  Quantum and private communications are affected by a fundamental limitation
which severely restricts the optimal rates that are achievable by two distant
parties. To overcome this problem, one needs to introduce quantum repeaters
and, more generally, quantum communication networks. Within a quantum network,
other problems and features may appear when we move from the basic unicast
setting of single-sender/single-receiver to more complex multi-end scenarios,
where multiple senders and multiple receivers simultaneously use the network to
communicate. Assuming various configurations, including multiple-unicast,
multicast, and multiple-multicast communication, we bound the optimal rates for
transmitting quantum information, distributing entanglement, or generating
secret keys in quantum networks connected by arbitrary quantum channels. These
bounds cannot be surpassed by the most general adaptive protocols of quantum
network communication.
