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.

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.