  We study the quantum phase synchronization of a driven two-level system (TLS)
coupled to a structured environment and demonstrate that quantum
synchronization can be enhanced by the classical driving field. We use the
Husimi $Q$-function to characterize the phase preference and find the in-phase
and anti-phase locking phenomenon in the phase diagram. Remarkably, we show
that the in-phase classical driving enables a TLS to reach stable anti-phase
locking in the Markovian regime. However, we find that the synergistic action
of classical driving and non-Markovian effects significantly enhances the
initial in-phase locking. By introducing the $S$-function and its maximal value
to quantify the strength of synchronization and sketch the synchronization
regions, we observe the typical signatures of the hollowed Arnold tongue in the
parameter regions of synchronization. In the hollowed Arnold tongue, the
synchronization regions exist both inside and outside the tongue while
unsynchronized regions only lie on the boundary line. We also provide an
intuitive interpretation of the above results by using the quasimode theory.
