Experimental two-way communication with one photon.

Superposition of two or more states is one of the fundamental concepts of quantum mechanics and provides the basis for several advantages quantum information processing offers. In this work, we experimentally demonstrate that quantum superposition permits two-way communication between two distant parties that can exchange only one particle once, an impossible task in classical physics. This is achieved by preparing a single photon in a coherent superposition of the two parties' locations. Furthermore, we show that this concept allows the parties to perform secure quantum communication, where the transmitted bits and even the direction of communication remain private. These important features can lead to the development of new quantum communication schemes, which are simultaneously secure and resource-efficient.