  We theoretically predict a nonequilibrium phase transition in quantum spin
systems induced by a laser, which provides a purely quantum-mechanical way of
coherently controlling magnetization. Namely, when a circularly polarized laser
is applied to a spin system, the magnetic component of a laser is shown to
induce a magnetization normal to the plane of polarization, leading to an
ultrafast phase transition. We first demonstrate this phenomenon numerically
for an $S=1$ antiferromagnetic Heisenberg spin chain, where a new state emerges
with magnetization perpendicular to the polarization plane of the laser in
place of the topologically ordered Haldane state. We then elucidate its
physical mechanism by mapping the system to an effective static model. The
theory also indicates that the phenomenon should occur in general quantum spin
systems with a magnetic anisotropy. The required laser frequency is in the
terahertz range, with the required intensity being within a prospective
experimental feasibility.
