  Nitrogen-vacancy (NV) centres in diamond hold promise in quantum sensing
applications. A major interest in them is an enhancement of their sensitivity
by the extension of the coherence time ($T_2$). In this report, we
experimentally generated more than four dressed states in a single NV centre in
diamond based on Autler-Townes splitting (ATS). We also observed the extension
of the coherence time to $T_2 \sim$ 1.5 ms which is more than two orders of
magnitude longer than that of the undressed states. As an example of a quantum
application using these results we propose a protocol of quantum sensing, which
shows more than an order of magnitude enhancement in the sensitivity.
