Hong-Ou-Mandel interferometry on a biphoton beat note

Hong-Ou-Mandel interference, the fact that identical photons that arrive simultaneously on different input ports of a beam splitter bunch into a common output port, can be used to measure optical delays between different paths. It is generally assumed that great precision in the measurement requires that photons contain many frequencies, i.e., a large bandwidth. Here we challenge this well-known assumption and show that the use of two well-separated frequencies embedded in a quantum entangled state (discrete color entanglement) suffices to achieve great precision. We determine optimum working points using a Fisher Information analysis and demonstrate the experimental feasibility of this approach by detecting thermally-induced delays in an optical fiber. These results will not only prove useful for facilitating the use of quantum interference for quantum sensing, by avoiding some stringent conditions such as the requirement for large bandwidth signals, but also indicate new directions towards harnessing multi-photon interference in general.