# The physics of thermal light second-order interference beyond coherence

## TL;DR

This paper explains a surprising second-order interference effect with thermal light that occurs beyond the coherence length, using a simple two-photon interference model to clarify the underlying physics.

## Contribution

It provides a straightforward physical explanation for second-order interference beyond coherence length in thermal light interferometry, expanding understanding of classical light behavior.

## Key findings

- Second-order interference persists beyond the coherence length.
- Two-photon interference explains the phenomenon.
- The effect is demonstrated in a novel thermal light interferometer.

## Abstract

A novel thermal light interferometer was recently introduced in V. Tamma and J. Seiler, New J. Phys. 18, 032002 (2016). Here, two classically correlated beams, obtained by beam splitting a thermal light beam, propagate through two unbalanced Mach-Zehnder interferometers. Remarkably, second-order interference between the long and the short paths in the two interferometers was predicted independently of how far, in principle, the length difference between the long and short paths is beyond the coherence length of the source. This phenomenon seems to contradict our common understanding of second order coherence. We provide here a simple description of the physics underlying this effect in terms of two-photon interference.