  Heterostructures comprising different monolayer semiconductors provide a new
system for fundamental science and device technologies, such as in the emerging
field of valleytronics. Here, we realize valley-specific interlayer excitons in
monolayer WSe2-MoSe2 vertical heterostructures. We create interlayer exciton
spin-valley polarization by circularly polarized optical pumping and determine
a valley lifetime of 40 nanoseconds. This long-lived polarization enables the
visualization of the expansion of a valley-polarized exciton cloud over several
microns. The spatial pattern of the polarization evolves into a ring with
increasing exciton density, a manifestation of valley exciton exchange
interactions. Our work lays a foundation for quantum optoelectronics and
valleytronics based on interlayer excitons in van der Waals heterostructures.
