Parabolic Diamond Scanning Probes for Single-Spin Magnetic Field Imaging

Enhancing the measurement signal from solid-state quantum sensors such as the nitrogen-vacancy (N-$V$) center in diamond is a challenge for sensing and imaging of condensed-matter systems. Here we design and fabricate diamond scanning probes with a truncated parabolic profile that optimizes the photonic signal from single, near-surface, embedded N-$V$ centers, forming a high-sensitivity probe for nanoscale magnetic field imaging. These structures operate over the full N-$V$ photoluminescence spectrum with the emission being directed into a measured numerical aperture of 0.44. Furthermore, they yield a median saturation count rate of 2.1 MHz, a fivefold improvement in measurement signal over the state of the art in scanning-probe-based N-$V$ sensors. We additionally introduce a charge-state-registered pulsed-excitation method for measuring the detection efficiency of photons from the N-$V$'s metrologically relevant negative charge state, finding an overall detection efficiency of $12\mathrm{%}$. We demonstrate the excellent properties of these diamond scanning probes by imaging thin-film ferromagnetic stripes with a spatial resolution better than 50 nm.