Steering light by electromagnetically-induced transparency

Frequency conversion and routing of quantum information carried by light is of great importance for future quantum communication networks. We experimentally demonstrate a communication protocol that enables frequency conversion and routing of quantum optical information in an adiabatic and thus robust way. The protocol is based on electromagnetically-induced transparency (EIT) in systems with multiple excited levels: transfer and/or distribution of optical states between different signal modes is implemented by adiabatically changing the control fields. Proof-of-principle experiments were performed using the hyperfine levels of Rb 87 atoms at the D1 line. First, we placed a signal pulse (resonant to the F=1, F'=2 transition) into the cell under EIT conditions created by a control laser (resonant to F=2, F'=2). This laser is adiabatically switched off while another control laser (resonant to F=2, F'=1) is switched on. The information carried by the state of the original signal pulse is then transferred to the optical mode resonant with the F=1, F'=1 transition. The evolution of the spatial characteristics of the EIT signal field is governed by the paraxial approximation, which is equivalent to the Schrödinger equation of a free particle in space. In an EIT system with multiple excited levels, in the presence of several control fields, the signal field is subjected to a unitary transformation, which, in analogy to gauge transformations, modifies the paraxial equation, making it analogous to that of a charged Schrödinger particle in an electromagnetic field, with the quasi-potentials related to the amplitudes and phases of the two pump fields. By choosing specific spatially inhomogeneous control fields, one can steer the EIT photon inside the cell.