[en] Full text: Optical transitions of erbium in silica enable telecom communication world wide and erbium in crystals such as YSO is a key candidate for quantum memories and repeater applications. Erbium embedded in silicon retains the same attractive telecommunication transitions but shows a broad spectrum due to large inhomogenous broadening. Here we present a technique based on optical excitation of erbium in silicon combined with single electron charge sensing of sufficiently high resolution that single sites can be identified in the broad spectrum. Different optical transitions and ions can be addressed in one device based on frequency selection with a narrow excitation source. Magnetic field rotation spectra allow the identification of the site symmetry. As an example, the spectrum of two coupled erbium ions is presented. Here the high resolution spectrum allows the direct comparison and parametrisation of a simple model with dipolar magnetic coupling. In addition, the excitation process is analysed based on sub 1us excitation pulses combined with single shot latched readout. Finally, the concept of erbium ions utilised as atomic-scale sensors is presented with high-resolution strain and electric field measurements that enable metrology applications in nano-transistors. (author)