Spin-photon interfaces play a crucial role in the realization of large-scale quantum computation and quantum communication. In this thesis, we investigate two types of spin-photon interfaces, Group-IV color centers in diamond and trapped Rubidium atoms, and evaluate their potential to enable long-distance quantum communication and modular quantum computation.

The technology of building quantum networks by interconnecting multiple quantum nodes has the potential to revolutionize the world, much like its classical equivalent - the internet - did. The possibility of processing quantum information on a shared network promises exciting applications that are not possible with today’s technologies. Quantum systems with stationary qubits and qubit-photon interfaces, like diamond color centers, are prime candidates for these quantum network nodes. Here, the qubit-photon interface efficiency is crucial for their connectivity, which propels the development of cavity systems that enhance this interface via the Purcell effect. This dissertation presents advances in coupling nitrogen-vacancy (NV) and tin-vacancy (SnV) centers in diamond to open microcavities and explores their capabilities for quantum networks.
The basis for experiments with cavity-coupled color centers is laid by the construction of a cryogenic fiber-based open microcavity system. This system uses a flat sample mirror opposite to a laser-ablated spherical fiber mirror to confine optical cavity modes. It is designed to achieve a low passive cavity length fluctuation level, microwave integration as well as full optical access through the fiber-mirror and free-space via the sample mirror. A closed-cycle optical cryostat hosts this system and enables continuous operation in a controlled high-vacuum environment.
Besides the microcavity developments, a novel patterning method for the fabrication of micrometer-thin diamond membranes is presented. The method involves laser-cutting to pattern diamonds with micrometer-scale feature sizes and subsequent bonding to a sample mirror. Comparing the laser-cutting method to established electron-beam lithography and a two-step transfer pattern process with a silicon nitride hard mask validates the fabrication of high-quality diamond devices for microcavity applications.
By integrating a diamond device hosting SnV centers into the cryogenic microcavity system, single cavity-coupled SnV centers are investigated. The coherent coupling regime is reached as a result of the achieved Purcell enhancement and the coherence of the optical transition. The coupled system of SnV center and cavity exhibits quantum nonlinear behavior, as evidenced by dips in the cavity transmission spectrum and changes in the photon statistics of the transmitted light. These effects can be exploited in remote entanglement protocols, underlining the potential of these systems to serve as quantum network nodes with Purcell-enhanced photonic interfaces.
Moreover, the cryogenic microcavity system is employed to equip NV centers with efficient qubit-photon interfaces. The cavity-coupling is used in combination with a crosspolarized resonant excitation and detection scheme to initialize and read out the NV center electron spin qubit. In addition, the electron spin is coherently controlled with on-chip delivered microwave pulses, and pulsed resonant excitation enables the generation of spin-photon correlated states. The quantum networking capabilities of the system are demonstrated by measuring heralded Z-basis correlations between photonic time-bin qubits and the spin qubit. In these experiments, a tenfold improvement in resonant photon detection probability is achieved over state-of-the-art NV center quantum network nodes, paving the way for cavity-enhanced quantum networking with NV centers.

The Quantum Internet is a complementary tool to the widely spread classical Internet, which has already revolutionized our everyday life. The promise is that the Quantum Internet will unlock new unprecedented capabilities and applications that span from secure communication, to distributed quantum computation and enhanced quantum sensing. The realization of such a powerful tool is the result of a joint effort among several fields, like computer science, physics, engineering, and materials science, which all rely on the fundamentals of quantum mechanics. The introduction of a new computational unit, the qubit, allows for the creation of superposition and entangled states, and the possibility of measuring such states. On a practical level, we can envision the Quantum Internet as a network of interconnected heterogeneous platforms aimed at solving different tasks, such as the processing of quantum information at the end nodes, and the storing and retrieval of quantum information in between end nodes to bridge long distances. The quantum information routing is governed and optimized by a dedicated software architecture that facilitates the user interface, removing the requirement of knowing the hardware’s physical principles for a general user.
In the hardware framework, the Nitrogen-Vacancy center in diamond represents a viable platform as processing end node, thanks to the high quality of its qubits and the capability of generating remote entanglement with other nodes in the network via its optical interface. These properties can be engineered to utilize the NV center as a test-bed for demonstrating crucial steps towards the Quantum Internet final goal.
We first employ a two-node NV quantum network in the laboratory to demonstrate the elementary building-blocks of distributed quantum computation: the generation of a distributed 4-partite Greenberger-Horne-Zeilinger state and the realization of a non-local Controlled-NOT gate between physically separated and non-interacting qubits.
In the long distance scenario, we use the NV center platform to study the photonic interface of solid-state qubits with time-bin qubits compatible with the emission from quantum memory platforms, such as Rubidium gas or Thulium-doped crystals. The interface is benchmarked with a quantum teleportation experiment. Quantum teleportation is the ultimate protocol that enables the transfer of quantum information from one physical point to another. We teleport a photonic time-bin qubit to the communication qubit of the NV center platform, establishing the primary form of communication between heterogeneous platforms in a quantum network.
Finally, the two-node NV network is used as reliable setup to demonstrate the first operating system for quantum network applications, QNodeOS. QNodeOS can schedule and manage quantum network applications in a multitasking fashion. It constitutes a software interface which enables facilitated access for users, boosting the research in quantum network applications and making a first step towards the deployment of such technology into society.

