Mode-locked two-photon states

The concept of mode locking in laser is applied to a two-photon state with frequency entanglement. Cavity enhanced parametric down conversion is found to produce exactly such a state. The mode-locked two-photon state exhibits a comblike correlation function. An unbalanced Hong-Ou-Mandel type interferometer is used to measure the correlation function. A revival of the typical interference dip is observed. We will discuss a scheme for engineering of quantum states in time domain.