  Exchange-mediated fine-structure splitting of bright excitons in an ensemble
of InAs quantum dots is studied using optical two-dimensional Fourier-transform
spectroscopy. By monitoring the non-radiative coherence between the bright
states, we find that the fine-structure splitting decreases with increasing
exciton emission energy at a rate of 0.1 $\mu$eV/meV. Dephasing rates are
compared to population decay rates to reveal that pure dephasing causes the
exciton optical coherences to decay faster than the radiative limit at low
temperature, independent of excitation density. Fluctuations of the bright
state transition energies are nearly perfectly-correlated, protecting the
non-radiative coherence from interband dephasing mechanisms.
