Hong–Ou–Mandel effect with two frequencyentangled photons of vastly different color

In the original Hong-Ou-Mandel (HOM) experiment -- when two identical photons are incident upon the input ports of a balanced beam-splitter -- they coalesce, always leaving via the same output port. It is often interpreted that this interference arises due to the indistinguishability of the single photons; the situation, however, is often more nuanced. Here, we demonstrate an analog of HOM interference between two photons of completely different color. To do so, we utilize a quantum frequency converter based on sum- and difference-frequency generation as an `active' beam-splitter -- coupling frequency-entangled red and telecom single photons with an octave-spanning energy difference of 282 THz. Uncorrected two-photon interference visibilities well above 90% were achieved. This work presents the first demonstration of HOM interference between two single photons of distinctly different color, deepening our understanding of what underlies quantum interference. It also suggests a novel approach to interfacing photonic qubits in heterogeneous quantum systems where frequency conversion and quantum interference are unified.