  Indirect excitons (IXs), also known as interlayer excitons, can form the
medium for excitonic devices whose operation is based on controlled propagation
of excitons. A proof of principle for excitonic devices was demonstrated in
GaAs heterostructures where the operation of excitonic devices is limited to
low temperatures. IXs in van der Waals transition-metal dichalcogenide (TMD)
heterostructures are characterized by high binding energies making IXs robust
at room temperature and offering an opportunity to create excitonic devices
operating at high temperatures suitable for applications. However, a
characteristic feature of TMD heterostructures is the presence of moir\'e
superlattice potentials, which are predicted to cause modulations of IX energy
reaching tens of meV. These in-plane energy landscapes can lead to IX
localization, making IX propagation fundamentally different in TMD and GaAs
heterostructures and making uncertain if long-range IX propagation can be
realized in TMD heterostructures. In this work, we realize long-range IX
propagation with the $1/e$ IX luminescence decay distances reaching 13 microns
in a MoSe$_2$/WSe$_2$ heterostructure. We trace the IX luminescence along the
IX propagation path. We also realize control of the long-range IX propagation:
the IX luminescence signal in the drain of an excitonic transistor is
controlled within 40 times by gate voltage. These data show that the long-range
IX propagation is possible in TMD heterostructures with the predicted moir\'e
superlattice potentials.
