All-optical sub-amu ion mass spectrometry*

Ions that are difficult to laser cool, and hence cannot be detected with fluorescence, are promising for fundamental physics research and quantum information science. We present a method to rapidly and nondestructively identify trapped dark ions by measuring the motional frequencies of dark ions co-trapped with laser-coolable bright ions. We use the $S_{1/2}$, $P_{1/2}$ and $D_{3/2}$ level structure of $\mathrm{Sr}^+$, and separately $\mathrm{Ra}^+$, to controllably amplify the ion’s motion by modifying the optical spectrum with coherent population trapping. The amplified ion motion modulates the scattered light, which we collect and Fourier transform to extract the motional frequencies, and from this the dark ion mass. The technique can be used with ions that have a lambda level structure, such as $\mathrm{Ca}^+$ and $\mathrm{Ba}^+$, and only utilizes the two lasers already required for laser cooling.