Solid-state spin-photon interfaces are crucial for quantum networks and sensors, requiring single-photon generation, long-lived spin coherence, and scalable integration, ideally at ambient conditions. While progress has been made with systems like diamond and silicon carbide, room-temperature quantum coherent single spins remain rare. Layered materials, such as hBN, offer a promising platform due to their suitability for large-area growth, deterministic defect creation, and hybrid device integration. hBN hosts various defects emitting across the visible and near-infrared, but previous studies on spin signatures were limited to ensembles with low optical quantum efficiency. This work focuses on visible-spectrum defects (~600 nm) known for bright, tunable, single-photon emission, aiming to achieve coherent spin control at room temperature.

Several systems, including defects in diamond and silicon carbide, have been extensively studied for their spin-photon interface properties. However, achieving quantum coherent spins at room temperature in these materials often presents challenges related to optical properties. The exploration of new material systems and engineering of existing candidates for improved performance remains a significant research direction. Layered materials, particularly hBN, have emerged as a promising platform for quantum technologies due to their inherent advantages in scalability. While some spin signatures in hBN defects have been observed, these were mainly on an ensemble level due to limitations in optical quantum efficiency. Previous reports on visible-spectrum defects showed spin signatures via optically detected magnetic resonance (ODMR) but only under a finite magnetic field.

Multilayer hBN was grown via metal-organic vapor phase epitaxy using a carbon precursor and ammonia. The carbon precursor flow rate controlled the defect density, yielding individually addressable defects. Confocal microscopy was used for photoluminescence (PL) measurements, with a 532 nm laser for excitation and avalanche photodiodes or a spectrometer for detection. Optically detected magnetic resonance (ODMR) was performed using a copper coil for microwave generation and a continuous-wave 70 Hz square-wave modulation or pulsed sequences for spin control and readout. Ramsey interferometry and dynamical decoupling pulse protocols were employed to characterize spin coherence times. Angle-resolved magneto-optical measurements were conducted to determine the defect's symmetry axis.

This study demonstrates room-temperature coherent spin control of individually addressable single-photon-emitting defects in hBN. The defects were identified as having a spin-triplet (S=1) ground-state spin manifold with a 1.96 GHz zero-field splitting. The principal symmetry axis of the defect lies in the hBN layer plane, suggesting a low-symmetry chemical structure. Microwave-based Ramsey interferometry revealed an inhomogeneous dephasing time (T2) of ~100 ns. Surprisingly, the continuously driven spin Rabi coherence time (TRabi) was prolonged beyond 1 µs at room temperature without an applied magnetic field. Dynamical decoupling pulse protocols further extended the spin-echo coherence time (T2echo) to ~200 ns, reaching ~1 µs with ten refocusing pulses. Analysis of ODMR signal fine structure suggests hyperfine coupling to only a few inequivalent nitrogen and boron nuclei. The T1 spin-lattice relaxation time varied between 35–200 µs, significantly longer than the excited-state lifetime, confirming the ground-state nature of the S=1 system. High-resolution ODMR spectroscopy, combined with magnetic field-dependent measurements, points toward hyperfine coupling to only two inequivalent nuclei, potentially two 14N atoms. This low-symmetry structure, combined with the observed coherence, suggests the defect is not a single-site vacancy, single-atom substituent or carbon tetramer, with carbon trimers and vacancy-substituent complexes being more plausible candidates.

The findings address the need for room-temperature, ambient-condition quantum coherent single spins for quantum technologies. The observed long coherence times, even without applied magnetic fields, are significant, highlighting the potential of hBN defects for scalable quantum repeaters and sensors. The identification of a specific spin-triplet defect with its characteristic hyperfine interactions provides valuable insights for theoretical modeling and the design of future quantum devices. The potential for integrating this system into nanostructures to further tune its properties makes it particularly attractive for practical applications.

This work demonstrates the remarkable quantum coherence properties of a novel spin-triplet defect in hBN at room temperature and zero magnetic field. The long coherence times achieved, along with the single-photon emission capabilities, pave the way for utilizing this system in various quantum technologies. Future research should focus on characterizing the precise chemical structure of the defect, exploring the possibilities of manipulating the nearby nuclear spins for quantum computation, and integrating these defects into nanoscale devices for applications in quantum sensing and communication.

The study focuses on a specific type of defect in hBN; the findings might not be generalizable to other types of defects in hBN or other layered materials. The precise chemical structure of the defect remains to be fully identified through further theoretical and experimental studies. The observed coherence times are influenced by the specific nuclear environment of each defect, suggesting some variability could be expected across different samples or fabrication processes.