Routing of optical states by atomic media

Electromagnetically induced transparency (EIT) is a quantum interference effect, in which a weak signal light field and a stronger control field drive atomic transitions with a common excited state. The quantum interference between both light-atom interactions leads to strong dispersion which causes phenomena such as slowdown and stopping of light and can be used for enhanced nonlinear interaction. We extended the standard quantum theory of EIT to accommodate for multiple excited levels and show experimentally that a transfer of optical quantum states between different signal modes can be implemented by an adiabatic change of the control fields. Raman adiabatic transfer of optical states resembles stimulated Raman adiabatic passage (STIRAP) but applies to optical rather than atomic states. It can be used to route and distribute optically encoded information in classical and quantum communication. We performed experiments using the hyperfine levels of Rb87 atoms at the D1 line: First, a signal pulse (resonant to the F=1, F'=1 transition) was placed into the cell under EIT conditions created by a control laser (resonant to F=2, F'=1). Then adiabatically this laser is switched off while another control laser (resonant to F=2, F'=2) is switched on. This procedure transfers the information carried by the state of the original signal pulse to the optical mode resonant with the F=1, F'=2 transition.