  Electromagnetically induced transparency allows for light transmission
through dense atomic media by means of quantum interference. Media exhibiting
electromagnetically induced transparency have very interesting properties, such
as extremely slow group velocities. Associated with the slow light propagation
are quasiparticles, the so-called dark polaritons, which are mixtures of a
photonic and an atomic contribution. We here demonstrate that these excitations
behave as particles with a nonzero magnetic moment, which is in clear contrast
to the properties of a free photon. It is found that light passing through a
rubidium gas cell under the conditions of electromagnetically induced
transparency is deflected by a small magnetic field gradient. The deflection
angle is proportional to the optical propagation time through the cell. The
observed beam deflection can be understood by assuming that dark state
polaritons have an effective magnetic moment. Our experiment can be described
in terms of a Stern-Gerlach experiment for the polaritons.
