  Exciton, trion and biexciton dephasing rates are measured within the
inhomogeneous distribution of an InAs quantum dot (QD) ensemble using
two-dimensional Fourier-transform spectroscopy. The dephasing rate of each
excitonic state is similar for all QDs in the ensemble and the rates are
independent of excitation density. An additional spectral feature -- too weak
to be observed in the time-integrated four-wave mixing signal -- appears at
high excitation density and is attributed to the $\chi^{\textrm{(5)}}$
biexcitonic nonlinear response.
