Storage and processing of quantum light

Summary form only given. For quantum communication, a light pulse is an ideal carrier of quantum information. In quantum communication protocols (quantum cryptography for instance), normally it is required to store and manipulate quantum information. However, light is very hard to be stored. Recently, the idea to transfer squeezing from light to collective atomic spins has been proposed [A. Kozhekin et al., 1999]. It is not clear, however, how to write an arbitrary unknown state of a light pulse with an arbitrary shape onto the atomic internal state, and then, after some storage time, to read out the state back onto a light pulse with the same or a different chosen shape. We propose a scheme with the following properties: (i) an arbitrary unknown state of the input light pulse with an arbitrary shape (which is only limited by the parameters obtainable in the lab) can be mapped to the collective internal atomic state with near-to-unit fidelity, and after storage the state can be perfectly transferred back to another light pulse with an arbitrary chosen shape; (ii) if the input light pulse consists of several mode shapes, the atomic ensemble can selectively store one mode and completely reflect other modes. The scheme is shown to be feasible with the current experimental technology.