Quantum technologies rely on materials that offer the central resource of quantum coherence, that allow one to control this resource, and that provide suitable interactions to create entanglement. Rare earth ions (REI) doped into solids have an outstanding potential in this context and could serve as a scalable, multi-functional quantum material. REI provide a unique physical system enabling a quantum register with a large number of qubits, strong dipolar interactions between the qubits allowing fast quantum gates, and coupling to optical photons – including telecom wavelengths – opening the door to connect quantum processors in a quantum network. This project aims at establishing individually addressable rare earth ions as a fundamental building block of a quantum computer, and to overcome the main roadblocks on the way towards scalable quantum hardware. The goal is to realize the basic elements of a multifunctional quantum processor node, where multiple qubits can be used for quantum storage, quantum gates, and for coherent spin-photon quantum state mapping. Novel schemes and protocols targeting a scalable architecture will be developed. The central photonic elements that enable efficient single ion addressing will be engineered into deployable technologies.

A quantum repeater-based internet can address our society’s need for secure communication and form the backbone for distributed quantum computing and sensing tasks [1,2]. In order to address the scalability challenge, three key technologies have yet to be demonstrated. On one side, quantum error correction [3] is required to maintain high quantum state fidelities in multi- node networks. On the other side, realistic rate improvements can be achieved by distributing quantum information with loss-resilient spin-entangled photonic cluster states [4,5]. Finally, these steps have to be integrated in robust, user-friendly and transportable systems. This project addresses these challenges using a solid-state system to demonstrate a quantum repeater, including spin-based quantum processing, and multi-photon state generation. Our advances in improving system robustness will be showcased by the integration into a real-field telecom fibre quantum link deployed over the French Riviera. The proposed quantum system is based on silicon vacancy (VSi) colour centres in semiconductor silicon carbide (SiC) [6–9]. Ground-breaking research from University of Stuttgart (US) and Linköping University (LIU) identified the system as truly unique as it combines all required features for demonstrating multi-spin- multi-photon quantum repeaters [6–9]. The expected technological advances are based on our waveguide integration of VSi centres [10] and fibre coupling with near-unity collection efficiency [11], which will be transferred into cryostats through synergies with the industrial partner Attocube Systems AG (AT). The collaboration between AT and the scientific partners will further lead to clear guidelines for next-generation compact, robust and transportable cryostat platforms, a critical requirement towards scalable quantum network architectures. The scientific goals comprise the improvement of our recent two-photon generation scheme [8] to higher photon numbers. We will also take advantage of the system’s uniquely high operation (T = 20 K) [12] to implement electron-nuclear spin control without the commonly observed heating-related issues. To make the VSi centre’s emission compatible with telecom networks, we will develop high- efficiency coherent quantum frequency converters [13,14] based on novel high index contrast lithium niobate (LN) devices, which has already been pioneered by the PIs from University of Iasi (UAIC) [15,16] and Université Côte d’Azur (INPHYNI). The unique synergies offered by the partners will allow us to embed an innovative and hybrid SiC-LN device into an inter-metropolitan fibre network. Besides the distribution of secret quantum keys, we will also show network-relevant quantum computational features, such as error correction and distribution of multi-photon states. Our quantum link will have a disruptive impact in the field of quantum communication and distributed quantum computing [17], thus providing substantial leaps forward towards establishing a European Quantum Internet. References 1. Wehner, S., et al., R. Science 362, 303 (2018). 2. Awschalom, et al., Nat. Photon. 12, 516 (2018). 3. Waldherr, G. et al. Nature 506, 204 (2014). 4. Borregaard, J. et al. Phys. Rev. X 10, 21071 (2020). 5. Michaels, C. P. et al. arXiv:2104.12619 (2021). 6. Nagy, R. et al. Phys. Rev. Appl. 9, 034022 (2018). 7. Nagy, R. et al. Nat. Commun. 10, 1954 (2019). 8. Morioka, N. et al. Nat. Commun. 11, 2516 (2020). 9. Nagy, R. et al. Appl. Phys. Lett. 118, 144003 (2021). 10. Babin, C. et al. arXiv2109.04737, Nat. Mater. to appear, (2021). 11. Bhaskar, M. K. et al. Nature 580, 60 (2020). 12. Udvarhelyi, P. et al. Phys. Rev. Appl. 13, 054017 (2020). 13. Tanzilli, S. et al. Nature 437, 116 (2005). 14. Kaiser, F. et al. Opt. Express 27, 25603 (2019). 15. Rambu, A. P. et al. J. Light. Technol. 36, 2675 (2018). 16. Rambu, A. P. et al. 8, 8 (2020). 17. Barz, S. et al. Science 335, 303 (2012).