  We have determined both the real and imaginary parts of the dielectric
polarizability of a single quantum dot. The experiment is based on the
observation and the manipulation of Rayleigh scattering at photon frequencies
near the resonance of an optical exciton transition in single self-assembled
InAs and InGaAs quantum dots. The interference between the narrow-band laser
field and the weak electromagnetic field coherently scattered by the quantum
dot is detected with a cryogenic Fabry-Perot setup by combined differential
transmission and reflectivity measurements.
