  The photon helicity may be mapped to a spin-1/2, whereby we put forward an
intrinsic interaction between a polarized light beam as a ``photon spin
current'' and a pure spin current in a semiconductor, which arises from the
spin-orbit coupling in valence bands as a pure relativity effect without
involving the Rashba or the Dresselhaus effect due to inversion asymmetries.
The interaction leads to circular optical birefringence, which is similar to
the Faraday rotation in magneto-optics but nevertheless involve no net
magnetization. The birefringence effect provide a direct, non-demolition
measurement of pure spin currents.
