  We exploit the nonlinearity arising from the spin-photon interaction in an
InAs quantum dot to demonstrate phase shifts of scattered light pulses at the
single-photon level. Photon phase shifts of close to 90 degrees are achieved
using a charged quantum dot in a micropillar cavity. We also demonstrate a
photon phase switch by using a spin-pumping mechanism through Raman transitions
in an in-plane magnetic field. The experimental findings are supported by a
theoretical model which explores the dynamics of the system. Our results
demonstrate the potential of quantum dot-induced nonlinearities for quantum
information processing.
