  Nitrogen-vacancy centers in diamond are attractive as quantum sensors owing
to their remarkable optical and spin properties under ambient conditions. Here
we experimentally demonstrated a hybrid fiber-based thermometer coupled with
nitrogen-vacancy center ensemble and a permanent magnet, where the temperature
sensitivity was improved by converting the temperature variation to the
magnetic field change based on the thermal-demagnetization of the permanent
magnet. We have achieved both large temperature working range (room temperature
to 373 K) and millikelvin sensitivity (1.6 mK/sqrt(Hz)), nearly 6-fold
improvement compared with conventional technique. This stable and compact
hybrid thermometer will enable a wide range of applications for large-area
detection and imaging with high temperature sensitivity.
