Solid-state spin-photon interfaces promise to scale quantum networks through on-chip photonic integration and multiplexed entanglement generation. To date, remote entanglement between integrated emitters has been realized only in cavity-enhanced systems, where fabrication yield and spectral matching remain major obstacles. Here we demonstrate heralded remote entanglement between diamond tin-vacancy spin qubits embedded in separate on-chip waveguides. Combining intrinsically efficient photon emission with a broadband waveguide architecture provides high device yield and obviates the need for spectral matching to cavity modes. We realize coherent optical and spin control and achieve high-visibility two-photon interference. By combining photon-mediated entanglement generation with real-time feedforward, we produce a consistent entangled state independent of the heralding pattern. These results establish waveguide-integrated tin-vacancy centers as a compelling platform for scalable quantum network nodes.

Quantennetzwerke versprechen eine Reihe aussichtsreicher Anwendungen, von Experimenten zu den Grundlagen der Quantenmechanik bis hin zum Zusammenschalten mehrerer Quantencomputer. Quanteninformation kann dabei mit Lichtteilchen über Glasfasern und einer optischen Schnittstelle zwischen lokalen Netzwerkknoten ausgetauscht werden. Unserem Team am niederländischen Forschungsinstitut QuTech ist es nun gelungen, mittels einer Spiegelkavität die optische Schnittstelle eines Farbzentrums zu verbessern und damit einen Schritt in Richtung leistungsfähigerer Quantennetzwerke zu machen.

The tin-vacancy center in diamond (SnV) has emerged as a compelling building block for realizing nextgeneration quantum networks thanks to its excellent optical and spin properties. Coupling to photonic crystal cavities (PCCs) promises to further enhance the SnV light-matter interface and unlock a diverse range of entanglement generation protocols. Recent pioneering experiments showing Purcell enhancement of SnV centers in PCCs underscore this potential. However, optical coupling that is coherent-the key ingredient for use in quantum protocols-has so far remained elusive. Here, we demonstrate aboveunity coherent cooperativity of SnV centers embedded in photonic crystal cavities. We fabricate freestanding PCCs using a quasi-isotropic undercut. Across two samples, we conduct room-temperature characterizations, measuring resonances for 327 cavities, with an average quality factor exceeding Q = 1.1(4) & times; 104. Two cavity-coupled emitters are examined in detail, exhibiting quality factors up to Q = 25.4(4) & times; 103 and Purcell-reduced lifetimes corresponding to cooperativities up to C = 20.3(11). Furthermore, the single SnVs are observed to strongly modulate the cavity transmission with an extinction contrast up to 98.8(4)%% on resonance. Finally, SnV linewidth measurements reveal above-unity coherent cooperativities in both devices, with the highest value being Ccoh = 8.3(12). These results open the door to using cavity-coupled SnV centers as efficient, coherent light-matter interfaces for future quantum networks.

Entanglement between ions connected by 10 kilometres of optical fibre is a step towards large-scale quantum communications networks. Entanglement between ions connected by 10 kilometres of optical fibre is a step towards large-scale quantum communications networks.

Quantum networks connecting quantum processing nodes via photonic links enable distributed and modular quantum computation. In this framework, quantum gates between remote qubits can be realized using quantum teleportation protocols. The essential requirements for such non-local gates are remote entanglement, local quantum logic within each processor, and classical communication between nodes to perform operations based on measurement outcomes. Here, we demonstrate an unconditional Controlled-NOT quantum gate between remote diamond-based qubit devices. The control and target qubits are Carbon-13 nuclear spins, while NV electron spins enable local logic, readout, and remote entanglement generation. We benchmark the system by creating a Greenberger-Horne-Zeilinger state, showing genuine 4-partite entanglement shared between nodes. Using deterministic logic, single-shot readout, and real-time feed-forward, we implement non-local gates without post-selection. These results demonstrate a key capability for solid-state quantum networks, enabling exploration of distributed quantum computing and testing of complex network protocols on full-stack systems.

