Quantum communications promises reliable transmission of quantum information,efficient distribution of entanglement and generation of completely securekeys. For all these tasks, we need to determine the optimal point-to-pointrates that are achievable by two remote parties at the ends of a quantumchannel, without restrictions on their local operations and classicalcommunication, which can be unlimited and two-way. These two-way assistedcapacities represent the ultimate rates that are reachable without quantumrepeaters. By constructing an upperbound based on the relative entropy ofentanglement and devising a dimension-independent technique dubbed"teleportation stretching", we establish these capacities for many fundamentalchannels, namely bosonic lossy channels, quantum-limited amplifiers, dephasingand erasure channels in arbitrary dimension. In particular, we determine thefundamental rate-loss trade-off affecting any protocol of quantum keydistribution. Our findings set the ultimate limits of point-to-point quantumcommunications and provide the most precise and general benchmarks for quantumrepeaters.
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