Shifts in the relative phases of quantum beats in the time-resolved four-wave-mixing signals from a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum well are observed. The relative phases are shown to depend strongly on the intensities of the incident pulses as well as their detuning from the heavy-hole exciton. A simple coupled six-level model shows that the phases of the quantum beats are directly determined by many-body effects such as density-dependent dephasing and the local-field correction.

• Received 21 September 1994

DOI:https://doi.org/10.1103/PhysRevB.51.4242