# Hong-Ou-Mandel interferometry on a biphoton beat note

## TL;DR

This paper demonstrates that high-precision Hong-Ou-Mandel interferometry can be achieved using biphoton states with only two well-separated frequencies, simplifying quantum sensing requirements.

## Contribution

It challenges the assumption that large bandwidth is necessary for precise measurements, showing that discrete color entanglement suffices.

## Key findings

- Achieved high-precision delay measurements with biphotons of two frequencies.
- Validated the approach experimentally by detecting delays in optical fibers.
- Used Fisher Information to optimize measurement points.

## Abstract

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 may significantly facilitate the use of quantum interference for quantum sensing, by avoiding some stringent conditions such as the requirement for large bandwidth signals.