  Nitrogen-vacancy (NV) center in diamond is an ideal candidate for quantum
sensors because of its excellent optical and coherence property. However,
previous studies are usually conducted at low or room temperature. The lack of
full knowledge of coherence properties of the NV center at high temperature
limits NV's further applications. Here, we systematically explore the coherence
properties of the NV center ensemble at temperatures from 300 K to 600 K.
Coherence time $T_2$ decreases rapidly from $184 \mu s$ at 300 K to $30 \mu s$
at 600 K, which is attributed to the interaction with paramagnetic impurities.
Single-quantum and double-quantum relaxation rates show an obvious
temperature-dependent behavior as well, and both of them are dominated by the
two phonon Raman process. While the inhomogeneous dephasing time $T_2^*$ and
thermal echo decoherence time $T_{TE}$ remain almost unchanged as temperature
rises. Since $T_{TE}$ changed slightly as temperature rises, a
thermal-echo-based thermometer is demonstrated to have a sensitivity of $41
mK/\sqrt{Hz}$ at 450 K. These findings will help to pave the way toward
NV-based high-temperature sensing, as well as to have a more comprehensive
understanding of the origin of decoherence in the solid-state qubit.
