The project has fully achieved two out of the three originally proposed objectives, and made decisive steps towards achieving the third. This will be explained in detail in the following:
(I) The first objective of the proposal was the implementation of a quantum spin-photon interface at a telecommunication wavelength. To this end, we have integrated a thin membrane of yttrium orthosilicate into a cryogenic Fabry-Perot resonator. We could demonstrate that our approach preserves the coherence of the dopants (Merkel et al. Phys. Rev. X 2020), and is compatible with microwave control (Cova Fariña et al., Phys. Rev. Applied 2021) and dynamical decoupling to further extend the coherence time (Merkel et al. Phys. Rev. Lett. 2021). In addition, we pioneered the use of silicon as an alternative host material (Weiss et al. Optica 2021) that can give access to higher rates and improved coherence (Gritsch et al. Phys. Rev. X. 2022) and is compatible with commercial semiconductor manufacturing (Rinner et al. Nanophotonics 2023). The process of emitter integration into silicon has been patented (WO2022258204A1) and first steps towards commercialization are currently taken by the project members.
(II) The second objective was the multiplexing of many quantum bits in one device via frequency-selective addressing. We could demonstrate this both in YSO crystals (Ulanowski et al. Sci. Adv. 2022) and in silicon (Gritsch et al. Optica. 2023). Recent results have increased the number of frequency-multiplexed and Purcell enhanced qubits to the current world-record value of 350 (Ulanowski et al. Advanced Optical Materials 2024).
(III) The third objective of the proposal was the implementation of remote entanglement swapping. This goal has not been achieved yet because of unexpectedly large spectral diffusion of the emitters. However, by now we have identified mechanisms how this can be overcome, and we have demonstrated several key steps towards this end: 1) The world’s first optical initialization, readout and control of spins in silicon (Gritsch et al, in preparation), and 2), the first demonstration of these techniques with single nuclear spins (Ulanowski et al, in preparation). These advancements bring achieving the final goal of the objective within reach.
Taken together, our progress in the new platform makes the implementation of large-scale quantum networks and repeaters based on erbium dopants a realistic perspective for the coming years.
To disseminate the results, the group has written review articles for a broader audience in quantum science (Reiserer, Rev. Mod. Phys. 2022, Gonzalez-Tudela et al. Nat. Rev. Phys. 2023), as well as popular research articles for the general public (Reiserer, Physik in unserer Zeit, 2022). In addition, press releases by the institutes and videos distributed via the social media have disseminated the project ideas and results to a broad audience.
