  Communication in a network generally takes place through a sequence of
intermediate nodes connected by communication channels. In the standard theory
of communication, it is assumed that the communication network is embedded in a
classical spacetime, where the relative order of different nodes is
well-defined. In principle, a quantum theory of spacetime could allow the order
of the intermediate points between sender and receiver to be in a coherent
superposition. Here we experimentally realise a table-top simulation of this
exotic possibility on a photonic system, demonstrating high-fidelity
transmission of quantum information over two noisy channels arranged in a
superposition of two alternative causal orders.
