Department of Computer Science and York Centre for Quantum Technologies, University of York, York YO10 5GH, UK.
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
Nat Commun. 2017 Apr 26;8:15043. doi: 10.1038/ncomms15043.
Quantum communications promises reliable transmission of quantum information, efficient distribution of entanglement and generation of completely secure keys. For all these tasks, we need to determine the optimal point-to-point rates that are achievable by two remote parties at the ends of a quantum channel, without restrictions on their local operations and classical communication, which can be unlimited and two-way. These two-way assisted capacities represent the ultimate rates that are reachable without quantum repeaters. Here, by constructing an upper bound based on the relative entropy of entanglement and devising a dimension-independent technique dubbed 'teleportation stretching', we establish these capacities for many fundamental channels, namely bosonic lossy channels, quantum-limited amplifiers, dephasing and erasure channels in arbitrary dimension. In particular, we exactly determine the fundamental rate-loss tradeoff affecting any protocol of quantum key distribution. Our findings set the limits of point-to-point quantum communications and provide precise and general benchmarks for quantum repeaters.
量子通信有望实现量子信息的可靠传输、纠缠的高效分发以及完全安全密钥的生成。对于所有这些任务，我们需要确定通过量子信道两端的两个远程方在不受其本地操作和经典通信限制的情况下（可以是无限和双向的），能够实现的最优点对点速率。这些双向辅助容量代表了在没有量子中继器的情况下可达到的最终速率。在这里，通过构建基于纠缠相对熵的上界，并设计一种称为“传送拉伸”的与维度无关的技术，我们为许多基本信道（即玻色损耗信道、量子限幅放大器、任意维度的退相和擦除信道）确定了这些容量。特别地，我们精确地确定了影响任何量子密钥分发协议的基本速率-损耗折衷。我们的发现为点对点量子通信设定了限制，并为量子中继器提供了精确和通用的基准。
