This is a schematic representation of the experimental set-up for photon-recoil spectroscopy. An auxiliary ion (the logic ion, blue) is trapped in an ion trap together with the ion to be investigated (the spectroscopy ion, red). The logic ion is cooled down to the ground state of motion by means of laser cooling (cooling laser and logic laser). Due to the strong coupling of the two ions, the spectroscopy ion is cooled along with the logic ion. The spectroscopy ion is then excited by means of laser pulses (spectroscopy/repump laser), which puts both ions into motion. This motion represents the spectroscopy signal and can be read out via the logic ion (logic laser, cooling laser) which becomes dark in the event of motion.