In working towards a quantum internet, nodes based on nitrogen-vacancy (NV) centres in diamond have shown great potential. A key challenge in scaling these networks is the low entanglement generation rate due to low coherent photon emission (≈ 3%) and limited collection efficiency (≈ 15% using state-of-the-art solid immersion lens setups). Both can be improved by integrating NV centres in an optical cavity. In this thesis NV centres coupled to an open Fabry-Pérot microcavity are investigated. The NV centres are integrated into the cavity by bonding a μm-thin diamond sample to one of the mirrors.
The goal of this thesis is to move towards the realisation of an efficient spin photon interface of NV centres in an open microcavity. To this end, short optical pulses for eventual spinphoton entanglement creation and microwave electronics for spin control are implemented.
A cavity is formed and characterised. A finesse of 3.3×103 is found, along with a quality factor of (3.14 ± 0.03)×105 and a mode volume of 83 𝜆3. From this, a theoretical outcoupled coherent photon fraction of 14% is determined. NV centres are found in the cavity, and their coupling strength is determined using off-resonant lifetime measurements. From this, the actual outcoupled coherent photon fraction is determined to be (12 ± 1) %. Which represents a more than 25 times improvement over NV centres in solid immersion lenses.
The electron spin resonance (ESR) of an NV centre is measured and a magnetic field strength aligned with its spin axis of (36 ± 1) G is found. A lifetime measurement of an NV centre using pulsed resonant excitation is shown. The last two measurements can be extended to achieve coherent control and resonant readout of the NV spin. Paving the way towards a more efficient spin-photon interface.

The nitrogen-vacancy (NV) center in diamond can be used as a quantum network node [1], but the low Debye-Waller factor β0 ≈ 3 % [3, 4] limits the entanglement rate of the system drastically. By embedding the color center into an open microcavity and utilizing the Purcell effect, this limitation can be reduced [3, 5]. Additionally, as coupling an emitter to an optical cavity enhances the collection efficiency of the emitted coherent photons, such a system is also interesting for emitters with a higher intrinsic Debye-Waller factor, as, for example, the tin-vacancy (SnV) center in diamond. The diamond sample between the cavity mirrors is an important building block of such a cavity system. In this thesis, the fabrication of singledigit micrometer-thin, color center-enriched diamond platelets with side lengths of tens of micrometers is studied. Additionally, their properties were characterized after bonding them to a Bragg mirror. Platelets containing NV centers and platelets with implanted SnV centers were produced. In addition to fabrication with electron beam (e-beam) lithography and dry etching, a new method is introduce utilizing laser cutting. A reasonable bonding yield was achieved for platelets from both fabrication methods, and a minimal surface roughness of Rq ≈ 0.2 nm was measured. The samples were studied in a low-temperature confocal microscopy setup to determine the color center’s optical linewidth of the zero-phonon line (ZPL) emission. For the SnV centers in the diamond platelets a close to lifetime-limited [6] dephasing linewidth of Γd ≈ 32.2(4) MHz and a minimum spectral diffusion linewidth of Γs ≈ 58(2) MHz was observed. For the NV center, only the spectral diffusion linewidth could be measured Γs < 80 MHz. Those measured linewidth lay within the bounds to achieve two-photon quantum interference of separate color centers [7]. Additionally, the diamond platelets were studied in an open microcavity setup. With an e-beam-lithography and dry etching fabricated, NV center-enriched sample, a maximum finesse of F ≈ 3100(300) was measured. Additional losses introduced by the diamond of Ladd,dia ≈ 1200(200) ppm could be estimated. For a laser cut fabricated, SnV center-enriched sample, a maximum finesse of F ≈ 2300(200) could be achieved. An estimation for the maximum achievable Purcell enhancement and resulting branching ratio into the cavity mode was calculated using the measured finesse and cavity parameters. For the SnV center, a maximum achievable Purcell factor of F ZPL P ≈ 15(2) was computed, resulting in a branching ratio into the cavity mode of βcav ≈ 85(2) %. In addition, an upper limit for the outcoupling percentage trough the plane mirror of βout < 62(13) % was estimated. For the NV center sample, F ZPL P ≈ 19(2), βcav ≈ 37(2) %, and βout ≈ 13(6) % were calculated.