Entangling different-color photons via time-resolved measurement and active feed forward.

Entangling independent photons is not only of fundamental interest but also of crucial importance for quantum-information science. Two-photon interference is a major method for entangling independent identical photons. If two photons are different in color, perfect two-photon coalescence can no longer happen, which makes the entangling of different-color photons difficult to realize. In this Letter, by exploring and developing time-resolved measurement and active feed forward, we have entangled two independent photons of different colors for the first time. We find that entanglement with a varying form can be identified for different two-photon temporal modes through time-resolved measurement. By using active feed forward, we are able to convert the varying entanglement into uniform entanglement. Adopting these measures, we have successfully entangled two photons with a frequency separation 16 times larger than their linewidths. In addition to its fundamental interest, our work also provides an approach for solving the frequency-mismatch problem for future quantum networks.