Demonstration of complementarity between path information and interference with thermal light

We report an experimental demonstration of complementarity between path information and interference in second-order correlation with thermal light. The key apparatus is a Sagnac interferometer in which an off-axis slit is inserted. Two orthogonal thermal light beams travel through the slit clockwise and anticlockwise in the interferometer. The second-order interference fringes vanish when the path information is acquired. The interference fringes are revived when the path information is effaced.