  Monolayers of semiconducting transition metal dichalcogenides exhibit
intriguing fundamental physics of strongly coupled spin and valley degrees of
freedom for charge carriers. While the possibility of exploiting these
properties for information processing stimulated concerted research activities
towards the concept of valleytronics , maintaining control over spin-valley
polarization proved challenging in individual monolayers. A promising
alternative route explores type II band alignment in artificial van der Waals
heterostructures. The resulting formation of interlayer excitons combines the
advantages of long carrier lifetimes and spin-valley locking . Here, we
demonstrate direct magnetic manipulation of valley polarization in a WSe2/MoSe2
heterostructure through giant valley Zeeman splitting of interlayer
transitions. Remarkably, even after non-selective injection, the observed $g$
factor as large as $-15$ induces near-unity polarization of long-lived excitons
with 100 ns lifetimes under magnetic fields. The demonstrated control of the
spin-valley physics highlights the exceptional aspects of novel, artificially
designed material systems and their promise for atomically-thin valleytronic
devices.
