High-precision nonlocal temporal correlation identification of entangled photon pairs for quantum clock synchronization

High-precision nonlocal temporal correlation identification in entangled photon pairs is critical to measure the time offset between remote
independent time scales for many quantum information applications. The first nonlocal correlation identification was reported in 2009,
which extracts the time offset via the algorithm of iterative fast Fourier transformations and their inverse. The best identification resolution
is restricted by the peak identification threshold of the algorithm, and thus the time offset calculation precision is limited. In this paper, an
improvement for the identification is presented both in resolution and precision via a modified algorithm of direct cross correlation extraction.
A flexible resolution down to 1 ps is realized, which is only dependent on the least significant bit resolution of the time-tagging device.
The attainable precision is shown to be mainly determined by the inherent timing jitter of single photon detectors, the acquired pair rate, and
acquisition time, and a sub-picosecond precision (0.72 ps) has been achieved at an acquisition time of 4.5 s. This high-precision nonlocal measurement
realization provides a solid foundation for the field applications of entanglement-based quantum clock synchronization, ranging,
and communications.