  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.
