  While optical technology provides the best solution for the transmission of
information, optical logics still calls for qualitative new concepts to be
explored. Exciton-polaritons are composite particles, resulting from the strong
coupling between excitons and photons, which have recently demonstrated
exceptional properties like huge non-linearities, long range coherence and
suppression of scattering. Here we demonstrate a switching scheme for
polaritons moving in the plane of a microcavity which satisfy all the
requirements for an all-optical transistor. Under resonant excitation, the
power threshold for the nonlinear increase of the polariton density is varied
by a weak control beam, obtaining up to 19 times amplification with switching
energies in the range of attojoule per square micron. Polariton propagation in
the plane of the microcavity is then used to control the switching of a second,
spatially separated transistor, opening the way to the implementation of
polariton integrated circuits.
