High frequency magnetometry with an ensemble of spin qubits in hexagonal boron nitride

Sensors based on spin qubits in 2D crystals offer the prospect of nanoscale proximities between sensor and source, which could provide access to otherwise inaccessible signals. For AC magnetometry, the sensitivity and frequency range are typically limited by the noise spectrum, which determines the qubit coherence time. We address this using phase modulated continuous concatenated dynamic decoupling, which extends the coherence time towards the T-1 limit at room temperature and enables tuneable narrowband AC magnetometry. Using an ensemble of negatively charged boron vacancies in hexagonal boron nitride, we detect out-of-plane AC fields in the range of similar to 10 - 150 MHz, and in-plane fields within +/- 150 MHz of the electron spin resonance. We measure an AC magnetic field sensitivity of similar to 1 root T/Hz at similar to 2.5 GHz, for a sensor volume of similar to 0.1 mu m(3). This work establishes the viability of spin defects in 2D materials for high frequency magnetometry, with wide-ranging applications across science and technology.