Entanglement generation between remote qubit systems is the central tasks for quantum communication. Future quantum networks will have to be compatible with low-loss telecom bands and operate with large separation between qubit nodes. Single-click heralding schemes can be used to increase entanglement rates at the cost of needing an optically phase-synchronized architecture. In this paper we present such a phase synchronization scheme for a metropolitan quantum network, operating in the low-loss telecom L band. To overcome various challenges such as communication delays and optical power limitations, the scheme consists of multiple tasks that are individually stabilized. We characterize each task, identify the main noise sources, motivate the design choices, and describe the synchronization schemes. The performance of each of the tasks is quantified by a transfer-function measurement that investigates the frequency response and feedback bandwidth. Finally we investigate the resulting optical phase stability of the fully deployed system over a continuous period of 10 h, reporting a short-term stability standard deviation of 𝜎 ≈30∘ and a long-term stability of the average optical phase to within a few degrees. The scheme presented served as a key enabling technology for a nitrogen-vacancy-center-based metropolitan quantum link. This scheme is of interest for other quantum network platforms that benefit from an extendable and telecom-compatible phase-synchronization solution.@en

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.@en

Many quantum entanglement generation protocols require phase stabilization between the nodes. For color centers that are embedded in a solid immersion lens (SIL) often a reflection from the SIL’s surface is input to an interferometer where it is mixed with a reference beam. However, the beam reflected beam by the SIL does not travel colinear with the photons that are emitted by the color center, which ultimately leads to a reduction of the interferometer’s signal-to-noise ratio (SNR). Additionally, imperfections of the SIL surface introduce aberrations into the reflected light, thereby further reducing the SNR. Through several design-iterations and extensive experience realizing phase stabilization on many different SIL’s we have come to an approach that significantly improves the SNR and enhances the operability of the quantum node. In this paper we report on our optical design and provide useful guidelines for the operation thereof.@en

A key challenge toward future quantum internet technology is connecting quantum processors at metropolitan scale. Here, we report on heralded entanglement between two independently operated quantum network nodes separated by 10 kilometers. The two nodes hosting diamond spin qubits are linked with a midpoint station via 25 kilometers of deployed optical fiber. We minimize the effects of fiber photon loss by quantum frequency conversion of the qubit-native photons to the telecom L-band and by embedding the link in an extensible phase-stabilized architecture enabling the use of the loss-resilient single-click entangling protocol. By capitalizing on the full heralding capabilities of the network link in combination with real-time feedback logic on the long-lived qubits, we demonstrate the delivery of a predefined entangled state on the nodes irrespective of the heralding detection pattern. Addressing key scaling challenges and being compatible with different qubit systems, our architecture establishes a generic platform for exploring metropolitan-scale quantum networks.

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We present a highly efficient low-noise quantum frequency converter from the visible range to telecom wavelengths, combining a pump laser at intermediate frequency resonantly enhanced in an actively stabilized cavity with a monocrystalline bulk crystal. A demonstrator for photons emitted by nitrogen-vacancy-center qubits achieves 43% external efficiency with a noise photon rate per wavelength (frequency) band of 2 s−1/pm(17 s−1/GHz) – reducing the noise by two orders of magnitude compared with current devices based on periodically poled crystals with waveguides. With its tunable output wavelength, this device enables the generation of indistinguishable telecom photons from different network nodes and is, as such, a crucial component for a future quantum internet based on optical fiber.@en

We report on a quantum interface linking a diamond NV center quantum network node and 795nm photonic time-bin qubits compatible with Thulium and Rubidium quantum memories. The interface makes use of two-stage low-noise quantum frequency conversion and waveform shaping to match temporal and spectral photon profiles. Two-photon quantum interference shows high indistinguishability between converted 795nm photons and the native NV center photons. We use the interface to demonstrate quantum teleportation including real-time feedforward from an unbiased set of 795nm photonic qubit input states to the NV center spin qubit, achieving a teleportation fidelity well above the classical bound. This proof-of-concept experiment shows the feasibility of interconnecting different quantum network hardware.@en

We show the latest progress towards establishing a solid-state, metropolitan quantum link, consisting of two remote Nitrogen Vacancy (NV)-centers and a central measurement station. The entanglement is generated by converting single emitted photons to the same frequency in the telecom L-band, guiding them to a central beamsplitter, where a joint Bell-state measurement projects the NV-centre spins in an entangled state.@en

Scaling current quantum communication demonstrations to a large-scale quantum network will require not only advancements in quantum hardware capabilities, but also robust control of such devices to bridge the gap in user demand. Moreover, the abstraction of tasks and services offered by the quantum network should enable platform-independent applications to be executed without the knowledge of the underlying physical implementation. Here we experimentally demonstrate, using remote solid-state quantum network nodes, a link layer, and a physical layer protocol for entanglement-based quantum networks. The link layer abstracts the physical-layer entanglement attempts into a robust, platform-independent entanglement delivery service. The system is used to run full state tomography of the delivered entangled states, as well as preparation of a remote qubit state on a server by its client. Our results mark a clear transition from physics experiments to quantum communication systems, which will enable the development and testing of components of future quantum networks.

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We demonstrate interference of photons emitted by remote, spectrally distinct NV-centers. Quantum frequency conversion at the nodes brings the photons to the same wavelength in the telecom L-band, compatible with entanglement generation at metropolitan scale.

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Entanglement distribution over quantum networks has the promise of realizing fundamentally new technologies. Entanglement between separated quantum processing nodes has been achieved on several experimental platforms in the past decade. To move toward metropolitan-scale quantum network test beds, the creation and transmission of indistinguishable single photons over existing telecom infrastructure is key. Here, we report the interference of photons emitted by remote spectrally detuned NV-center-based network nodes, using quantum frequency conversion to the telecom L band. We find a visibility of 0.79±0.03 and an indistinguishability between converted NV photons around 0.9 over the full range of the emission duration, confirming the removal of the spectral information present. Our approach implements fully separated and independent control over the nodes, time multiplexing of control and quantum signals, and active feedback to stabilize the output frequency. Our results demonstrate a working principle that can be readily employed on other platforms and shows a clear path toward generating metropolitan-scale solid-state entanglement over deployed telecom fibers.

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Quantum communication brings radically new capabilities that are provably impossible to attain in any classical network. Here, we take the first step from a physics experiment to a quantum internet system. We propose a functional allocation of a quantum network stack, and construct the first physical and link layer protocols that turn ad-hoc physics experiments producing heralded entanglement between quantum processors into a well-defined and robust service. This lays the groundwork for designing and implementing scalable control and application protocols in platform-independent software. To design our protocol, we identify use cases, as well as fundamental and technological design considerations of quantum network hardware, illustrated by considering the state-of-the-art quantum processor platform available to us (Nitrogen-Vacancy (NV) centers in diamond). Using a purpose built discrete-event simulator for quantum networks, we examine the robustness and performance of our protocol using extensive simulations on a supercomputing cluster. We perform a full implementation of our protocol in our simulator, where we successfully validate the physical simulation model against data gathered from the NV hardware. We first observe that our protocol is robust even in a regime of exaggerated losses of classical control messages with only little impact on the performance of the system. We proceed to study the performance of our protocols for 169 distinct simulation scenarios, including trade-offs between traditional performance metrics such as throughput, and the quality of entanglement. Finally, we initiate the study of quantum network scheduling strategies to optimize protocol performance for different use cases.

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