  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. By constructing an upperbound 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 determine the
fundamental rate-loss trade-off affecting any protocol of quantum key
distribution. Our findings set the ultimate limits of point-to-point quantum
communications and provide the most precise and general benchmarks for quantum
repeaters.
