Quantum sensing with a spin ensemble in a two-dimensional material
Quantum sensing with solid-state spin defects has transformed nanoscale metrology, offering sub-wavelength spatial resolution with exceptional sensitivity to multiple signal types. Maximizing these advantages requires minimizing both the sensor-target separation and the detectable signal threshold. However, leading platforms such as nitrogen-vacancy (NV) centers in diamond suffer from performance degradation near surfaces or in nanoscale volumes, motivating the search for optically addressable spin sensors in atomically thin, two-dimensional (2D) van der Waals materials. Here, we present a comprehensive experimental framework to probe a 2D spin ensemble, including its Hamiltonian, coherent sensing dynamics, and noise environment. Using a central spin system in a hexagonal boron nitride (hBN) crystal, we fully map the hyperfine interactions with proximal nuclear spins, demonstrate switchable magnetic and electric noise sensing, and introduce a method to accurately reconstruct the environmental noise spectrum while explicitly accounting for quantum control imperfections. We achieve a record coherence time of $80~μ\mathrm{s}$ under dynamical decoupling, enabling sub-microtesla AC magnetic sensitivity at a $10~\mathrm{nm}$ target distance. Leveraging the broad opportunities for defect engineering in atomically thin hosts, these results lay the foundation for next-generation quantum sensors with ultrahigh sensitivity, tunable noise selectivity, and versatile functionalities.