Quantum sensing with a spin ensemble in a van der Waals 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 detectable signal threshold. However, leading platforms such as nitrogen-vacancy centers in diamond suffer performance degradation near surfaces or in nanoscale volumes, motivating the search for optically addressable spin sensors in atomically thin, two-dimensional (2D) materials. Here, we present an experimental framework to probe a novel 2D spin ensemble, including its Hamiltonian, coherent sensing dynamics, and noise environment. Using a central spin system in a 2D hexagonal boron nitride (hBN) crystal, we fully map hyperfine interactions with proximal nuclear spins, demonstrate programmable switching between magnetic- and electric-field noise decoupling, and introduce a robust method for reconstructing the environmental noise spectrum that explicitly accounts for quantum control imperfections. We achieve a record coherence time of 80 μs and nanotesla-level AC magnetic sensitivity within a 10-nm-thick hBN host. 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 quantum functionalities.