  The nonlinear, parametric coupling between two harmonic oscillators has been
used in the field of optomechanics for breakthrough experiments regarding the
control and detection of mechanical resonators. Although this type of
interaction is an extremely versatile resource and not limited to coupling
light fields to mechanical resonators, there have only been, very few reports
of implementing it within other systems so far. Here, we present a device
consisting of two superconducting LC circuits, parametrically coupled to each
other by a magnetic flux-tunable photon-pressure interaction. We observe
dynamical backaction between the two circuits, photon-pressure-induced
transparency and absorption, and enter the parametric strong-coupling regime,
enabling switchable and controllable coherent state transfer between the two
modes. As result of the parametric interaction, we are also able to amplify and
observe thermal current fluctuations in a radio-frequency LC circuit close to
its quantum ground-state. Due to the high design flexibility and precision of
superconducting circuits and the large single-photon coupling rate, our
approach will enable new ways to control and detect radio-frequency photons and
allow for experiments in parameter regimes not accessible to other platforms
with photon-pressure interaction.
