Abstract . The basic principle of quantum mechanics 1 guarantees the unconditional security of quantum key distribution (QKD) 2 , 3 , 4 , 5 , 6 at the cost of forbidding the amplification of a quantum state. As a result, and despite remarkable progress in worldwide metropolitan QKD networks 7 , 8 over the past decades, a long-haul fibre QKD network without a trusted relay has not yet been achieved. Here, through the sending-or-not-sending protocol 9 , we achieve t...