Nanophotonics with Diamond Color Centers: Quantum Optics and Entanglement Protocols

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.