  Excitons are pairs of electrons and holes bound together by the Coulomb
interaction. At low temperatures, excitons can form a Bose-Einstein condensate
(BEC), enabling macroscopic phase coherence and superfluidity. An electronic
double layer (EDL), in which two parallel conducting layers are separated by an
insulator, is an ideal platform to realize a stable exciton BEC. In an EDL
under strong magnetic fields, electron-like and hole-like quasi-particles from
partially filled Landau levels (LLs) bind into excitons and condense. However,
in semiconducting double quantum wells, this magnetic-field-induced exciton BEC
has been observed only in sub-Kelvin temperatures due to the relatively strong
dielectric screening and large separation of the EDL. Here we report exciton
condensation in bilayer graphene EDL separated by a few atomic layers of
hexagonal boron nitride (hBN). Driving current in one graphene layer generates
a quantized Hall voltage in the other layer, signifying coherent superfluid
exciton transport. Owing to the strong Coulomb coupling across the atomically
thin dielectric, we find that quantum Hall drag in graphene appears at a
temperature an order of magnitude higher than previously observed in GaAs EDL.
The wide-range tunability of densities and displacement fields enables
exploration of a rich phase diagram of BEC across Landau levels with different
filling factors and internal quantum degrees of freedom. The observed robust
exciton superfluidity opens up opportunities to investigate various quantum
phases of the exciton BEC and design novel electronic devices based on
dissipationless transport.
