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  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  and fibre coupling with near-unity collection efficiency , 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  to higher photon numbers. We will also take advantage of the system’s uniquely high operation (T = 20 K)  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 , thus providing substantial leaps forward towards establishing a European Quantum Internet.
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).
Monsieur Florian Kaiser (University of Stuttgart)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
attocube systems AG
US University of Stuttgart
INPHYNI Institut de Physique de Nice
"Alexandru Ioan Cuza" University of Iasi, Research Center on Advanced Materials and Technologies - RAMTECH
Help of the ANR 271,426 euros
Beginning and duration of the scientific project: April 2022 - 36 Months