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.Comment: 6 pages and 3 figure
