  Hybrid organic-inorganic halide perovskites have shown remarkable
optoelectronic properties (1-3), believed to originate from correlated motion
of charge carriers and the polar lattice forming large polarons (4-7). Few
experimental techniques are capable of probing these correlations directly,
requiring simultaneous sub-meV energy and femtosecond temporal resolution after
absorption of a photon (8). Here we use transient multi-THz spectroscopy,
sensitive to the internal motions of charges within the polaron, to temporally
and energetically resolve the coherent coupling of charges to longitudinal
optical phonons in single crystal CH3NH3PbI3 (MAPI). We observe room
temperature quantum beats arising from the coherent displacement of charge from
the coupled phonon cloud. Our measurements provide unambiguous evidence of the
existence of polarons in MAPI.