Quantum networks hold the potential to enable new applications, such as secure key distribution, high-precision distributed sensing, and distributed quantum computing. A central functionality of a quantum network is the distribution of entanglement between remote parties. Since experimental implementations remain in an early stage, it is important to understand both the capabilities and limitations of near-term architectures. However, characterising quantum network performance is challenging, due to the complex, stochastic nature of even simple architectures. Analytical studies can therefore play a crucial role: they not only reduce computational cost but also reveal fundamental relationships between performance, the choice of entanglement distribution protocols, and properties of quantum network hardware. In this thesis, we develop analytical methods to study quantumnetwork performance in several important scenarios.

We firstly analyse entanglement buffers, which are systems designed to generate and store high-quality entangled states to be consumed at any time. For this setting, we derive analytical expressions for two key performance metrics. The solutions are computationally efficient, make no restrictive assumptions about the entanglement purification protocol, and allow general insights: for example, that simple purification schemes can outperformmore complex ones previously considered “optimal” in different contexts.

Then,we turn to the problemof entanglement packet generation,where multiple entangled states of sufficient quality must be established simultaneously between network users. The fast generation of entanglement packets is an essential capability for many quantum network protocols. We obtain analytical results for the entanglement packet generation rate under a constant entanglement generation scheme and later extend the analysis to adaptive schemes, where entanglement parameters are tuned dynamically. Using parameter regimes motivated by current experiments, we show that adaptivity can enhance the entanglement packet generation rate by up to a factor of twenty.

Finally, we examine a standard assumption in performance analyses: that the initial states in a quantum repeater chain can be approximated by a symmetrised, or “twirled”, form. We investigate this assumption in the contexts of postselected and non-postselected entanglement swapping, where postselection is performed based on the Bell-state measurement outcomes at the repeaters. A central result is that, in many relevant cases, the twirled approximation is exact for non-postselected swapping. More generally, we provide a systematic framework to determine when the twirled approximation is valid for the initial states of a repeater chain.

In this thesis, we explore the fabrication methods aimed at engineering integrated spin-photon interfaces based on SnV centers in diamond nanostructures for future quantum networks. We first introduce the group-IV color centers, elaborating on their spin and optical properties as future quantum networks end-node candidates. Specifically, the inversion symmetry of these color centers opens the path towards nanophotonic integration, a key step to enable scalability. However, integration of tin-vacancy (SnV) color centers in diamond and fabrication of suspended nanophotonic devices is challenging....

Quantum networks are expected to enable applications that are provably impossible with classical communication alone, such as generation of secret keys for secure communication and high-precision distributed sensing. A fundamental resource needed for many of these applications is shared entanglement among distant parties. Hence, the viability of an application relies on the underlying protocol for entanglement distribution. Existing protocols often suffer from long waiting times, as they rely on the success of multiple random events, each with a low probability of success. Moreover, pre-distribution of entanglement is difficult, since entanglement degrades over time when stored in memory, eventually becoming unusable. In this thesis, we address these challenges by designing efficient entanglement distribution protocols and architectures.