Diamond tin-vacancy (SnV) centers are promising candidates for building quantum network nodes. However, their native photon emission at 619 nm is incompatible with metropolitan-scale networks operating at low-loss telecom wavelengths. To address this, we demonstrate highly efficient, low-noise quantum frequency conversion (QFC) of 619 nm photons to the telecom S-band at 1480 nm. The conversion process combines 619 nm photons with 1064 nm pump light in an actively stabilized cavity containing a bulk monocrystalline potassium titanyl arsenate (KTA) crystal. We achieve an internal (external) conversion efficiency of (48 ± 3) % ((28 ± 2) %) and a noise photon rate per wavelength of (2.2 ± 0.9) cts/s/pm, which is spectrally flat in the investigated frequency range of 40 GHz. Furthermore, we demonstrate that the efficiency remains above 80% of its maximum over a frequency range of 70 GHz. Finally, we generate a string of photons from a single waveguide-embedded SnV − center using a train of excitation pulses and send these through the QFC. After the QFC, we observe telecom light displaying the SnV lifetime, confirming successful conversion. These results represent a critical step towards metropolitan-scale fiber-based quantum networks using SnV − centers.

In this paper, we present a novel single stage quantum frequency conversion (QFC) scheme based on an enhancement cavity, which allows for high conversion efficiencies with record low noise levels, 100 times smaller than in previous systems and close to the thermal background. Our approach represents a significant improvement over traditional QFC methods that rely on periodically poled waveguide crystals, which typically produce high levels of noise with input wavelengths in the visible spectrum. One of the key challenges in QFC is the generation of noise photons - photons that are not part of the desired output state. Those noise photons are one of the major limitations of today’s quantum network experiments. It has been widely believed that single stage QFC of qubit photons in the visible range would result in too many noise photons in the telecom band, because of SPDC induced by the short wavelength driver laser. However, using the enhancement cavity design, we are able to overcome this challenge and demonstrate high-fidelity QFC from the visible to telecom band. Our results have important implications for the development of practical QFC devices for a range of applications, including quantum communication and quantum networks, where low noise levels are critical.

The negatively charged tin-vacancy (SnV-) center in diamond has emerged as a promising platform for quantum computing and quantum networks. To connect SnV- qubits in large networks, in-situ tuning and stabilization of their optical transitions are essential to overcome static and dynamic frequency offsets induced by the local environment. Here we report on the large-range optical frequency tuning of diamond SnV- centers using micro-electro-mechanically mediated strain control in photonic integrated waveguide devices. We realize a tuning range of >40 GHz, covering a major part of the inhomogeneous distribution. In addition, we employ real-time feedback on the strain environment to stabilize the resonant frequency and mitigate spectral wandering. These results provide a path for on-chip scaling of diamond SnV-based quantum networks.

Quantum networks are based on shared remote entanglement between local nodes by exchanging indistinguishable photons. We show Two-Photon Quantum Interference between tin-vacancy centers in diamond-waveguides and report on the progress towards remote entanglement generation.

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. Published by the American Physical Society 2025

An efficient interface between a spin qubit and single photons is a key enabling system for quantum science and technology. We report on a coherently controlled diamond nitrogen-vacancy center electron spin qubit that is optically interfaced with an open microcavity. Through Purcell enhancement and an asymmetric cavity design, we achieve efficient collection of resonant photons, while on-chip microwave lines allow for spin qubit control at a 10 MHz Rabi frequency. With the microcavity tuned to resonance with the nitrogen-vacancy center's optical transition, we use excited state lifetime measurements to determine a Purcell factor of 7.3 $\pm$ 1.6. Upon pulsed resonant excitation, we find a coherent photon detection probability of 0.5 % per pulse. Although this result is limited by the finite excitation probability, it already presents an order of magnitude improvement over the solid immersion lens devices used in previous quantum network demonstrations. Furthermore, we use resonant optical pulses to initialize and read out the electron spin. By combining the efficient interface with spin qubit control, we generate two-qubit and three-qubit spin-photon states and measure heralded Z-basis correlations between the photonic time-bin qubits and the spin qubit.

Quantum technologies have made impressive progress over the past decade. In some areas, such as quantum sensing and key distribution, these technologies are moving from the laboratory to enable real-world applications. However, for areas such as quantum computing, entanglement-enhanced sensing, and a global quantum internet, we are in an equivalent of the early transistor age, and hardware breakthroughs are required in multiple arenas to reach the performance necessary for the envisioned applications. In this Review, we assess the current state of the art of quantum information hardware and identify key challenges and opportunities ahead. We draw inspiration from the history of scaling and development of classical electronics and photonics to anticipate progress in the field.

