  Security of modern classical data encryption often relies on computationally
hard problems, which can be trivialized with the advent of quantum computers. A
potential remedy for this is quantum communication which takes advantage of the
laws of quantum physics to provide secure exchange of information. Here,
quantum key distribution (QKD) represents a powerful tool, allowing for
unconditionally secure quantum communication between remote parties. At the
same time, microwave quantum communication is set to play an important role in
future quantum networks because of its natural frequency compatibility with
superconducting quantum processors and modern near-distance communication
standards. To this end, we present an experimental realization of a
continuous-variable QKD protocol based on propagating displaced squeezed
microwave states. We use superconducting parametric devices for generation and
single-shot quadrature detection of these states. We demonstrate unconditional
security in our experimental microwave QKD setting. We show that security
performance can be improved by adding finite trusted noise to the preparation
side. Our results indicate feasibility of secure microwave quantum
communication with the currently available technology in both open-air (up to
$\sim$ 80 m) and cryogenic (over 1000 m) conditions.
