Single-Photon Switch Based on Rydberg Blockade
An atomic ensemble can serve not only as a quantum memory but also as a processing unit for single photons. We realized an all-optical switch in which the presence or absence of a gate light pulse determines whether a target light pulse is transmitted or not. Our experiment takes this to the quantum regime, where the incoming gate pulse contains only one photon on average. This pulse is stored in the atomic gas in the form of a Rydberg excitation with principal quantum number 100 using a slow-light technique based on electromagnetically induced transparency (EIT). If a gate photon is stored, Rydberg blockade suppresses the transmission of the subsequent target pulse by a factor of 20. If no gate photon is stored, the transmission of the target pulse is high due to Rydberg EIT. We manage to retrieve the gate excitation after the target pulse passed the medium. This retrieved signal shows that phase coherence was preserved and it serves as a herald to indicate successful storage.