  Nitrogen-vacancy (NV) centers in diamond are promising quantum sensors for
their long spin coherence time under ambient conditions. However, their spin
resonances are relatively insensitive to non-magnetic parameters such as
temperature. A magnetic-nanoparticle-nanodiamond hybrid thermometer, where the
temperature change is converted to the magnetic field variation near the Curie
temperature, was demonstrated to have enhanced temperature sensitivity (11 mK
Hz^{-1/2}) [Phys. Rev. X 8, 011042 (2018)], but the sensitivity was limited by
the large spectral broadening of ensemble spins in nanodiamonds. To overcome
this limitation, here we showed an improved design of a hybrid nanothermometer
using a single NV center in a diamond nanopillar coupled with a single magnetic
nanoparticle of copper-nickel alloy, and demonstrated a temperature sensitivity
of 76 uK Hz^{-1/2}. This hybrid design enabled detection of 2 millikelvins
temperature changes with temporal resolution of 5 milliseconds. The
ultra-sensitive nanothermometer offers a new tool to investigate thermal
processes in nanoscale systems.
