Title: Microscale Fiber-Integrated Vector Magnetometer with On-Tip Field Biasing using NV Ensembles in Diamond Microcystals

Abstract: In quantum sensing of magnetic fields, ensembles of NV centers in diamond offer high sensitivity, high bandwidth and outstanding spatial resolution while operating in harsh environments. Moreover, the orientation of defect centers along four crystal axes forms an intrinsic coordinate system, enabling vector magnetometry within a single diamond crystal. While most vector magnetometers rely on a known bias magnetic field for full recovery of three-dimensional field information, employing external 3D Helmholtz coils or permanent magnets results in bulky, laboratory-bound setups, impeding miniaturization of the device. Here, a novel approach is presented that utilizes a fiber-integrated microscale coil at the fiber tip to generate a localized uniaxial magnetic field. The same fiber-tip coil is used in parallel for spin control by combining DC and microwave signals in a bias tee. To implement vector magnetometry using a uniaxial bias field, we preselect the orientation of the diamond crystal and then fully characterize it by rotating a static magnetic field in three planes of rotation. We demonstrate the measurement of vector magnetic fields in the full solid angle with a shot-noise limited sensitivity of $19.4\:\textrm{nT/Hz}^{1/2}$ and microscale spatial resolution while achieving a cross section of the fiber sensor head below $1\:\textrm{mm}^2.$