  Controlling the flow of charge neutral interlayer exciton (IX) quasiparticles
can potentially lead to low loss excitonic circuits. Here, we report
unidirectional transport of IXs along nanoscale electrostatically defined
channels in an MoSe$_2$-WSe$_2$ heterostructure. These results are enabled by a
lithographically defined triangular etch in a graphene gate to create a
potential energy ''slide''. By performing spatially and temporally resolved
photoluminescence measurements, we measure smoothly varying IX energy along the
structure and high-speed exciton flow with a drift velocity up to 2 * 10$^6$
cm/s, an order of magnitude larger than previous experiments. Furthermore,
exciton flow can be controlled by saturating exciton population in the channel
using a second laser pulse, demonstrating an optically gated excitonic
transistor. Our work paves the way towards low loss excitonic circuits, the
study of bosonic transport in one-dimensional channels, and custom potential
energy landscapes for excitons in van der Waals heterostructures.