First, we focus on on-demand entanglement distribution, in which the entanglement distribution process is initiated only after some users request it. We find optimal protocols that minimize the waiting time for distributing entanglement among two users that are connected by a chain of two-way quantum repeaters. The performance of these protocols sets a benchmark for on-demand distribution of quantum states. We also study a multi-user network of one-way quantum repeaters, and we conclude that finite waiting times are only achievable when the users are at most a few kilometers apart from each other, irrespective of the number of repeaters available.

Next, we examine protocols for continuous entanglement distribution, in which the distribution process is initiated before any user requests. While these protocols can sometimes lead to resource wastage – as noise in memory renders the entanglement
unusable if distributed too early –, they offer the potential to reduce expected waiting times compared to on-demand methods. Surprisingly, we find that, when the time required to distribute entanglement follows a broad probability distribution, initiating the process preemptively can actually result in longer expected waiting times compared to an on-demand approach.

Lastly, we propose an architecture for buffering high-quality entanglement, ensuring it is readily available for use when needed. A key feature of this system is the use of purification subroutines to prevent the buffered entanglement from degrading over time due to quantum decoherence. Among other findings, we show that maximizing entanglement quality upon consumption requires frequent purification, even if this process often fails and results in the loss of high-quality buffered entanglement. The results presented in this dissertation were obtained mostly analytically, leveraging tools from performance analysis, including queueing theory and renewal theory, and supported by extensive discrete-event simulations. Our theoretical insights provide benchmarks and identify fundamental limitations of quantum networks, offering valuable guidance for the design of reliable entanglement distribution systems.

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.

Quantum networks promise exciting applications that are not possible with their classical counterparts, such as distributed quantum computation or fundamentally secure quantum communication. Optically active spins in solid states are among the prime candidates for realizing quantum network nodes, while photons are used to distribute entanglement between nodes. The nitrogen-vacancy (NV) center in diamond is a pioneering system with the demonstrations of teleportation of quantum states within a three-node network and metropolitan-scale heralded entanglement. However, scaling to more nodes or longer distances is limited by the low extraction of resonant photons, creating a high interest in optical resonators (cavities) to improve the optical interface. Moreover, the diamond tin-vacancy (SnV) center with better optical properties emerged as a promising alternative candidate for quantum network nodes.
This dissertation presents the building blocks and the experimental realization of an open, fiber-based, cryogenic Fabry-Pérot microcavity enabling the Purcell enhancement of diamond NV and SnV centers, incorporated into the microcavity via a diamond membrane. The background on color centers in the context of quantum networking (Chapter 2) and optical cavities for diamond color centers is summarized (Chapter 3). A novel laser-cutting patterning method is introduced, which can be used to fabricate micrometer-thin diamond devices with arbitrary lateral shapes in the range of tens to hundreds of micrometers (Chapter 4). Microdevices fabricated by this method are characterized by scanning cavity microscopy, revealing a high cavity finesse. Furthermore, SnV and NV centers in the microdevices maintain bulk-like optical properties, which are required for quantum networking. Next to the sample fabrication, the detailed design, construction, and operation of a cryogenic microcavity system is presented, reaching a reproducible cavity length stability level of around 25 picometer with a sample temperature of about 8 kelvin on the cavity mirror (Chapter 5), a prerequisite for the following cavity experiments. Two cavity quantum optics experiments are conducted, exploring the regimes of coherent cavity coupling and efficient photon extraction with the cavity. In the first experiment, a single SnV center is coupled to the cavity, achieving a coherent cooperativity of 0.7. This enables the observation of nonlinear quantum effects, such as the modulation of the cavity resonance by an individual SnV center and the altered photon statistics of light transmitted through the cavity (Chapter 6). In the second experiment, a single NV center is coupled to the cavity, and the Purcell enhancement combined with a high cavity outcoupling leads to a resonant photon extraction (end-to-end) efficiency of 0.5 %. The NV center’s electron spin qubit initialization, manipulation with an on-chip microwave stripline, and readout are utilized to generate spin-photon correlated states, a precondition for remote entanglement with a second color center (Chapter 7).
The presented platform combines an efficient optical interface with microwave control of the spin state and can be used for the exploration of optically active defects in solid states, as a bright source of single photons, and for cavity-enhanced quantum networking.

