  Communication over proven-secure quantum channels is potentially one of the
most wide-ranging applications of currently developed quantum technologies. It
is generally envisioned that in future quantum networks, separated nodes
containing stationary solid-state or atomic qubits are connected via the
exchange of optical photons over large distances. In this work we explore an
intriguing alternative for quantum communication via all-microwave networks. To
make this possible, we describe a general protocol for sending quantum states
through thermal channels, even when the number of thermal photons in the
channel is much larger than one. The protocol can be implemented with
state-of-the-art superconducting circuits and enables the transfer of quantum
states over distances of ~100 m via microwave transmission lines cooled to only
T=4K. This opens up completely new possibilities for quantum communication
within and across buildings, and consequently, for the implementation of
intra-city quantum networks based on microwave technology only.