We demonstrate large-range tuning of the optical transition of Tin-Vacancies (SnV) in diamond using electro-mechanical-induced strain, realizing > 40 GHz tuning. We employ real-time feedback on the strain environment to stabilize the resonant frequency.

Diamond photonic crystal cavity parameters are measured at cryogenic temperatures. In-situ resonance frequency tuning through gas desorption allows us to probe the SnV-cavity system.

Micrometer-scale thin diamond devices are key components for various quantum sensing and networking experiments, including the integration of color centers into optical microcavities. In this work, we introduce a laser-cutting method for patterning microdevices from millimeter-sized diamond membranes. The method can be used to fabricate devices with micrometer thicknesses and edge lengths of typically 10–100 µ m. We compare this method with an established nanofabrication process based on electron-beam lithography, a two-step transfer pattern utilizing a silicon nitride hard mask material, and reactive ion etching. Microdevices fabricated using both methods are bonded to a cavity Bragg mirror and characterized using scanning cavity microscopy. We record two-dimensional cavity finesse maps over the devices, revealing insights about the variation in diamond thickness, surface quality, and strain. The scans demonstrate that devices fabricated by laser-cutting exhibit similar properties to devices obtained by the conventional method. Finally, we show that the devices host optically coherent Tin- and Nitrogen-Vacancy centers suitable for applications in quantum networking.

Solid-state quantum registers consisting of optically active electron spins with nearby nuclear spins are promising building blocks for future quantum technologies. For electron spin-1 registers, dynamical decoupling (DD) quantum gates have been developed that enable the precise control of multiple nuclear spin qubits. However, for the important class of electron spin-1/2 systems, this control method suffers from intrinsic selectivity limitations, resulting in reduced nuclear spin gate fidelities. Here, we demonstrate improved control of single nuclear spins by an electron spin-1/2 using dynamically decoupled radio-frequency (DDRF) gates. We make use of the electron spin-1/2 of a diamond tin-vacancy center, showing high-fidelity single-qubit gates, single-shot readout, and spin coherence beyond a millisecond. The DD control is used as a benchmark to observe and control a single ^{1}3C nuclear spin. Using the DDRF control method, we demonstrate improved control on that spin. In addition, we find and control an additional nuclear spin that is insensitive to the DD control method. Using these DDRF gates, we show entanglement between the electron and the nuclear spin with 72(3)% state fidelity. Our extensive simulations indicate that DDRF gate fidelities well in excess are feasible. Finally, we employ time-resolved photon detection during readout to quantify the hyperfine coupling for the electron’s optically excited state. Our work provides key insights into the challenges and opportunities for nuclear spin control in electron spin-1/2 systems, opening the door to multiqubit experiments on these promising qubit platforms.

We report on the realization of a fiber-based microcavity, exhibiting low cavity length fluctuations in combination with full spatial and spectral tunability. The microcavity is used to demonstrate Purcell-enhancement of diamond Tin-Vacancy centers.

We present our optimized diamond fabrication process based on quasi-isotropic crystal-plane-dependent reactive-ion-etching at low and high temperature plasma regime. We demonstrate successful integration of SnV centers in diamond waveguides showing quantum non-linear effects. We report on our latest results on all-diamond photonic crystal cavities.

The goal of future quantum networks is to enable new internet applications that are impossible to achieve using solely classical communication. Up to now, demonstrations of quantum network applications and functionalities on quantum processors have been performed in ad-hoc software that was specific to the experimental setup, programmed to perform one single task (the application experiment) directly into low-level control devices using expertise in experimental physics. Here, we report on the design and implementation of the first architecture capable of executing quantum network applications on quantum processors in platform-independent high-level software. We demonstrate the architecture's capability to execute applications in high-level software, by implementing it as a quantum network operating system – QNodeOS – and executing test programs including a delegated computation from a client to a server on two quantum network nodes based on nitrogen-vacancy (NV) centers in diamond. We show how our architecture allows us to maximize the use of quantum network hardware, by multitasking different applications on a quantum network for the first time. Our architecture can be used to execute programs on any quantum processor platform corresponding to our system model, which we illustrate by demonstrating an additional driver for QNodeOS for a trapped-ion quantum network node based on a single ^40Ca^+ atom. Our architecture lays the groundwork for computer science research in the domain of quantum network programming, and paves the way for the development of software that can bring quantum network technology to society.

Generating 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, of which many 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.