The development of a quantum internet is crucial for the advancement of quantum computing, enabling secure and distributed quantum computing capabilities. Despite significant progress in building small-scale quantum networks in lab environments, a fundamental barrier remained: the inability to communicate between quantum computers over metropolitan-scale distances. This thesis tackles this challenge and shows the development of a scalable quantum network hardware platform for long-distance deployment using existing internet fiber connections (Part I). To further widen the understanding of the future implications of a quantum internet, the second part of this thesis explores the social and business implications of future quantum networks (Part II). The results of thesis aim to accelerate the commercialization of a quantum internet and improve our understanding of its implications in business and society.

The ability to send quantum information over long distances can enable fundamentally new applications, such as intrinsically secure communication, enhanced metrology, and distributed quantumcomputation. Entangled links serve as powerful resources for sending quantum information between nodes in a quantum network. However, generating entanglement in sufficient quantity and quality across a network, such that they can be used for applications, remains an open challenge.

In this thesis, we explore the use of color centers in diamond as network nodes. Their electron spin serves as a matter qubit with an optical interface, enabling the entanglement of two distant color centers, mediated by photons. The surrounding nuclear spins are used as memory qubits for local computation and entanglement storage. In this thesis, we investigate both the well-established nitrogen-vacancy (NV) center in diamond and the recently discovered tin-vacancy (SnV) center in diamond. The physics and control methods for both types of color centers are discussed in Chapter 2.

Remote entanglement between matter qubits can be achieved with many different entanglement protocols. In Chapter 3, we present a framework that explains and categorizes different protocols and quantum network hardware components. This framework is then used to compare the performance of various protocols while using similar hardware.

There have been many realisations of rudimentary network links between two network nodes using various quantum hardware. In Chapters 4 and 5, we realize the first entanglement-based three-node quantum network employing NV centers in diamond. In this network, we demonstrate fundamental network capabilities such as the creation of a remote three-party Greenberger–Horne–Zeilinger (GHZ) state and entanglement swapping to connect non-neighboring network nodes, as detailed in Chapter 4. These advancements are facilitated by storing an entangled state in a network node while generating a second entangled link. In Chapter 5 we demonstrate quantum teleportation between two non-neighboring network nodes by adding a fifth qubit to the network and utilizing the entangled link generated through the entanglement swapping.

The three-node network experimentswere enabled by the nuclear spin memory, emphasizing the importance of nuclear spin control for quantum networks based on color centers. In Chapter 6, we explore nuclear spin control with the SnV center in diamond. This recently discovered color center promises enhanced entanglement rates compared to the NV center due to its superior optical interface. We control single nuclear spins and show entanglement between the electron and nuclear spin. These experiments provide insights into the challenges and opportunities of controlling nuclear spins using an electron spin-1/2.

In modern-day research, magnetometry provides valuable information for a wide range of studies. Among all the different forms of magnetometers, the nitrogenvacancy (NV) lattice defect in diamond has emerged as a powerful magnetic field sensor thanks to the combination of sensitivity, spatial resolution and versatile capabilities. High-fidelity microwave control and optical readout of the NV spin over a wide range of conditions has enabled applications in condensed matter physics, chemistry, biology, geoscience and many more. In particular, its capability of visualizing magnetic phenomena with high spatial resolution has proven to be a powerful tool in both fundamental physics and applied sciences. Advances in NV magnetometry in the past decade have led to numerous breakthroughs, especially in revealing the nanoscale physics of condensed matter systems. However, the free-space optics generally used for optical interrogation of the NV spins are challenging to realize in cryogenic, intra-cellular, or other hard-to-reach environments. As such, realizing robust all-fiber-based NV probes with efficient optical readout could enable new measurements in low-temperature (quantum) or biological systems...

This thesis explores how artificial intelligence (AI) can be used to bridge the gap between simulation and experiment in nanoscience. As both theoretical modeling and experimental techniques in nanoscience become increasingly sophisticated, AI is emerging as a powerful tool to tackle challenges such as tuning experiments, accelerating simulations, generating synthetic data, and automating data analysis. This work presents applications of AI in three main domains: neuroscience, quantum computing, and condensed matter physics.

In neuroscience, we address the problem of efficiently simulating neuronal activity data by implementing a quantum machine learning model that uses a reduced number of trainable parameters. On the experimental side, we develop a computational package that automates the analysis of micro-electrode array data using a neural network trained to replicate human expert detection of burst patterns from spiking activity.

