  Van der Waals heterostructures of atomically thin layers with rotational
misalignments, such as twisted bilayer graphene, feature interesting structural
moir\'e superlattices. Due to the quantum coupling between the twisted atomic
layers, light-matter interaction is inherently chiral; as such, they provide a
promising platform for chiral plasmons in the extreme nanoscale. However, while
the interlayer quantum coupling can be significant, its influence on chiral
plasmons still remains elusive. Here we present the general solutions from full
Maxwell equations of chiral plasmons in twisted atomic bilayers, with the
consideration of interlayer quantum coupling. We find twisted atomic bilayers
have a direct correspondence to the chiral metasurface, which simultaneously
possesses chiral and magnetic surface conductivities, besides the common
electric surface conductivity. In other words, the interlayer quantum coupling
in twisted van der Waals heterostructures may facilitate the construction of
various (e.g., bi-anisotropic) atomically-thin metasurfaces. Moreover, the
chiral surface conductivity, determined by the interlayer quantum coupling,
determines the existence of chiral plasmons and leads to a unique phase
relationship (i.e., +/-{\pi}/2 phase difference) between their TE and TM wave
components. Importantly, such a unique phase relationship for chiral plasmons
can be exploited to construct the missing longitudinal spin of plasmons,
besides the common transverse spin of plasmons.
