  We introduce a novel two-dimensional electronic system with ultrastrong
interlayer interactions, namely twisted bilayer graphene with a large twist
angle, as an ideal ground for realizing interlayer-coherent excitonic
condensates. In these systems, subnanometer atomic separation between the
layers allows significant interlayer interactions, while interlayer electron
tunneling is geometrically suppressed due to the large twist angle. By fully
exploiting these two features we demonstrate that a sequence of odd-integer
quantum Hall states with interlayer coherence appears at the second Landau
level (N = 1). Notably the energy gaps for these states are of order 1 K, which
is several orders of magnitude greater than those in GaAs. Furthermore, a
variety of quantum Hall phase transitions are observed experimentally. All the
experimental observations are largely consistent with our phenomenological
model calculations. Hence, we establish that a large twist angle system is an
excellent platform for high-temperature excitonic condensation.
