  In classical optics, Young's double-slit experiment with colored coherent
light gives rise to individual interference fringes for each light frequency,
referring to single-photon interference. However, two-photon double-slit
interference has been widely studied only for wavelength-degenerate biphoton,
known as subwavelength quantum lithography. In this work, we report double-slit
interference experiments with two-color biphoton. Different from the degenerate
case, the experimental results depend on the measurement methods. From a
two-axis coincidence measurement pattern we can extract complete interference
information about two colors. The conceptual model provides an intuitional
picture of the in-phase and out-of-phase photon correlations and a complete
quantum understanding about the which-path information of two colored photons.
