An all-optical quantum router

Just as the internet uses routers to ensure that packets of data reach their intended recipient, quantum networks will need routers that can direct the flow of quantum bits of information between nodes. The information could potentially be carried by photons, and Barak Dayan (Weizmann Institute of Science in Rehovot, Israel) and coworkers have now demonstrated a practical way to route them. In the researchers’ scheme, illustrated here, photons get injected into an optical fiber at one end or the other, and a rubidium atom coupled with a microsphere optical resonator directs the traffic. The scheme takes advantage of a special property of the resonator’s mode that links photons’ propagation direction with their polarization—photons coming from one input are right-hand circularly polarized (σ⁺), and photons coming from the other are left-hand polarized (σ⁻). One photon acts as a control: Depending on its polarization, it switches the atom between two degenerate ground states, ↓ and ↑. The state then determines the path of a second photon, the target. The ↓ state transmits σ⁻ photons, reflects σ⁺ photons, and therefore routes the target—irrespective of its initial propagation direction—to output 1. The ↑ state, which has the opposite properties, steers targets to output 2. The scheme has a key advantage over competing approaches to photon manipulation: Because it relies only on fiber-guided photons, requires no external control fields, and can be fashioned out of a microchip, it is compatible with scalable architectures for future quantum networks. (I. Shomroni et al., Science, in press .)