Experimental measuring of the coherence length of a single photon generated via a degenerated optical parametric oscillator far below threshold

In this work, we report on experimental measuring of the coherence length of a single photon, which is generated via a degenerated optical parametric oscillator (OPO) operated far below threshold. Although the bandwidth of the OPO cavity is about 7.8 MHz, the measured coherence length of the single photon is only about 90 µm via a Michelson interferometer, due to the multimode property of the photon generated in the experiment. Therefore, further reducing the bandwidth of the photon is necessary for its application in some fields, such as transferring the information between the photon and an atomic system.