Experimental distribution of entanglement via separable states

Information gain in communication is bounded by the information encoded in the physical systems exchanged between sender and receiver. Surprisingly, this does not hold for quantum entanglement, which can increase even though the communicated system carries no entanglement at all. Here we demonstrate this phenomenon in a four-photon experiment where two parties sharing initially separable (unentangled) state get entangled by exchanging a photon that is at all times not entangled with either of them. Our result validates a long-standing assert in quantum information and has important practical implications in quantum networking, where entanglement must be reliably distributed across many nodes at low resource-cost.