The nitrogen vacancy (NV) center in diamond is an atom-sized defect in diamond whose spin exhibits remarkable quantum coherence in a uniquely accessible way: at room temperature, in ambient atmosphere, and with a simple optical microscope. Furthermore the NV center is an excellent sensor of magnetic, electric, thermal, and strain fields on the nanoscale, and hence it is being actively explored for a variety of quantum technologies. In this talk, I describe the development of an NV-based scanning probe magnetometer and its application to studying nanoscale magnetism in condensed matter systems. Importantly the magnetometer operates at liquid helium temperatures, opening up the possibility of exploring various condensed matter phenomena at low temperatures. Our magnetic images of superconductors and helical magnets indicate our scanning magnetometer has nanometer-scale stability and spatial resolution. I also present results of a diamond hybrid quantum system in which spin, photons, and phonons are coupled. Using high-quality diamond mechanical resonators strain-coupled to embedded NV centers, we show that coherent mechanical control of individual diamond spins and their optical properties is possible. These results are encouraging for proposals to use such a spin-mechanical platform for spin-squeezing, phonon-mediated spin-spin interactions, and phonon cooling and lasing.
