  As one of the most promising Kitaev quantum-spin-liquid (QSL) candidates,
$\alpha$-RuCl$_3$ has received a great amount of attention. However, its ground
state exhibits a long-range zigzag magnetic order, which defies the QSL phase.
Nevertheless, the magnetic order is fragile and can be completely suppressed by
applying an external magnetic field. Here, we explore the evolution of magnetic
excitations of $\alpha$-RuCl$_3$ under an in-plane magnetic field, by carrying
out inelastic neutron scattering measurements on high-quality single crystals.
Under zero field, there exist spin-wave excitations near the $M$ point and a
continuum near the $\mit\Gamma$ point, which are believed to be associated with
the zigzag magnetic order and fractional excitations of the Kitaev QSL state,
respectively. By increasing the magnetic field, the spin-wave excitations
gradually give way to the continuous excitations. On the verge of the critical
field $\mu_0H_{\rm c}=7.5$ T, the former vanish and only the latter is left,
indicating the emergence of a pure QSL state. By further increasing the field
strength, the excitations near the $\mit\Gamma$ point become more intense. By
following the gap evolution of the excitations near the $\mit\Gamma$ point, we
are able to establish a phase diagram composed of three interesting phases,
including a gapped zigzag order phase at low fields, possibly-gapless QSL phase
near $\mu_0H_{\rm c}$, and gapped partially polarized phase at high fields.
These results demonstrate that an in-plane magnetic field can drive
$\alpha$-RuCl$_3$ into a long-sought QSL state near the critical field.
