To entangle independent photons of different color, researchers use down-conversion to generate two pairs of polarization-entangled photons, labeled (A, a) and (B, b). The frequencies of each pair differ by either 40 or 80 MHz. Photons a and b enter a beam splitter, resulting in a “hypoentangled” state of polarization and frequency. Two detectors record the arrival times, t1 and t2, of the two photons, and this information is fed forward to a device that applies a time-dependent phase shift to photon A. In the end, the entanglement of a and b is swapped to A and B.