Over the years, an enormous effort has been made to establish nitrogen vacancy (N-V) centers in diamond as easily accessible and precise magnetic field sensors. However, most of their sensing protocols rely on the application of bias magnetic fields, preventing their usage in zero- or low-field experiments. We overcome this limitation by exploiting the full spin S=1 nature of the N-V center, allowing us to detect nuclear spin signals at zero and low field with a linearly polarized microwave field. As conventional dynamical decoupling protocols fail in this regime, we develop robust pulse sequences and optimize pulse pairs, which allow us to sense temperature and weak ac magnetic fields and achieve an efficient decoupling from environmental noise. Our work allows for much broader and simpler applications of N-V centers as magnetic field sensors in the zero- and low-field regime and can be further extended to three-level systems in ions and atoms.

Nuclear spin hyperpolarization provides a promising route to overcome the challenges imposed by the limited sensitivity of nuclear magnetic resonance. Here we demonstrate that dissolution of spin-polarized pentacene-doped naphthalene crystals enables transfer of polarization to target molecules via intermolecular cross-relaxation at room temperature and moderate magnetic fields (1.45 T). This makes it possible to exploit the high spin polarization of optically polarized crystals, while mitigating the challenges of its transfer to external nuclei. With this method, we inject the highly polarized mixture into a benchtop NMR spectrometer and observe the polarization dynamics for target ¹H nuclei. Although the spectra are radiation damped due to the high naphthalene magnetization, we describe a procedure to process the data to obtain more conventional NMR spectra and extract the target nuclei polarization. With the entire process occurring on a time scale of 1 min, we observe NMR signals enhanced by factors between −200 and −1730 at 1.45 T for a range of small molecules.