  Advances in opto-electronics require the development of materials with novel
and engineered characteristics. A class of materials that has garnered
tremendous interest is metal-halide perovskites, stimulated by meteoric
increases in photovoltaic efficiencies of perovskite solar cells. In addition,
recent advances have applied perovskite nanocrystals (NCs) in light-emitting
devices. It was discovered recently that, for cesium lead-halide perovskite
NCs, their unusually efficient light-emission may be due to a unique excitonic
fine-structure composed of three bright triplet states that minimally interact
with a proximal dark singlet state. To study this fine-structure without
isolating single NCs, we use multi-dimensional coherent spectroscopy at
cryogenic temperatures to reveal coherences involving triplet states of a
CsPbI$_3$ NC ensemble. Picosecond timescale dephasing times are measured for
both triplet and inter-triplet coherences, from which we infer a unique exciton
fine-structure level-ordering comprised of a dark state energetically
positioned within the bright triplet manifold.
