Two-dimensional single-valley exciton qubit and optical spin magnetization generation*

Creating and manipulating coherent qubit states are actively pursued in two-dimensional (2D) materials research. Significant efforts have been made towards the realization of two-valley exciton qubits in monolayer transition-metal dichalcogenides (TMDs), based on states from their two distinct valleys in k-space. Here, we propose a new scheme to create qubits in 2D materials utilizing a novel kind of degenerate exciton states in a single valley. Combining group theoretic analysis and ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations, we demonstrate such novel qubit states 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 along with the symmetry leads to a pair of degenerate 1s exciton states with opposite spin configurations. Specific coherent linear combinations of the two degenerate excitons in a single valley can be excited with light polarizations, enabling full manipulation of the exciton qubits and their spin configurations. In particular, a net spin magnetization can be generated. Our finding opens new routes to create and manipulate qubit systems in 2D materials.