  Perovksite semiconductors have shown promise for low-cost solar cells, lasers
and photodetectors, yet their fundamental photophysical properties are not well
understood. Recent observations of a low ($\sim$few meV) exciton binding energy
and evidence of hot phonon effects in the room temperature phase suggest that
perovskites are much closer to inorganic semiconductors than the absorber
layers in traditional organic photovoltaics, signaling the need for experiments
that shed light on the placement of perovskite materials within the spectrum of
semiconductors used in optoelectronics and photovoltaics. Here we use four-wave
mixing (FWM) to contrast the coherent optical response of CH$_3$NH$_3$PbI$_3$
thin films and crystalline GaAs. At carrier densities relevant for solar cell
operation, our results show that carriers interact surprisingly weakly via the
Coulomb interaction in perovskite, much weaker than in inorganic
semiconductors. These weak many-body effects lead to a dephasing time in
CH$_3$NH$_3$PbI$_3$ $\sim$3 times longer than in GaAs. Our results also show
that the strong enhancement of the exciton FWM signal tied to
excitation-induced dephasing in GaAs and other III-V semiconductors does not
occur in perovskite due to weak exciton-carrier scattering interactions.
