Measurement-device-independent quantum key distribution coexisting with classical communication
The prospect of building a quantum internet, which promises information-theoretic secure communication  as well as blind or networked quantum computing , is generating a rapidly increasing amount of academic and corporate development efforts . To minimise operating costs and hence facilitate deployment, it is important to benefit as much as possible from existing infrastructure. Starting in 1995, this has encouraged many experiments with deployed elecommunication fibre [4-6],and, since 1997, demonstrations of quantum key distribution (QKD) - the most mature application of quantum networks - together with classical data on the same fibre . However, to-date, comprehensive studies of the latter have been limited to so-called prepare & measure QKD, whose security is threatened by blinding attacks - quantum hacking that exploits vulnerabilities of single-photon detectors . Here, we experimentally demonstrate that measurement-device independent (MDI)QKD , which is impervious to such attacks, can operate simultaneously with at least five 10 Gbps bidirectional classical communication channels over 40 km of spooled fibre, and we project communication rates in excess of 10 THz when moving the quantum channel from the third to the second telecommunication window. Due to the similarity between MDI-QKD and quantum-repeaters ,our investigation not only establishes the possibility of cost-effective quantum-secured communication,but also shows that classical and quantum networks can more generally be operated using the same fibre infrastructure. This will facilitate the adoption of the new quantum technology, and hence constitutes an important step towards a world in which quantum information processing will help meeting challenges in secure data transmission, and will provide opportunities for unparalleled data processing.