A large-scale quantum network, where many nodes are connected via entanglement and can share and process quantum information, will allow applications that are now unattainable. While some are known, ranging from distributed quantum computing to enhanced quantum sensing and quantum communication, the full potential of such a fundamentally new technology is yet to be discovered. Color centers in diamond, with their excellent optical properties, long spin coherence times, and versatile control over local nuclear spins, are a leading platform for building quantum network nodes and recently allowed crucial proof-of-principle experiments. Going beyond experiments and towards a large and functional network requires faster entanglement generation and a scalable architecture. Integration of color centers in nanophotonic structures paves the way to solving these challenges, enhancing the interface between the spins and photons that carry quantum information across the network. Furthermore, combining diamond nanophotonic devices with integrated photonic and electronic circuits is a promising way to realize a large-scale quantum system. This thesis is about integrated spin-photon interfaces in diamonds and how to generate entanglement between them and includes experimental and theoretical results toward this goal. First, we introduce the Group-IV color centers in diamond, which thanks to their symmetry properties are robust against electric field noise and compatible with integration in nano-scale structures. Focusing on the tin-vacancy (SnV) center, we discuss the main features and effects that play a role in its use as a spin-photon interface. We then present experimental results going from the fabrication and characterization of bulk diamond samples with SnV centers to the integration in nanophotonic waveguides, where we show stable and coherent optical lines and measure the extinction of the transmission signal caused by a single SnV center with contrast up to 35%. Then, we investigate the interaction between a single SnV center and a weak coherent light field in a single-mode waveguide. We perform spectroscopy of the transmitted and reflected signals, and demonstrate the single-photon nature of the interaction by measuring the effect on the photon statistics in both fields. Finally, we introduce a theoretical framework for photon-mediated entanglement generation protocols between spin-based quantum systems. This allows us to understand, categorize, and construct entanglement protocols in terms of abstract building blocks, that can be combined with hardware modeling for a more detailed description of the protocols. To showcase the framework, we analyze three different entanglement protocols, considering silicon-vacancy (SiV) centers coupled to photonic crystal cavities as the hardware, and we quantitatively compare them using a software package built to match the structure of the framework.@en

We fabricate single tin-vacancy centres in diamond, we perform spectroscopy and coherent population trapping to verify optical driving of the spin states. We investigate the integration in diamond waveguides to realise an efficient spin-photon interface.@en

The generation of entanglement between distant quantum systems is at the core of quantum networking. In recent years, numerous theoretical protocols for remote-entanglement generation have been proposed, many of which have been experimentally realized. Here, we provide a modular theoretical framework to elucidate the general mechanisms of photon-mediated entanglement generation between single spins in atomic or solid-state systems. Our framework categorizes existing protocols at various levels of abstraction and allows for combining the elements of different schemes in new ways. These abstraction layers make it possible to readily compare protocols for different quantum hardware. To enable the practical evaluation of protocols tailored to specific experimental parameters, we have devised numerical simulations based on the framework with our codes available online.@en

Diamond tin-vacancy centers have emerged as a promising platform for quantum information science and technology. A key challenge for their use in more-complex quantum experiments and scalable applications is the ability to prepare the center in the desired charge state with the optical transition at a predefined frequency. Here we report on heralding such successful preparation using a combination of laser excitation, photon detection, and real-time logic. We first show that fluorescence photon counts collected during an optimized resonant probe pulse strongly correlate with the subsequent charge state and optical-transition frequency, enabling real-time heralding of the desired state through threshold photon counting. We then implement and apply this heralding technique to photoluminescence-excitation measurements, coherent optical driving, and an optical Ramsey experiment, finding strongly increased optical coherence with increasing threshold. Finally, we demonstrate that the prepared optical frequency follows the probe laser across the inhomogeneous linewidth, enabling tuning of the transition frequency over multiple homogeneous linewidths.@en