The development of novel technologies, which are based on fundamental principles of quantum mechanics like superpositions or entanglement, is currently very actively pursued. Not only researchers, but also more and more companies devote considerable efforts to the realization of quantum computers, secure quantum communication networks, or quantum simulators for material science and chemistry applications. This development is pursed in parallel with a variety of different physical systems including, for example, single photons, trapped atoms and ions, defects centers in solids or superconducting quantum circuits. The interdisciplinary field of hybrid quantum systems aims at the integration of different optical, atomic and solid-state systems to harness their combined functionalities in an optimal way. For example, optical photons are excellent information carriers, while electronic spins and superconducting circuits are ideally suited for storing and processing quantum information, respectively. However, these systems do not naturally interact with each other and therefore new schemes for artificially interfacing quantum systems of different types must be explored.
In the current project we have theoretically analyzed a set of hybrid quantum systems involving superconducting circuits, electronic spins in solids and tiny mechanical resonators. The original objective was to investigate, how the combined functionalities of these system can be used for enhancing magnetometry applications and for realizing new types of quantum simulators for unconventional many-body interactions. In this context we have specifically investigated new efficient schemes for coupling a single electronic spin to the quantized motion of a mechanical nanoresonator. This interface constitutes a basic building block for hybrid quantum systems, where, for example, quantum information is stored in the spin-quantum memory, while the mechanical system is used as an interconnection to superconducting circuits or optical photons. We further showed, how superconducting circuits can be used to simulate light-matter interactions in the so-called ultrastrong coupling regime, which is not accessible with real atoms and photons. In this work we discovered a new quantum many-body effect, which was overlooked in related studies before and can be used as a natural entanglement resource. Finally, we found an unexpected noise-evasion mechanism for quantum communication schemes, which enables a faithful transmission of quantum information through noisy channels. This mechanism makes quantum communication over electric microwave channels possible, where otherwise the weak quantum signal would be washed out by an unavoidable background of thermal microwave photons.
In conclusion, in this project several important results for the further development of hybrid quantum systems and quantum technologies based on superconducting quantum circuits have been obtained. This concerns, in particular, our quantum communication protocol for noisy channels, which enables a completely approach for intra-city quantum networks based on microwave technology only.
