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 31C 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.

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 ± 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.

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 resonance frequency and mitigate spectral wandering. These results provide a path for on-chip scaling of diamond SnV-based quantum networks.

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 µm. 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.

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.

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.

The goal of future quantum networks is to enable new internet applications that are impossible to achieve using only classical communication1, 2–3. Up to now, demonstrations of quantum network applications4, 5–6 and functionalities7, 8, 9, 10, 11–12 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 an architecture capable of executing quantum network applications on quantum processors in platform-independent high-level software. We demonstrate the capability of the architecture 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 server13 on two quantum network nodes based on nitrogen-vacancy (NV) centres in diamond14,15. We show how our architecture allows us to maximize the use of quantum network hardware by multitasking different applications. Our architecture can be used to execute programs on any quantum processor platform corresponding to our system model, which we illustrate by demonstrating an extra driver for QNodeOS for a trapped-ion quantum network node based on a single 40Ca+ atom16. Our architecture lays the groundwork for computer science research in quantum network programming and paves the way for the development of software that can bring quantum network technology to society.

The ability to sense and control nuclear spins near solid-state defects might enable a range of quantum technologies. Dynamically decoupled radio-frequency (DDrf) control offers a high degree of design flexibility and long electron-spin coherence times. However, previous studies have considered simplified models and little is known about optimal gate design and fundamental limits. Here, we develop a generalized DDrf framework that has important implications for spin sensing and control. Our analytical model, which we corroborate by experiments on a single NV center in diamond, reveals the mechanisms that govern the selectivity of gates and their effective Rabi frequencies, and enables flexible detuned gate designs. We apply these insights to numerically show a 60× sensitivity enhancement for detecting weakly coupled spins and study the optimization of quantum gates in multiqubit registers. These results advance the understanding for a broad class of gates and provide a toolbox for application-specific design, enabling improved quantum control and sensing.

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.

We show coupling of an SnV center to a diamond waveguide of 20% with almost transform-limited optical transitions. Besides, we show control over the SnV spin qubit and extend its coherence to over a millisecond.

We demonstrate heralded initialization of charge state and optical transition frequency of diamond tin-vacancy centers, using (off-)resonant lasers, photon detection and real-time logic. Using this, we show frequency tunability > 100 MHz and strongly improved optical coherence.

The generation of 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, many of which 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.

Open microcavities offer great potential for the exploration and utilization of efficient spin-photon interfaces with Purcell-enhanced quantum emitters thanks to their large spectral and spatial tunability combined with high versatility of sample integration. However, a major challenge for this platform is the sensitivity to cavity length fluctuations in the cryogenic environment, which leads to cavity resonance frequency variations and thereby a lowered averaged Purcell enhancement. This work presents a closed-cycle cryogenic fiber-based microcavity setup, which is in particular designed for a low passive vibration level, while still providing large tunability and flexibility in fiber and sample integration, and high photon collection efficiency from the cavity mode. At temperatures below 10 K, a stability level of around 25 pm is reproducibly achieved in different setup configurations, including the extension with microwave control for manipulating the spin of cavity-coupled quantum emitters, enabling a bright photonic interface with optically active qubits.

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 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.

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.

We demonstrate coherent coupling of a single diamond Tin-Vacancy center to a fiber-based microcavity, showing a cavity transmission dip of 50 % on resonance, and altered photon statistics in cavity transmission.