In spin-based quantum computing, we develop a computational package that simulates charge stability diagrams (CSDs) based on device characteristics, enabling the efficient creation of synthetic datasets. We also demonstrate how machine learning models can be used to filter high-quality CSDs for building experimental datasets, and we present the first implementation of diffusion models to complete partially measured CSDs, an approach that can be integrated into measurement routines to accelerate the process.

In condensed matter physics, we leverage neural quantum states to detect phase transitions by analyzing the evolution of neural network weights, without the need to calculate order parameters, and discuss potential directions for combining this technique with neural quantum states trained on experimental data.

These studies show that AI can accelerate simulations and data analysis, while also supporting the design and interpretation of experiments, highlighting its growing and essential role in the future of nanoscience.

In working towards a quantum internet, nodes based on nitrogen-vacancy (NV) centres in diamond have shown great potential. A key challenge in scaling these networks is the low entanglement generation rate due to low coherent photon emission (≈ 3%) and limited collection efficiency (≈ 15% using state-of-the-art solid immersion lens setups). Both can be improved by integrating NV centres in an optical cavity. In this thesis NV centres coupled to an open Fabry-Pérot microcavity are investigated. The NV centres are integrated into the cavity by bonding a μm-thin diamond sample to one of the mirrors.
The goal of this thesis is to move towards the realisation of an efficient spin photon interface of NV centres in an open microcavity. To this end, short optical pulses for eventual spinphoton entanglement creation and microwave electronics for spin control are implemented.
A cavity is formed and characterised. A finesse of 3.3×103 is found, along with a quality factor of (3.14 ± 0.03)×105 and a mode volume of 83 𝜆3. From this, a theoretical outcoupled coherent photon fraction of 14% is determined. NV centres are found in the cavity, and their coupling strength is determined using off-resonant lifetime measurements. From this, the actual outcoupled coherent photon fraction is determined to be (12 ± 1) %. Which represents a more than 25 times improvement over NV centres in solid immersion lenses.
The electron spin resonance (ESR) of an NV centre is measured and a magnetic field strength aligned with its spin axis of (36 ± 1) G is found. A lifetime measurement of an NV centre using pulsed resonant excitation is shown. The last two measurements can be extended to achieve coherent control and resonant readout of the NV spin. Paving the way towards a more efficient spin-photon interface.

The Nitrogen-Vacancy (NV) center has demonstrated great potential as a quantum networks platform. While many milestones have been reached, the current hardware implementations have reached their limit in terms of remote entanglement generation rates, which hinders the scalability of the platform. The most dominant limits are that only ∼ 3% of the emission is coherent, and the outcoupling efficiency in state-of-the-art network experiments is ∼ 15%. The coherent emission can be increased via the Purcell effect, which can be achieved by the use of an open Fabry-Pérot microcavity. In such a setup the NV center is embedded in a micrometer-thin diamond membrane, thereby retaining its favourable optical properties. The open microcavity has the ability of in situ spectral and spatial tunability. However, this is accompanied by a high susceptibility to vibrations, which needs to be considered in the experimental design. The goal of this thesis is to simultaneously increase the coherent emission and the outcoupling efficiency by coupling NV centers to an open Fabry-Pérot microcavity. An optimized cavity is found by characterizing different fiber tip mirrors, reaching a bare cavity finesse of 9500. The vibrations in the bare cavity are analyzed, revealing a root mean square cavity length detuning of 22 pm while operating at low temperature. A hybrid cavity is formed with a diamond membrane with NV centers, resulting in cavity with a finesse of 2100, quality factor of 266000, mode volume of 108 λ 3 , and outcoupling efficiency ∼ 30%. Due to a different setup configuration, an increased root mean square cavity length detuning of 190 pm is measured, resulting in a lowered Purcell factor. Off-resonant measurements demonstrate the coupling of NV centers to the cavity. Moreover, excited state lifetimes are measured to obtain a Purcell factor of 2.6 ± 0.5, with a corresponding enhanced coherent emission ratio of 0.07 ± 0.01. This result is in good agreement with simulations that include the effect of vibrations. The ZPL branching ratio and outcoupling efficiency are improved by a factor two compared to state-of-the-art confocal microscope setups. For the first time in open microcavities, the ability to drive the spin of NV centers is shown by an optically detected magnetic resonance measurement with a contrast of (28 ± 2)%. By improving the setup to the already obtained vibration level, and a realistic finesse of 5000, an expected Purcell factor of ∼ 15.7 and outcoupling efficiency of ∼ 80% can be reached, paving the way to the next-generation of quantum network nodes.

