Electromagnetically induced transparency allows light transmission through dense atomic media by means of quantum interference¹. Media with electromagnetically induced transparency have very interesting properties, such as extremely slow group velocities^2,3,4. Quasiparticles, the so-called dark polaritons, which are mixtures of a photonic and an atomic contribution⁵, are associated with slow light propagation. Here, we demonstrate that these excitations behave as particles with a non-zero 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 beam deflection observed 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.

We report the trapping of ultracold neutral Rb atoms and ${\mathrm{Ba}}^{+}$ ions in a common optical potential in absence of any radio frequency (rf) fields. We prepare ${\mathrm{Ba}}^{+}$ at $370\mu \mathrm{K}$ and demonstrate efficient sympathetic cooling by $100\mu \mathrm{K}$ after one collision. Our approach is currently limited by the Rb density and related three-body losses, but it overcomes the fundamental limitation in rf traps set by rf-driven, micromotion-induced heating. It is applicable to a wide range of ion-atom species, and may enable novel ultracold chemistry experiments and complex many-body dynamics.