  Quantum control of electromagnetic fields was initially established in the
optical domain and has been advanced to lower frequencies in the gigahertz
range during the past decades extending quantum photonics to broader frequency
regimes. In standard cryogenic systems, however, thermal decoherence prevents
access to the quantum regime for photon frequencies below the gigahertz domain.
Here, we engineer two superconducting LC circuits coupled by a photon-pressure
interaction and demonstrate sideband cooling of a hot radio frequency (RF)
circuit using a microwave cavity. Because of a substantially increased coupling
strength, we obtain a large single-photon quantum cooperativity
$\mathcal{C}_{\mathrm{q}0} \sim 1$ and reduce the thermal RF occupancy by 75%
with less than one pump photon. For larger pump powers, the coupling rate
exceeds the RF thermal decoherence rate by a factor of 3, and the RF circuit is
cooled into the quantum ground state. Our results lay the foundation for RF
quantum photonics.
