  Multiple quantum beats of a system of the coherently excited quantum confined
exciton states in a high-quality heterostructure with a wide InGaAs/GaAs
quantum well are experimentally detected by the spectrally resolved pump-probe
method for the first time. The beat signal is observed as at positive as at
negative delays between the pump and probe pulses. A theoretical model is
developed, which allows one to attribute the QBs at negative delay to the
four-wave mixing (FWM) signal detected at the non-standard direction. The beat
signal is strongly enhanced by the interference of the FWM wave with the
polarization created by the probe pulse. At positive delay, the QBs are due to
the mutual interference of the quantum confined exciton states. Several QB
frequencies are observed in the experiments, which coincide with the interlevel
spacings in the exciton system. The decay time for QBs is of order of several
picoseconds at both the positive and negative delays. They are close to the
relaxation time of exciton population that allows one to consider the exciton
depopulation as the main mechanism of the coherence relaxation in the system
under study.
