  Van der Waals heterostructures offer attractive opportunities to design
quantum materials. For instance, transition metal dichalcogenides (TMDs)
possess three quantum degrees of freedom: spin, valley index, and layer index.
Further, twisted TMD heterobilayers can form moir\'e patterns that modulate the
electronic band structure according to atomic registry, leading to spatial
confinement of interlayer exciton (IXs). Here we report the observation of
spin-layer locking of IXs trapped in moir\'e potentials formed in a
heterostructure of bilayer 2H-MoSe$_2$ and monolayer WSe$_2$. The phenomenon of
locked electron spin and layer index leads to two quantum-confined IX species
with distinct spin-layer-valley configurations. Furthermore, we observe that
the atomic registries of the moir\'e trapping sites in the three layers are
intrinsically locked together due to the 2H-type stacking characteristic of
bilayer TMDs. These results identify the layer index as a useful degree of
freedom to engineer tunable few-level quantum systems in two-dimensional
heterostructures.
