Quantum router: Storing and redirecting light at the photon level

Abstract

We propose a method for spatially rerouting single photons or light in a coherent state with a small average photon number by purely electronic means, i.e., without using mechanical devices such as micromirror arrays. The method is based on mapping the quantum state of the incoming light onto a spin-wave in an atomic ensemble, as is done in quantum memories of light. Then the wave vector of the spin-wave is modified in a controlled way by an applied magnetic field gradient. Finally, by reapplying the same control beam as for storing, the signal pulse is released in a new direction that depends on the deflected wave vector of the spin-wave. We show by numerical simulation that efficiencies can be achieved for arbitrary deflection angles in the plane that are comparable with simple photon storage and reemission in the forward direction, and we propose a method for eliminating the stored momentum as a source of decoherence in the quantum memory. In a reasonable parameter regime, the rerouting should be achievable on a timescale on the order of a few to $\ensuremath{\sim}100$ microseconds, depending on the deflection angle. The shifts in the wave vector that can be achieved using the Zeeman effect, with otherwise minimal changes to the spin-wave, can also be used to complement existing ac-Stark spin-wave manipulation methods.