High-performance in-vacuum optical system for quantum optics experiments in a Penning-trap

Accurate measurements with implications in many branches in Physics have been accessed using Penning traps and conventional techniques within a temperature regime where each eigenmotion of a charged particle is still a classical harmonic oscillator. Cooling the particle directly or indirectly with lasers allows reaching the quantum regime of each oscillator, controlling subtle effects in the precision frontier by detecting photons instead of electric current. In this paper, we present a new in-vacuum optical system designed to detecting 397-nm fluorescence photons from individual calcium ions and Coulomb crystals in a 7-T Penning trap. Based on the outcome of computer simulations, our design shows diffraction-limited performance. The system has been characterized using a single laser-cooled ion as a point-like source, reaching a final resolution of 3.69(3) $\mu$m and 2.75(3) $\mu$m for the trap's axial and radial directions, respectively, after correcting aberrations.