A fast scanning ultrafast phase-dependent reflectivity technique is us
ed to measure the coherent properties of multiple quantum wells at car
rier densities below 10(8) cm(-2). Signal to noise ratios in the refle
cted intensity >80 dB can be achieved allowing underlying coherences t
o be measured at time delays beyond 30 ps. An unexpected 1.5 ps rise t
ime of the coherence is resolved before a nonexponential decay and asc
ribed to interference of polaritons. By time-resolving the pair of pul
ses reflected off the sample, a complete history of energy flow in the
quantum wells can be identified. Both spontaneous and stimulated cohe
rent reemission are observed in good agreement with models based on th
e Bloch equations.