All photons are equal but some photons are more equal than others

TL;DR

This paper introduces a method to quantify photon distinguishability in quantum experiments by analyzing coincidence probabilities, revealing how different degrees of freedom affect photon identity.

Contribution

It provides a novel experimental parameter, the rate of distinguishability, to measure how quantum states and degrees of freedom influence photon distinguishability.

Findings

Photon distinguishability depends on specific degrees of freedom.

The rate of distinguishability can be experimentally measured.

Coincidence probability distribution reveals distinguishability characteristics.

Abstract

Two photons are said to be identical when they are prepared in the same quantum state. Given the latter, there is a unique way to achieve this. Conversely, there are many different manners to prepare two non-identical photons: they may have different frequency, polarization, amplitude, etc. Therefore, photon distinguishability depends upon the specific degree of freedom being varied. By means of a careful analysis of the coincidence probability distribution in a Hong-Ou-Mandel experiment, we can show that photon distinguishability can be actually quantified by the rate of distinguishability of photons, an experimentally measurable parameter that crucially depends on both the photon quantum state and the degree of freedom under control.