  Quantum sensors provide unprecedented magnetic field detection sensitivities,
enabling these to extend the common magnetometry range of applications and
environments of operation. In this framework, many applications also require
high spatial resolution magnetic measurements for biomedical research,
environmental monitoring and industrial production. In this regard, Optically
Pumped Magnetometers (OPMs) are considered as prominent candidates, but are
impaired in size with micrometer scale magnetic particles, e.g. magnetic
droplets. In order to address this limitation, here we study the effects of
adding a micrometer-to-millimeter magnetic guide to a miniature OPM. This
device is applied to detect Fe3O4 magnetic droplets flowing at rates up to $25$
drop./s in a microfluidic channel. The computed spatial resolution is $300$
$\mu$m and the measured SNR is larger than $15$ dB for the different sizes of
considered magnetic droplets.

Applied Physics;Quantum Physics