Cavity-enhancement of a low-noise single-photon emitter in diamond

With the immense progress demonstrated in recent years in the field of quantum communication, the use of quantum networks in applications ranging from secure communication to distributed quantum computing is within reach. Systems meeting the requirements set for distant, long-lived network nodes are continuously being identified and developed. The nitrogen-vacancy center (NV) in diamond is a solid-state defect fulfilling many of these conditions, one being its spin-state-dependent fluorescence that provides a direct way to distribute entanglement across a quantum network. Further advantages include record-long spin coherence times at low- and room temperature, spin-state control via electric, magnetic, strain and optical fields, and nearby nuclear spins that can be used for the storage and manipulation of quantum states. However, NVs also display a number of limitations related to their photonic properties. An inefficient photon extraction from the diamond host material, a small fraction of coherent emission, a long radiative lifetime as well as broadened optical linewidths in microstructured diamond all impose limitations on experimental implementations of NVs as nodes in a quantum network.
In this work, we address each of the above-mentioned problems. We develop two improved methods for NV formation - laser writing and carbon implantation post-fabrication (IPF) - and show that they result in reduced NV optical linewidths, both in bulk and in microstructured diamond. This indicates a lower charge-noise level and reduced spectral diffusion compared to other approaches. Most importantly, the majority of the created NVs exhibit a linewidth below 150 MHz, implying a minimized need for NV pre-selection and a reduced experimental overhead in spin-photon entanglement operations.
We incorporate diamond platelets with NVs created via carbon IPF into an open microcavity and make use of the Purcell enhancement resulting from tuning the cavity onto resonance with specific NV transitions to increase the coherent photon flux. We successfully operate the system at cryogenic temperatures, achieving high finesse values of up to 7900 on the diamond, and detect photon count rates under off-resonant excitation four times higher than the current state-of-the-art. For the first time for an NV in diamond, we successfully measure resonance fluorescence with a signal-to-background ratio higher than 1 without relying on temporal filtering. Projecting the current system efficiency to applications relying on two-mode interference would result in entanglement rates increased by more than an order of magnitude. Even higher gains are within reach after reasonable system improvements. The low-charge-noise NVs and the ability to resonantly generate single photons and with high probability establish the NV-cavity platform as an attractive photonic interface, paving the way for faster and more efficient long-distance quantum communication based on defects in diamond.