# Observation of second-order temporal interference with thermal light: Interference beyond the coherence time

## TL;DR

This study demonstrates that second-order interference with thermal light persists beyond the coherence length and depends on phase differences, revealing new insights into multi-photon interferometry beyond traditional coherence limits.

## Contribution

It provides the first observation of second-order interference with thermal light that remains stable beyond the coherence length, contrasting with entangled-photon systems.

## Key findings

- Second-order interference with thermal light shows 1/3 visibility.
- Interference persists regardless of path length differences beyond coherence length.
- Interference depends on phase difference, not sum of phases.

## Abstract

We report observation of a counter-intuitive phenomenon in multi-path correlation interferometry with thermal light. The intensity correlation between the outputs of two unbalanced Mach-Zehnder interferometers (UMZI) with two classically correlated beams of thermal light at the input exhibits genuine second-order interference with the visibility of $1/3$. Surprisingly, the second-order interference does not degrade at all no matter how much the path length difference in each UMZI is increased beyond the coherence length of the thermal light. Moreover, the second-order interference is dependent on the difference of the UMZI phases. These results differ substantially from those of the entangled-photon Franson interferometer which exhibits two-photon interference dependent on the sum of the UMZI phases and the interference vanishes as the path length difference in each UMZI exceeds the coherence length of the pump laser. Our work offers deeper insight into the interplay between interference and coherence in multi-photon interferometry.