  Excitonic insulators (EIs), arising in semiconductors when the electron-hole
binding energy exceeds the band gap, are a solid-state prototype for bosonic
phases of matter. Unlike the charged excitations that are frozen and unable to
transport current, the neutral electron-hole pairs (excitons) are free to move
in EIs. However, it is intrinsically difficult to demonstrate exciton transport
in bulk EI candidates. The recently emerged dipolar EIs based on
Coulomb-coupled atomic double layers open the possibility to realize exciton
transport across the insulator because separate electrical contacts can be made
to the electron and hole layers. Here we show that the strong interlayer
excitonic correlation at equal electron and hole densities in the MoSe2/WSe2
double layers separated by a 2-nm barrier gives rise to perfect Coulomb drag. A
charge current in one layer induces an equal but opposite drag current in the
other. The drag current ratio remains above 0.9 up to about 20 K for low
exciton densities. As exciton density increases above the Mott density, the
excitons dissociate into the electron-hole plasma abruptly, and only weak Fermi
liquid frictional drag is observed. Our experiment moves a step closer to
realizing exciton circuitry and superfluidity.
