Deflection of slow light in a Stern-Gerlach magnetic field

Electromagnetically induced transparency allows for light transmission through dense atomic media by means of quantum interference of absorption amplitudes. Media exhibiting electromagnetically induced transparency have interesting properties, such as very slow group velocities. Associated with the slow light propagation are quasiparticles, so-called dark polaritons, which are mixtures of a photonic and an atomic contribution. Here, we 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 circularily polarized 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 propagation time of an optical pulse through the cell. The observed beam deflection can be understood by assuming that dark state polaritons have an effective magnetic moment. This is attributed to the spin wave contribution that develops upon entry of light into the medium. Our experiment can be described in terms of a Stern-Gerlach experiment for the dark polaritons.