Three Megahertz Photon Collection Rate from an NV Center with Millisecond Spin Coherence
This paper introduces a novel approach to enhancing photon collection efficiency from nitrogen-vacancy (NV) centers in diamond, a crucial component for advancing quantum technologies. The authors present a circular "bullseye" diamond grating design that significantly improves the photon collection rate to (3.0±0.1)×106 counts per second from a single NV center, with a millisecond spin coherence time of 1.7 ms. This development addresses the challenge of efficiently capturing NV photoluminescence (PL), which is essential for applications such as quantum information processing and sensing.
The efficient extraction of NV photoluminescence is impeded by the high refractive index of diamond, causing significant internal reflection. Prior attempts to resolve this have involved solid immersion lenses, vertical pillars, optical antennas, and silicon dioxide gratings. However, the reported photon collection rates from these methods remained limited. The introduction of a planar circular "bullseye" diamond grating marks a substantial improvement, offering the highest photon collection rate achieved so far for a single NV center.
The bullseye grating fabrication involved creating concentric slits in a diamond membrane, aligned to satisfy the second-order Bragg condition for optimal scattering and constructive interference. The process required thinning diamond membranes, grown with a controlled concentration of NV centers, and transferring them onto a substrate facilitating optical and spin characterization. Scanning electron microscopy and photoluminescence scans confirmed the device's robustness and accuracy, with experimental validation through back-focal-plane (BFP) imaging revealing significant intensity at low numerical apertures, corroborating theoretical simulations.
Furthermore, the paper details the experimental setup, including a confocal microscope for PL analysis and a CCD camera coupled with a 'Bertrand lens' for imaging the BFP. The saturation curve of the NV PL indicated a substantial improvement in photon collection efficiency, with estimated photon count rates deduced from reliable empirical models, demonstrating marked improvements over unpatterned diamond or alternative geometries.
In terms of quantum coherence, the NV centers within the bullseye structures maintained coherence times comparable to those in unprocessed diamond, as demonstrated by a series of optically detected magnetic resonance and Ramsey measurements. A Hahn echo pulse sequence revealed long phase coherence times, and dynamic decoupling techniques extended these, underscoring the suitability of the bullseye device for maintaining quantum coherence within these intricate quantum systems.
The potential implications of this research are significant, impacting both theoretical and applied domains. The enhanced photon collection efficiency and retention of coherence times suggest that such diamond grating structures could be integrated into quantum networks, single-photon sources, and advanced sensing applications. The on-chip compatibility of the planar bullseye design paves the way for seamless integration with other photonic components, thereby representing a forward step in scalable, high-performance quantum device manufacture.
Looking to the future, this research lays a foundation for further refinement of NV center integration into multifunctional quantum systems. Enhanced efforts in material processing, combined with strategic engineering of diamond photonic structures, may lead to even greater efficiencies and broader applicability across the quantum technology landscape. This may facilitate more advanced applications in quantum information science and lead to the development of novel quantum devices that push the boundaries of current technology.