Quantum interference between two single photons of different microwave   frequencies
  We have measured quantum interference between two single microwave photons trapped in a superconducting resonator, whose frequencies are initially about 6 GHz apart. We accomplish this by use of a parametric frequency conversion process that mixes the mode currents of two cavity harmonics through a superconducting quantum interference device, and demonstrate that a two-photon entanglement operation can be performed with high fidelity. 
Fran\c{c}ois Nguyen, Eva Zakka-Bajjani, Raymond W. Simmonds, and   Jos\'e Aumentado
cond-mat.mes-hall quant-ph