Control of the optical interface of color centers in diamond

Abstract

Quantum networks offer capabilities beyond the reach of classical systems, enabling intrinsically secure communication, enhanced sensing, and the sharing of quantum states for distributed quantum computation. Optically active spins in solids, where stationary qubits are entangled with flying photonic qubits, enable the creation of entangled links for transmitting quantum information over large distances. However, realizing such links at high rates and with high fidelity remains a central challenge, while both are essential for scaling to practical quantum applications. Although the nitrogen-vacancy (NV) center in diamond has been extensively studied for such networks, its limited coherent photon emission rate constrains entanglement generation rates and thus network scalability. The tin-vacancy (SnV) center in diamond offers significant advantages: it intrinsically emits a higher fraction of coherent photons, and its inversion symmetry renders it first-order insensitive to charge noise, enabling integration into nanophotonic structures that can further enhance its coherent emission. This makes the SnV center a promising platform for building large-scale quantum networks.
This thesis addresses four major challenges in realizing scalable quantum networks with SnV centers. First, to further enhance the coherent photon emission, a novel laser-cutting technique is developed to realize micrometer-thin diamond devices, which are laser-cut into arbitrary lateral shapes. These devices are bonded to cavity mirrors and placed inside a cryogenic, fiber-based Fabry-Pérot microcavity, which can increase the coherent emission and thus the entanglement rate. The optical properties of the embedded color centers remained preserved during the fabrication.
In the second project, high-fidelity initialization of the negatively charged state and optical transition frequency is achieved using a real-time logic decision scheme based on photon counting during resonant excitation, enabling heralded initialization and improved optical coherence verified by optical Ramsey interferometry, as well as tuning of the optical frequency over the inhomogeneous linewidth of an individual SnV center.
Third, local strain engineering of suspended diamond waveguides allows shifting of optical resonances of SnV centers over a significant portion of the inhomogeneous distribution, while real-time feedback on the applied strain stabilizes the resonance frequency and mitigates spectral wandering over time. This allows for the generation of indistinguishable photons from different SnV centers.
Lastly, a highly efficient, low-noise quantum frequency converter is implemented to shift single photons from the visible 619 nm to 1480 nm in the telecom S-band, enabling low-loss transmission of photons entangled with the spin of the SnV center over long distances. Together, these advances move the SnV center closer to practical deployment in large-scale quantum networks.