Hybrid quantum circuits interfacing rare earth spin ensembles with microwave resonators are a promising approach for application as coherent quantum memory and frequency converter. In this thesis, hybrid circuits based on Er and Nd ions doped into Y SiO and YAlO crystals are investigated by optical and on-chip microwave spectroscopy. Coherent strong coupling between the microwave resonator and spin ensemble as well as a multimode memory for weak coherent microwave pulses are demonstrated.

While the universal quantum computer seems not in reach for the near future, this work focusses on analog quantum simulation of intriguing quantum models of light-matter interactions, with the goal of achieving a computational speed-up as compared to classical hardware. Existing building blocks of quantum hardware are used from superconducting circuits, that have proven to be a very suitable experimental platform for the implementation of model Hamiltonians at a high degree of controllability.

We worked on different magnetic molecules containing 3d and 4f magnetic centers. Their growth on metallic surfaces, topographies, spin states, magnetic properties and electron transport were locally investigated by using scanning tunneling microscopy (STM) at temperatures down to 30mK. The main achievement of this dissertation reveals the abrupt switching of crystal fields during formation of molecular contacts.

Die Schallgeschwindigkeit in metallischen Gläsern und die Permittivität von dielektrischen Aluminiumoxid-Schichten zeigen ein für amorphe Festkörper typisches Tieftemperaturverhalten. Akustische Messungen von wenigen kHz bis GHz zeigen den Einfluss der Leitungselektronen in einem massiven metallischen Glas. Messungen der Permittivität bei einigen kHz zeigen überaschenderweise ebenfalls einen Einfluss der Elektronen auf die Eigenschaften einer isolierenden Schicht. The speed of sound in metallic glasses and the permittivity of dielectric alumina layers show the typical low-temperature...

Within this work, the pairing mechanism of conventional (Pb) and unconventional superconductors (SrFe2(As1-xPx )2 , FeSe, FeSe/STO) was investigated experimentally by means of elastic and inelastic tunneling spectroscopy at temperatures down to 30 mK. The distinction between elastic and inelastic contributions to tunneling data was elaborated. The results help to identify conventional (phonon-mediated) and unconventional (e.g. spin- uctuation mediated) superconductivity.

This work presents the design and commissioning of a new low-temperature Scanning Tunnelling Microscope equipped with an innovative light collection setup using an integrated, micro-fabricated mirror tip. Commissioning experiments demonstrate the capabilities of this new instrument and reproduce known effects regarding gap plasmons on noble-metal surfaces. Furthermore, different contrasts in the plasmon-mediated light emission from Cobalt nano-islands on a Copper (111) substrate are reported.

We investigate superconducting granular aluminum (grAl) as a promising material for quantum circuits. We show that grAl strips can reach kinetic inductances up to nH/sq, while maintaining small microwave frequency losses. We identify excess quasiparticles as a limiting loss mechanism, and find quasiparticle relaxation times on the order of seconds. By fabricating a fluxonium quantum bit with a grAl superinductor, we demonstrate that grAl is a viable material for superconducting quantum circuits.

Despite tremendous progress of quantum computation with superconducting qubits, up-scaling for practical applications is hindered by decoherence and fluctuations induced by material defects. In this work, a qubit interface has been developed to study the microscopic nature of individual defects in a probe material. Further, a portable method has been developed to find locations of individual defects in ready-made qubit samples, which offers to test and improve micro-fabrication of qubits.