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Note: Optical fiber two-way time transfer based on single photon counting approach.

We are reporting on time transfer parameters of a newly presented method of the Two-Way Time Transfer (TWTT) using the single photon counting approach. The method utilizes two Single Photon Avalanche Diode (SPAD) detectors located on both ends of the optical channel. Each of the SPAD detectors measures the common signal taken from both communication directions on its location. The mixed signal is then mathematically decomposed back to the original, separated signals, whose time shifts are then measured, and the absolute time shift of the two distant time scales is calculated. The photon counting approach was utilized due to its great beneficence for the TWTT as most of the systematic errors can be eliminated, and, additionally, an optical power of only about -100 dBm is necessary for the measurement. It makes the method usable in optical fiber channels with the lengths of more than 500 km without any amplification. The presented TWTT method can be performed for any communication wavelength when proper SPAD detectors are used. In our free-air experiment, we utilized two commonly used multimode small form factor pluggable laser modules and the SPAD detector developed by our group. The long-term timing stability of the SPAD detectors and the timing system used is better than 100 fs. The ultimate precision of the time transfer of less than 1 ps for averaging times of hours was achieved, and the reliability of the new TWTT method was proven.