The progress in optical spectroscopy has made it is possible to study individual quantum emitters. However, only a few select “bright” emitters have been detected so far, leaving a large gap in the choice of critical parameters such as wavelength, coherence time, and energy level schemes. In this project, we develop methods for the detection of single emitters with long fluorescence lifetimes. In particular, we concentrate on rare earth ions embedded in crystals, which are of great technological and fundamental interest. To achieve this goal, we exploit methods from ultrahigh resolution microscopy, laser spectroscopy, scanning probe technology, cavity quantum electrodynamics, and plasmonics.The first approach to the detection of single ions at cryogenic temperatures will be to perform direct fluorescence excitation as well as absorption spectroscopy to address single Pr3+ ions spectrally within the inhomogeneous line of the sample. Here, we will develop a tunable laser system with sub-kHz linewidth for probing the narrow transitions of the ions. We expect a signal-to-noise ratio of about 10 in this first step. In order to improve this, we will enhance the emission of ions by pursuing two strategies. In the first case, we shall embed doped crystalline films in monolithic Bragg microcavities. In the second approach, we use plasmonic nanoantennas to reduce the radiative lifetime of the ions in the near field. The well-defined energy levels of ions provide ways for the preparation of long-lived coherent states for use in quantum information processing. Furthermore, access to the homogeneous spectra of ions at different temperatures and doping concentrations will shed light on fundamental open questions regarding their interaction with their matrices.