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.

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.

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.

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.

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.

Efficient coupling of optically active qubits to optical cavities is a key challenge for solid-state-based quantum optics experiments and future quantum technologies. Here we present a quantum photonic interface based on a single tin-vacancy center in a micrometer-thin diamond membrane coupled to a tunable open microcavity. We use the full tunability of the microcavity to selectively address individual tin-vacancy centers within the cavity mode volume. Purcell enhancement of the tin-vacancy center optical transition is evidenced both by optical excited state lifetime reduction and by optical linewidth broadening. As the emitter selectively reflects the single-photon component of the incident light, the coupled emitter-cavity system exhibits strong quantum nonlinear behavior. On resonance, we observe a transmission dip of 50% for low incident photon number per Purcell-reduced excited state lifetime, while the dip disappears as the emitter is saturated with higher photon number. Moreover, we demonstrate that the emitter strongly modifies the photon statistics of the transmitted light by observing photon bunching. This work establishes a versatile and tunable platform for advanced quantum optics experiments and proof-of-principle demonstrations on quantum networking with solid-state qubits.

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.

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 show diamond Tin-Vacancy centers, coherently-coupled to a tunable microcavity. The exceptional optical properties of this emitter in combination with a stable, high quality cavity enables a cavity transmission signal modulated by a single emitter.