  The unavoidable interaction between thermal environments and quantum systems
typically leads to the degradation of the quantum coherence, which can be
fought against by reservoir engineering. We propose that a thermal coherent
state can be realized using a thermally driven two-level system longitudinally
coupled to a resonator. Using the master equation approach to describe the open
system dynamics, we obtain the steady-state solution of the master equation for
the two-level system and resonator. We find that the state of the resonator is
a thermal coherent state, while the two-level system remains thermal. This
observation is verified by evaluating the second-order correlation coefficient
and photon number statistics of the resonator. Moreover, we reveal the
potential benefits of employing the thermal coherent state of the resonator in
quantum thermometry. In this context, the resonator functions as a probe to
measure the unknown temperature of a bath mediated by a two-level system,
strategically bridging the connection between the two. Our findings elucidate
that using an ancilla-assisted probe may enhance precision and broaden the
applicable temperature range.
