Atomic many-body effects in the propagation of slow light through atomic gases

A quantum memory for photonic qubits is an essential tool for quantum information processing because it would be vital for long-distance quantum communication. A strong candidate for quantum memories are atomic gases that exhibit electromagnetically induced transparency (EIT). In such a medium the information about a photon can be reversibly stored in form of a spin-wave. The common quantum optical calculations suggest that ultracold gases of high density would be most suitable for this task, but these theories ignore the dipole-dipole interaction (DDI) between atoms that become relevant at high densities. We have investigated the effect of DDI on EIT and found significant differences between hot and ultracold atomic gases. In a dense gas bosonic atoms close to condensation, DDI changes the refractive index more significantly and also leads to collective decoherence effects. Our theoretical approach combines dressed-states of quantum optics with non-equilibrium many-body techniques (Keldysh diagrams).