwood1998a]. It is also known that optical pumping into dark superposition states between the $^2D_{3/2}$ sublevels can occur when the magnetic field is weak and that this can be prevented for example by modulating the polarization of the repumper [@Barwood1998a; @Berkeland1998b].

![\[fig:SrII\]Lowest energy levels of $^{88}\mathrm{Sr}^+$. Cooling and repumping transitions are shown as arrows together with their respective wavelengths.](Fig01-SrII-levels-gs){width="0.67\columnwidth"}

Berkeland and Boshier [@Berkeland2002a] have theoretically studied how to destabilize dark states in Zeeman degenerate systems using an external magnetic 