  We propose a simple circuit quantum electrodynamics (QED) experiment to test
the generation of entanglement between two superconducting qubits. Instead of
the usual cavity QED picture, we study qubits which are coupled to an open
transmission line and get entangled by the exchange of propagating photons. We
compute their dynamics using a full quantum field theory beyond the
rotating-wave approximation and explore a variety of regimes which go from a
weak coupling to the recently introduced ultrastrong coupling regime. Due to
the existence of single photons traveling along the line with finite speed, our
theory shows a light cone dividing the spacetime in two different regions. In
one region, entanglement may only arise due to correlated vacuum fluctuations,
while in the other the contribution from exchanged photons shows up.
