Second-Order Temporal Interference with Thermal Light: Interference Beyond the Coherence Time

We report an experimental demonstration for a counter-intuitive phenomenon in multi-path interferometer with thermal light. The intensity correlation between the outputs of two unbalanced Mach-Zehnder interferometers (UMZIs) with two correlated beams of thermal light at the input exhibits genuine second-order interference with the visibility of 1/3. Here, 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.