  The growing field of quantum information technology requires propagation of
information over long distances with efficient readout mechanisms. Excitonic
quantum fluids have emerged as a powerful platform for this task due to their
straightforward electro-optical conversion. In two-dimensional transition metal
dichalcogenides, the coupling between spin and valley provides exciting
opportunities for harnessing, manipulating and storing bits of information.
However, the large inhomogeneity of single layers cannot be overcome by the
properties of bright excitons, hindering spin-valley transport. Nonetheless,
the rich band structure supports dark excitonic states with strong binding
energy and longer lifetime, ideally suited for long-range transport. Here we
show that dark excitons can diffuse over several micrometers and prove that
this repulsion-driven propagation is robust across non-uniform samples. The
long-range propagation of dark states with an optical readout mediated by
chiral phonons provides a new concept of excitonic devices for applications in
both classical and quantum information technology.
