Point quantum defects in diamond for measuring deuterium-hydrogen ratio in water

Point quantum defects in diamonds are the most promising qubit architecture to design the next generation of sensors that go beyond the boundaries imposed by classical physics. Identifying the deuterium-to-hydrogen (D/H) isotope ratio in water from exoplanets, with a typical range of 10^-4 - 10^-3, is an example of these limitations. In this work, we present a compact, room-temperature μ-Nuclear Magnetic Resonance (NMR) sensor that converts the weak magnetization of thermally polarized H and D nuclei into bright optical signals via ensembles of nitrogen-vacancy centers in (001) diamond without complex hyperpolarization methods. Our design consists of a microfluidic chip and a chassis containing the main hardware to perform NMR measurements (such as a calibration coil, NMR coils, and an MW antenna), centered in an optimized and miniaturized bias magnetic array which provides a highly homogeneous magnetic field larger than 200 mT. This novel magnetic source has an inner (outer) diameter of 4 cm (8 cm), has autoshielding features (4 times lower magnetic field at 15cm above the bore), is tunable, and guarantees a robust range of operation with no power consumption. Here, we detail sensor design, fabrication workflows, and magnet/diamond alignment strategies, along with mitigation plans for optical crosstalk, microwave heating, and fluidic sealing to design a full μ-NMR sensor based on point quantum defects in diamond.