  We report circular-to-linear and linear-to-circular conversion of optical
polarization by semiconductor quantum dots. The polarization conversion occurs
under continuous wave excitation in absence of any magnetic field. The effect
originates from quantum interference of linearly and circularly polarized
photon states, induced by the natural anisotropic shape of the self assembled
dots. The behavior can be qualitatively explained in terms of a pseudospin
formalism.
