  As part of a circuit QED architecture, we investigate photon emission from a
Cooper pair splitter composed of two double quantum dots, each coupled to a
microwave transmission line. We demonstrate the capability to generate
frequency-entangled photon pairs in the left and right transmission lines,
specifically a superposition of two photon wavepackets at different
frequencies. The photon frequencies are determined by the energy detuning in
each double quantum dot and by the nonlocal superconducting pairing amplitude,
which results from the tunnel coupling with the superconducting nanocontact
placed between the two double quantum dots. The frequency entanglement of the
two photons arises from the particle-hole coherent superposition involving the
delocalized entangled spin singlet. We also estimate an upper bound for the
efficiency of entangled photon-pair generation, accounting for the presence of
non-radiative processes such as phonon emissions. Using parameters from
cutting-edge circuit QED devices with quantum dots, our proposal is realistic
and could be implemented with state-of-the-art microwave nanoscale engineering.
