The results of the research re-ported in this work show that tunable gap flux qubits have a potential for building quantum registers. Cavities coupled to flux qubits can be used for in-formation storage and transfer between qubits. SFS π-shifters provide a simple approach to bias multi-qubit circuits. A possibility to change the qubit resonance frequency while preserving qubit coherence enables implementation of switchable coupling between qubits and cavities.

Quantum sensing is a vast and emerging field enabling in-situ studies of quantum systems and hence the development of quantum hybrid systems. This work creates the fundament of direct superconducting-magnetic hybrid systems by developing a local microwave sensing scheme and studying the influence of a static magnetic field on a superconducting qubit. Finally, a proof-of-principle hybrid system is demonstrated, which opens the path towards superconducting-magnetic quantum circuits.

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.

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 thesis deals with the nonlinear aspects of superconducting quantum interference devices (SQUIDs) as magnetic meta-atoms. Such meta-atoms are usually resonant structures that constitute the basic building blocks of a metamaterial with the purpose of giving the material unconventional magnetic properties. Due to their intrinsic nonlinearity, SQUIDs exhibit a number of phenomena such as frequency tunability and multi-stability that make them attractive as controllable meta-atoms.

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.

Nowadays, the development of magnetic data storage devices technique faces fundamental limitations which lead to a slow-down of the increase of areal storage density. Therefore, magnetoelectric coupling which allows to change the magnetic state of matter by means of an electric field attracted significant interest. In this work, scanning tunneling microscopy was used to show for the first time that magnetic information can be written on the nanometer scale by the application of electric fields.

In this work, magnetic atoms on surfaces are studied with low-temperature scanning tunneling microscopy. Motivated by the idea to use single atoms as magnetic bits, the factors that allow or prevent long-term stability of their magnetic moments are investigated. Lifetimes of up to several minutes can be achieved for the magnetic moments of holmium atoms on a Pt(111) surface, resulting from the combined symmetries of the system. Corresponding theoretical calculations are presented and evaluated.

In this book, hybrid systems based on yttrium-iron-garnet (YIG), three dimensional microwave cavity resonators, and superconducting transmon qubits, are investigated by continuous wave and pulsed microwave spectroscopy. Limitations to the magnetic linewidth in the quantum regime are identified and coherent exchange between a magnon and a superconducting qubit are demonstrated. Finally, a first step towards a strongly coupled hybrid system containing all three components is demonstrated.