Hybrid quantum systems leverage the complementary strengths of different physical systems to overcome their intrinsic limitations. Rare-earth ion-doped crystals combined with silicon nanophotonic structures provide a particularly versatile platform. Erbium ions offer long optical and spin coherence times together with emission in the telecom C-band, corresponding to the lowest-loss window of optical fibers. At the same time, the solid-state host environment provides tunability and scalable integration. Mature silicon nanofabrication enables high-quality optical and mechanical resonators with strong field confinement.
Two complementary implementations are explored in this thesis. First, silicon photonic crystal cavities are combined with erbium-doped lithium niobate, enabling Purcell-enhanced single-photon emission from individual ions. The optical frequency of the ions is tuned via the linear Stark effect, a key step toward the generation of indistinguishable photons. Second, erbium ions are implanted directly in silicon nanostructures. The optical transition dipole properties are investigated under different magnetic-field regimes, an essential ingredient for optimizing cavity-ion coupling. A general framework is established to determine the transition dipole polarization in spin-1/2 solid-state defects, overcoming limitations of existing approaches. Furthermore, this method enables the determination of the strain-orbital coupling tensor, representing an important step toward coupling erbium spins to mechanical modes supported by our nanostructures.
Overall, this work establishes the experimental foundations for hybrid quantum systems based on erbium ions coupled to silicon nanostructures, providing key building blocks for efficient spin-photon and spin-phonon interfaces and opening new opportunities for nonlinear quantum optomechanics.

A tunable magnetic field at low temperatures is essential for numerous applications, including spintronics, magnetic resonance imaging, and condensed matter physics. While commercial superconducting vector magnets are available, they are complex, expensive, and often not adaptable to specific experimental needs. As a result, simple in-house designs are often being used in research environments. However, no comprehensive step-by-step guide for their construction currently exists. In this work, we provide a detailed manual for designing and building a cryogenically compatible three-axis vector magnet. The system is tested at the mixing chamber of a dilution refrigerator at temperatures ranging from 15 mK to 4 K, with no significant increase in base temperature. Safety measures are implemented to mitigate heating from quenching. The coils are successfully driven with DC currents as high as 3 A, generating magnetic fields of up to 2.5 T in the bobbin’s bore and 0.4 T at the sample position. Magnetic field measurements using Hall sensors demonstrate good agreement with the predictions of the designed performance.

Single quantum emitters embedded in solid-state hosts are an ideal platform for realizing quantum information processors and quantum network nodes. Among the currently investigated candidates, Er3+ ions are particularly appealing due to their 1.5 μm optical transition in the telecom band as well as their long spin coherence times. However, the long lifetimes of the excited state - generally in excess of 1 ms - along with the inhomogeneous broadening of the optical transition result in significant challenges. Photon emission rates are prohibitively small, and different emitters generally create photons with distinct spectra, thereby preventing multiphoton interference - a requirement for building large-scale, multinode quantum networks. Here we solve this challenge by demonstrating for the first time linear Stark tuning of the emission frequency of a single Er3+ ion. Our ions are embedded in a lithium niobate crystal and couple evanescently to a silicon nanophotonic crystal cavity that provides a strong increase of the measured decay rate. By applying an electric field along the crystal c axis, we achieve a Stark tuning greater than the ion's linewidth without changing the single-photon emission statistics of the ion. These results are a key step towards rare earth ion-based quantum networks.