  Quantum nonlinear interferometers (QNIs) can measure the infrared physical
quantities of a sample by detecting visible photons. A QNI with Michelson
geometry based on the spontaneous parametric down-conversion in a second-order
nonlinear crystal is studied systematically. A simplified theoretical model of
the QNI is presented. The interference visibility, coherence length,
equal-inclination interference, and equal-thickness interference for the QNI
are demonstrated theoretically and experimentally. As an application example of
the QNI, the refractive index and the angle between two surfaces of a BBO
crystal are measured using equal-inclination interference and equal-thickness
interference.
