Fig. 1. Schematic diagram of the scheme. Frequency anticorrelated photon pairs are generated from the spontaneous parametric downconversion source [nonlinear crystal (NLC)]. The signal and the idler photons are sent into an unbalanced MZ interferometer. In the signal arm, a tunable time delay τ1 is introduced outside the MZ interferometer. Photon pairs are combined at the last beam splitter (BS), and we can observe the interference of quantum beats by observing the coincidence count rates between detectors D1 and D2. IF1 and IF2 are filters with different central frequencies set in front of the detectors. M represents the reflecting mirrors.

Fig. 2. Normalized coincidence count rate, which shows three quantum beats with the same interval of τ2=6  ps when the two filters in front of the detectors have different central frequencies. The three central dips are at the position of τ1=−6  ps, τ1=0  ps, and τ1=6  ps.

Fig. 3. Feynman’s path diagrams in different regions of . (a) , where each photon has two alternatives before arriving at the beam splitter; (b), where each photon only has one choice before arriving at the beam splitter in order to produce interference.

Fig. 4. Normalized coincidence count rate when the two filters have the same central frequencies. It shows three dips with the same interval of τ2=6  ps. The three central dips are at the positions of τ1=−6  ps, τ1=0  ps, and τ1=6  ps.