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.@en

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

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 evaluate the status of the development of a responsible future quantum internet (QI). Through horizon scanning, domain expert trend analysis and guided workshops, we present a desired future (DF) conceptualized by stakeholders within the scope of the ethical, legal, societal aspects (ELSA) and policy implications (ELSPI) of the QI. We examine the alignment of the present situation and the DF of the QI ELSPI to the ideals of the ‘ten principles for responsible quantum innovation’ developed by [Mauritz Kop et al. 2024 Quantum Sci. Technol. 9 035013]. Most principles in the DF are well aligned to the ideal, except for the misalignment in ‘intellectual property’ (IP) and ‘dual use’, revealing the precarious balance of well intended policy suggestions and effective outcomes. Additionally, there is an overemphasis placed on the principal of societal relevance in the DF, risking overseeing other principles in the future steering of the ELSPI. The present situation is in moderate alignment with the principles, however trending to misalignment on IP and international collaboration due to QI commercialization and the push for geopolitical sovereignty. For continued success of a responsible quantum internet, we recommend further investigation on the prevention of dual use quantum internet applications, closer involvement of commercial entities in ELSPI ideation, continued recognition of base-layer technology research and stakeholder education on QI applications.@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 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

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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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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