The advent of the quantum gas microscope allowed for the in situ probing of ultracold gaseous matter on an unprecedented level of spatial resolution. However, the study of phenomena on ever smaller length scales, as well as the probing of three-dimensional systems, is fundamentally limited by the wavelength of the imaging light for all techniques based on linear optics. Here, we report on a high-resolution ion microscope as a versatile and powerful experimental tool to investigate quantum gases. The instrument clearly resolves atoms in an optical lattice with a spacing of 532 nm over a field of view of 50 sites and offers an extremely large depth of field on the order of at least $70\mu \mathrm{m}$. With a simple model, we extract an upper limit for the achievable resolution of approximately 200 nm from our data. We demonstrate a pulsed operation mode enabling 3D imaging and allowing for the study of ionic impurities and Rydberg physics.

We investigate the dynamics of an ion sympathetically cooled by another laser-cooled ion or small ion crystal. To this end, we develop simple models of the cooling dynamics in the limit of weak Coulomb interactions. Experimentally, we create a two-ion crystal of Ca⁺ and Al⁺ by photo-ionization of neutral atoms produced by laser ablation. We characterize the velocity distribution of the laser-ablated atoms crossing the trap by time-resolved fluorescence spectroscopy. We observe neutral atom velocities much higher than the ones of thermally heated samples and find as a consequence long sympathethic cooling times before crystallization occurs. Our key result is a new technique for detecting the loading of an initially hot ion with energy in the eV range by monitoring the motional state of a Doppler-cooled ion already present in the trap. This technique not only detects the ion but also provides information about the dynamics of the sympathetic cooling process.

Experiments on one- and two-photon laser excitation of 232Th+ ions in a radio-frequency ion trap are reported. As the first excitation step, the strongest resonance line at 402 nm from the (6d27s)J=3/2 ground state to the (6d7s7p)J=5/2 state at 24874~cm$-${}1 is driven by radiation from an extended-cavity diode laser. Spontaneous decay of the intermediate state populates a number of low-lying metastable states, thus limiting the excited state population and fluorescence signal obtainable with continuous laser excitation. We study the collisional quenching efficiency of helium, argon, and nitrogen buffer gases, and the effect of repumping laser excitation from the three lowest-lying metastable levels. The experimental results are compared with a four-level rate equation model, which allows us to deduce quenching rates for these buffer gases. Using laser radiation at 399 nm for the second step, we demonstrate two-photon excitation to the state at 49\,960~cm$-${}1, among the highest-lying classified levels of Th+. This is of interest as a test case for the search for higher-lying levels in the range above 55\,000~cm$-${}1 which can resonantly enhance the excitation of the 229Th+ nuclear resonance through an inverse two-photon electronic bridge process.

Using resonant two-step laser excitation of trapped ${}^{232}$Th${}^{+}$ ions, we observe 43 previously unknown energy levels within the energy range from $7.3$ to $8.3$ eV. The high density of states promises a strongly enhanced electronic bridge excitation of the ${}^{229m}$Th nuclear state that is expected in this energy range. From the observation of resonantly enhanced three-photon ionization of Th${}^{+}$, the second ionization potential of thorium can be inferred to lie within the range between 11.9 and 12.3 eV. Pulsed laser radiation in a wide wavelength range from 237 to 289 nm is found to provide efficient photodissociation of molecular ions that are formed in reactions of Th${}^{+}$ with impurities in the buffer gas, leading to a significantly increased storage time for Th${}^{+}$ in the ion trap.