  Magnetic sensing and imaging instruments are important tools in biological
and material sciences. There is an increasing demand for attaining higher
sensitivity and spatial resolution, with implementations using a single qubit
offering potential improvements in both directions. In this article we describe
a scanning magnetometer based on the nitrogen-vacancy center in diamond as the
sensor. By means of a quantum-assisted readout scheme together with advances in
photon collection efficiency, our device exhibits an enhancement in signal to
noise ratio of close to an order of magnitude compared to the standard
fluorescence readout of the nitrogen-vacancy center. This is demonstrated by
comparing non-assisted and assisted methods in a $T_1$ relaxation time
measurement.
