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.

Quantum networks promise exciting applications that are not possible with their classical counterparts, such as distributed quantum computation or fundamentally secure quantum communication. Optically active spins in solid states are among the prime candidates for realizing quantum network nodes, while photons are used to distribute entanglement between nodes. The nitrogen-vacancy (NV) center in diamond is a pioneering system with the demonstrations of teleportation of quantum states within a three-node network and metropolitan-scale heralded entanglement. However, scaling to more nodes or longer distances is limited by the low extraction of resonant photons, creating a high interest in optical resonators (cavities) to improve the optical interface. Moreover, the diamond tin-vacancy (SnV) center with better optical properties emerged as a promising alternative candidate for quantum network nodes.
This dissertation presents the building blocks and the experimental realization of an open, fiber-based, cryogenic Fabry-Pérot microcavity enabling the Purcell enhancement of diamond NV and SnV centers, incorporated into the microcavity via a diamond membrane. The background on color centers in the context of quantum networking (Chapter 2) and optical cavities for diamond color centers is summarized (Chapter 3). A novel laser-cutting patterning method is introduced, which can be used to fabricate micrometer-thin diamond devices with arbitrary lateral shapes in the range of tens to hundreds of micrometers (Chapter 4). Microdevices fabricated by this method are characterized by scanning cavity microscopy, revealing a high cavity finesse. Furthermore, SnV and NV centers in the microdevices maintain bulk-like optical properties, which are required for quantum networking. Next to the sample fabrication, the detailed design, construction, and operation of a cryogenic microcavity system is presented, reaching a reproducible cavity length stability level of around 25 picometer with a sample temperature of about 8 kelvin on the cavity mirror (Chapter 5), a prerequisite for the following cavity experiments. Two cavity quantum optics experiments are conducted, exploring the regimes of coherent cavity coupling and efficient photon extraction with the cavity. In the first experiment, a single SnV center is coupled to the cavity, achieving a coherent cooperativity of 0.7. This enables the observation of nonlinear quantum effects, such as the modulation of the cavity resonance by an individual SnV center and the altered photon statistics of light transmitted through the cavity (Chapter 6). In the second experiment, a single NV center is coupled to the cavity, and the Purcell enhancement combined with a high cavity outcoupling leads to a resonant photon extraction (end-to-end) efficiency of 0.5 %. The NV center’s electron spin qubit initialization, manipulation with an on-chip microwave stripline, and readout are utilized to generate spin-photon correlated states, a precondition for remote entanglement with a second color center (Chapter 7).
The presented platform combines an efficient optical interface with microwave control of the spin state and can be used for the exploration of optically active defects in solid states, as a bright source of single photons, and for cavity-enhanced quantum networking.

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.

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.

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 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.

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.