A miniaturized magnetic field sensor based on nitrogen-vacancy centers

The nitrogen-vacancy (NV) center in diamond is a prime candidate for quantum sensing technologies. Ongoing miniaturization calls for ever-smaller sensors maintaining good measurement performance. Here, we present a fully integrated mechanically robust fiber-based endoscopic sensor capable of $5.9\,\mathrm{nT}/ \sqrt{\mathrm{Hz}}$ magnetic field sensitivity utilizing $15\,\mathrm{\mu m}$ sized microdiamonds at a microwave power of $50\,\mathrm{mW}$ and optical power of $2.15\,\mathrm{mW}$. A direct laser writing process is used to localize a diamond containing NV centers above the fiber's core by a polymer structure. This structure enables stable optical access and independent guiding of excitation and fluorescent light in different optical fibers. This separation strongly reduces the contribution of autofluorescence from the excitation light in the optical fiber. Moreover, a metallic direct laser written antenna structure is created next to the fibers' facet, allowing microwave manipulation of the NV centers' spins. The fabricated endoscopic sensor provides a robust platform with a tip diameter of $1.25\,\mathrm{mm}$. The device enables remote optical and microwave access to perform the full range of coherent spin measurements with NV centers at a spatial resolution of $15\,\mathrm{\mu m}$. We demonstrate the capability of vector magnetic field measurements in a magnetic field as used in state-of-the-art ultracold quantum gas experiments, opening a potential field in which high resolution and high sensitivity are necessary.