Heralded amplification of path entangled quantum states

Device-independent quantum key distribution (DI-QKD) represents one of the most fascinating challenges in quantum communication, exploiting concepts of fundamental physics, namely Bell tests of nonlocality, to ensure the security of a communication link.This requires the loophole-free violation of a Bell inequality, which is intrinsically difficult due to losses in fibre optic transmission channels.Heralded photon amplification is a teleportation-based protocol that has been proposed as a means to overcome transmission loss for DI-QKD.Here we demonstrate heralded photon amplification for path entangled states and characterise the entanglement before and after loss by exploiting a recently developed displacement-based detection scheme.We demonstrate that by exploiting heralded photon amplification we are able to reliably maintain high fidelity entangled states over loss-equivalent distances of more than 50 km.