• The problem/issue being addressed?
The laws of quantum mechanics allow for fundamentally new applications, specifically for sensing and information processing. However, interactions with the environment easily destroy the fragile quantum states. Thus, engineering of controlled interactions between individual quantum systems, while isolating them from their environment, is paramount to realizing a quantum network and to harness the laws of quantum mechanics for new applications. In addition, the challenges that arise in creating large quantum systems are not only a technical nuisance that needs to be overcome but is also intimately related to the underlying laws of physics. Therefore, the development of novel quantum networks is both of fundamental interest and of practical importance. This project aims at engineering interactions between quantum systems in the solid state and thereby realizing a quantum network.
• Importance for society?
Already today, quantum technologies impact everyone’s lives. Take for instance the omnipresent laser. One can hardly overestimate the economic value that this technology is generating, ranging from scanners at the supermarket to eye surgery and optical welding. Similarly, the impact of large scale quantum networks will be huge. First applications will be most likely in sensing, for instance as a tool for materials characterization, which subsequently will trigger the development of new materials. More advanced applications will include quantum simulation of small molecules, for instance for drug development, or the implementation of quantum optimization algorithms that will solve some of today’s intractable problems.
• Overall objectives?
In particular for quantum information processing, it is desirable to realize a quantum network in the solid state that is compatible with established nano-fabrication techniques, since this allows to scale the technology to the size that is required for the most ambitious applications. A prominent solid-state quantum system is the so-called NV-center. The NV-center is a special defect in diamond with remarkable properties that make it very interesting for quantum applications. It behaves very similar to an atom that and, thus, possess all the essential elements for quantum science, including storage, logic, and communication of quantum information. Quantum information can be stored in the electron spin of the NV or the nuclear spin of nearby atoms, with second lifetimes even at room temperature. In addition, spin quantum information can be extracted via spin-dependent fluorescence intensity, resulting in a source of spin-photon entangled pairs.
The objective of this research is to connect spatially separate NV spins into a network. In principle, the NV’s natural spin-optical interface allows to establish this link optically. However, the optical transition is not very efficient. Our approach connects the NVs’ spins directly through a common mechanical mode. The spin-mechanical coupling is established via a magnet that is attached to the mechanical resonator. By coupling multiple NV spins to the same mechanical resonator mode, one can establish a long-range spin-spin interaction. We followed two complementary implementations. The first is based on nano-fabricated mechanical resonators. It is compatible with large scale integration and fabrication. The second uses magnetic levitation of nanomagnets. Here, the interaction strength is much stronger, and its position can be controlled in situ.
• Summary of dissemination
The results were published as 9 peer reviewed articles (incl. 4x Phys. Rev. Lett., 1 Nat. Comm.), 1 book chapter and 1 perspective and disseminated through more than 25 oral presentations at international conferences, workshops and seminars. A topical workshop was organized with 80 international participants. The public was informed via social media (twitter) and a radio interview.