The efforts to bring quantum states, fundamental building blocks of nature, from research labs into the outside world are intensifying. The generation and processing of remote quantum states between nodes in a network would allowfor new applications such as distributed quantum computing, quantumenhanced sensing and quantum communication. Various demonstrations of such a quantum network have been shown in a lab setting, such as the generation of a three node GHZ-state, device-independent quantum key distribution and memory enhanced quantumcommunication. Color centers in diamond have been at the forefront of these developments due to their optically active spin interface, long coherence times and nuclear spin registers. The Nitrogen Vacancy (NV-) center is the color center of choice in this thesis, which we describe in Chapter 2. We explain what an NV-center is, how to control it and how we can use them to generate remote entanglement.

Solid-state defects in diamond and silicon carbide have emerged as a promising platform for exploring various quantum technologies, such as distributed quantum computing, quantum simulations of many-body physics, and nano-scale nuclear magnetic resonance. The noise environment surrounding such defects, consisting of magnetic and electrical impurities, directly impacts the spin and optical coherence, posing a key challenge for advancing quantum technologies. Systematic study of these spins and charges is crucial for mitigating their noise contribution. In some cases, establishing control over the environment can even convert it into a resource, to be used for storing, or processing (quantum) information. In this thesis, we develop experimental and analytical tools that enable a more detailed study of the defect spin and charge environment, and can be exploited to manipulate its microscopic configuration.

Electron-spin qubits associated to solid-state defects can exhibit exceptional optical and spin coherence. Additionally, magnetic interactions with surrounding spins presents a resource for multi-qubit registers. Combined, this makes such solid-state defect systems promising for quantum network applications. In this thesis, we first realize a toolbox to control electron-nuclear spin qubits surrounding an NV-center in diamond and investigate such spins as potential qubits by generating an entangled state shared amongst two spins. Secondly, we improve the robustness of a nuclear-spin memory qubit while emulating remote entanglement generation protocols. Ancillary (spectator) qubits subject to noise correlated to the memory qubit are measured and subsequent feedforward allows to mitigate this noise in real-time. Thirdly, we investigate spectral diffusion dynamics in commercially available silicon carbide and demonstrate (near-)Fourier linewidth limited optical transitions within a broad inhomogeneously broadened spectrum. Finally, an outlook on electron spin clusters and photonic integration of electron spins is presented.

For many quantum applications we require high-fidelity entanglement between multiple pairs of solid state qubits at a distance. To achieve a high fidelity, we have to minimize the time during which the generated qubits need to stay coherent. Entanglement protocols often used in practice only generate one qubit at the same time. To generate multiple entangled pairs, the protocol is repeated. However during the time it takes for all pairs to be generated, the memory qubits will dephase. The required coherence time increases with the inverse transmission probability of the photons, which decreases exponentially with distance. This thesis is concerned with entanglement generation protocols that herald multiple entangled pairs simultaneously and in general herald N-dimensional entangled bipartite states. The main advantage of using more than 2 dimensions is that the qudits only dephase during the time in which the protocol executes. With simulations we show that the fidelity of the entangled pairs created with our protocols is higher than the fidelity of pairs created by protocols that heralds one entangled pair for distances L > 10 km. We also show a polynomial relation between the total success probability of the tailored protocol with dimension, which is an exponential improvement with respect to previous works.

