  Microcavity polaritons are composite half-light half-matter quasi-particles,
which have recently been demonstrated to exhibit rich physical properties, such
as non-equilibrium Bose-Einstein condensation, parametric scattering and
superfluidity. At the same time, polaritons have some important advantages over
photons for information processing applications, since their excitonic
component leads to weaker diffraction and stronger inter-particle interactions,
implying, respectively, tighter localization and lower powers for nonlinear
functionality. Here we present the first experimental observations of bright
polariton solitons in a strongly coupled semiconductor microcavity. The
polariton solitons are shown to be non-diffracting high density wavepackets,
that are strongly localised in real space with a corresponding broad spectrum
in momentum space. Unlike solitons known in other matter-wave systems such as
Bose condensed ultracold atomic gases, they are non-equilibrium and rely on a
balance between losses and external pumping. Microcavity polariton solitons are
excited on picosecond timescales, and thus have significant benefits for
ultrafast switching and transfer of information over their light only
counterparts, semiconductor cavity lasers (VCSELs), which have only nanosecond
response time.
