Quantum sensors have attracted broad interest in the quest towards sub-micronscale NMR spectroscopy. Such sensors predominantly operate at low magnetic fields. Instead, however, for high resolution spectroscopy, the high-field regime is naturally advantageous because it allows high absolute chemical shift discrimination. Here we demonstrate a high-field spin magnetometer constructed from an ensemble of hyperpolarized 13C nuclear spins in diamond. They are initialized by Nitrogen Vacancy (NV) centers and protected along a transverse Bloch sphere axis for minute-long periods. When exposed to a time-varying (AC) magnetic field, they undergo secondary precessions that carry an imprint of its frequency and amplitude. For quantum sensing at 7T, we demonstrate detection bandwidth up to 7 kHz, a spectral resolution < 100mHz, and single-shot sensitivity of 410pT$$/\sqrt{{{{{{{{\rm{Hz}}}}}}}}}$$ / Hz . This work anticipates opportunities for microscale NMR chemical sensors constructed from hyperpolarized nanodiamonds and suggests applications of dynamic nuclear polarization (DNP) in quantum sensing.

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Quantum sensors have attracted broad interest in the quest towards sub-micronscale NMR spectroscopy. Such sensors predominantly operate at low magnetic fields. Instead, however, for high resolution spectroscopy, the high-field regime is naturally advantageous because it allows high absolute chemical shift discrimination. Here we propose and demonstrate a high-field spin magnetometer constructed from an ensemble of hyperpolarized 13 C nuclear spins in diamond. The 13 C nuclei are initialized via Nitrogen Vacancy (NV) centers and protected along a transverse Bloch sphere axis for minute-long periods. When exposed to a time-varying (AC) magnetic field, they undergo secondary precessions that carry an imprint of its frequency and amplitude. The method harnesses long rotating frame 13 C sensor lifetimes 𝑇 2 >20s, and their ability to be continuously interrogated. For quantum sensing at 7T and a single crystal sample, we demonstrate spectral resolution better than 100 mHz (corresponding to a frequency precision <1ppm) and single-shot sensitivity better than 70pT. We discuss advantages of nuclear spin magnetometers over conventional NV center sensors, including deployability in randomly-oriented diamond particles and in optically scattering media. Since our technique employs densely-packed 13 C nuclei as sensors, it demonstrates a new approach for magnetometry in the "coupled-sensor" limit. This work points to interesting opportunities for microscale NMR chemical sensors constructed from hyperpolarized nanodiamonds and suggests applications of dynamic nuclear polarization (DNP) in quantum sensing.