  Manipulating quantum states through light-matter interactions has been
actively pursued in two-dimensional (2D) materials research. Significant
progress has been made towards the optical control of the valley degrees of
freedom in semiconducting monolayer transition-metal dichalcogenides (TMD),
based on doubly degenerate excitons from their two distinct valleys in
reciprocal space. Here, we introduce a novel kind of optically controllable
doubly degenerate exciton states that come from a single valley, dubbed as
single-valley exciton doublet (SVXD) states. They are unique in that their
constituent holes originate from the same valence band, making possible the
direct optical control of the spin structure of the excited constituent
electrons. Combining ab initio GW plus Bethe-Salpeter equation (GW-BSE)
calculations and a newly developed theoretical analysis method, we demonstrate
such novel SVXD in substrate-supported monolayer bismuthene -- which has been
successfully grown using molecular beam epitaxy. In each of the two distinct
valleys in the Brillouin zone, strong spin-orbit coupling and $C_{3v}$ symmetry
lead to a pair of degenerate 1s exciton states (the SVXD states) with opposite
spin configurations. Any coherent linear combinations of the SVXD in a single
valley can be excited by light with a specific polarization, enabling full
manipulation of their internal spin configurations. In particular, a
controllable net spin magnetization can be generated through light excitation.
Our findings open new routes to control quantum degrees of freedom, paving the
way for applications in spintronics and quantum information science.