The nitrogen-vacancy (NV) center in diamond can be used as a quantum network node [1], but the low Debye-Waller factor β0 ≈ 3 % [3, 4] limits the entanglement rate of the system drastically. By embedding the color center into an open microcavity and utilizing the Purcell effect, this limitation can be reduced [3, 5]. Additionally, as coupling an emitter to an optical cavity enhances the collection efficiency of the emitted coherent photons, such a system is also interesting for emitters with a higher intrinsic Debye-Waller factor, as, for example, the tin-vacancy (SnV) center in diamond. The diamond sample between the cavity mirrors is an important building block of such a cavity system. In this thesis, the fabrication of singledigit micrometer-thin, color center-enriched diamond platelets with side lengths of tens of micrometers is studied. Additionally, their properties were characterized after bonding them to a Bragg mirror. Platelets containing NV centers and platelets with implanted SnV centers were produced. In addition to fabrication with electron beam (e-beam) lithography and dry etching, a new method is introduce utilizing laser cutting. A reasonable bonding yield was achieved for platelets from both fabrication methods, and a minimal surface roughness of Rq ≈ 0.2 nm was measured. The samples were studied in a low-temperature confocal microscopy setup to determine the color center’s optical linewidth of the zero-phonon line (ZPL) emission. For the SnV centers in the diamond platelets a close to lifetime-limited [6] dephasing linewidth of Γd ≈ 32.2(4) MHz and a minimum spectral diffusion linewidth of Γs ≈ 58(2) MHz was observed. For the NV center, only the spectral diffusion linewidth could be measured Γs < 80 MHz. Those measured linewidth lay within the bounds to achieve two-photon quantum interference of separate color centers [7]. Additionally, the diamond platelets were studied in an open microcavity setup. With an e-beam-lithography and dry etching fabricated, NV center-enriched sample, a maximum finesse of F ≈ 3100(300) was measured. Additional losses introduced by the diamond of Ladd,dia ≈ 1200(200) ppm could be estimated. For a laser cut fabricated, SnV center-enriched sample, a maximum finesse of F ≈ 2300(200) could be achieved. An estimation for the maximum achievable Purcell enhancement and resulting branching ratio into the cavity mode was calculated using the measured finesse and cavity parameters. For the SnV center, a maximum achievable Purcell factor of F ZPL P ≈ 15(2) was computed, resulting in a branching ratio into the cavity mode of βcav ≈ 85(2) %. In addition, an upper limit for the outcoupling percentage trough the plane mirror of βout < 62(13) % was estimated. For the NV center sample, F ZPL P ≈ 19(2), βcav ≈ 37(2) %, and βout ≈ 13(6) % were calculated.

The future quantum internet promises to create shared quantum entanglement between any two points on Earth, enabling applications such as provably-secure communication and connecting quantum computers. A popular method for distributing entanglement is by sending entangled photons through optical fiber. However, the probability of successful transmission decreases exponentially with the fiber length. This makes it challenging to realize large fiber-based quantum networks that create shared entanglement, let alone the construction of a quantum internet. Quantum repeaters have been proposed as a solution to mitigate losses by acting as intermediary nodes that divide long optical fibers into smaller segments. The required technology, however, is still under development. In this thesis we aim to expedite the realization of fiber-based quantum networks by identifying shortcuts towards that end.

One way in which we look for shortcuts is by identifying the technological advances that are required to build such networks. To achieve this we translate performance demands on the network to requirements on individual components, such as quantum repeaters. This way we are not only able to indicate how much development current-day technology still requires before functional quantum networks can be built, but also what specific set of improvements could be applied to state-of-the-art hardware to get there as soon as possible.

A specific promising shortcut that we investigate in this thesis is the construction of quantum networks using existing fiber infrastructure. As deploying optical fiber is costly, an economical method for building quantum networks would be to incorporate fiber that has already been placed in the field. Existing infrastructure however imposes restrictions on quantum networks, in particular on the possible locations where quantum hardware could be installed. An important question to answer is then how severe the effects of these restrictions are. We address this question by investigating the performance degradation caused by displacing nodes from their optimal location, and the increase in required technological advances when restrictions are taken into account. Additionally, we provide tools for choosing where to deploy quantum repeaters when subject to placement restrictions.

Finally, we also address the fact that quantum networks may need to provide entanglement to more than just two parties. When a network has many end nodes that require bipartite entanglement between different pairs of them, it is important that it is designed such that every end node is sufficiently connected to every other end node. We provide conditions to judge whether this is the case and a method to ensure the conditions are met. Alternatively end nodes could require multipartite entangled states shared by more than two of them, in which case specialized nodes may need to be included in the network. We investigate what such a node could look like and perform a thorough performance analysis.