  Degenerate extrema in the energy dispersion of charge carriers in solids,
also referred to as valleys, can be regarded as a binary quantum degree of
freedom, which can potentially be used to implement valleytronic concepts in
van der Waals heterostructures based on transition metal dichalcogenides. Using
magneto-photoluminescence spectroscopy, we achieve a deeper insight into the
valley polarization and depolarization mechanisms of interlayer excitons formed
across a MoS$_2$/MoSe$_2$/MoS$_2$ heterostructure. We account for the
non-trivial behavior of the valley polarization as a function of the magnetic
field by considering the interplay between exchange interaction and phonon
mediated intervalley scattering in a system consisting of Zeeman-split energy
levels. Our results represent a crucial step towards the understanding of the
properties of interlayer excitons, with strong implications for the
implementation of atomically thin valleytronic devices.
