Implementation of field two-way quantum synchronization of distant clocks across a 7 km deployed fiber link.

The two-way quantum clock synchronization has been shown not only providing femtosecond-level synchronization capability but also secure against symmetric delay attacks, thus becomes a prospective method to compare and synchronize distant clocks with both enhanced precision and security. In this letter, a field two-way quantum synchronization between a H-maser and a Rb clock linked by a 7 km-long deployed fiber is implemented. Limited by the frequency stability of the Rb clock, the achieved time stability at 30 s was measured as 32 ps. By applying a fiber-optic microwave frequency transfer technology, the dominance of the Rb clock was effectively overcome and the corresponding stability was more than one-magnitude improved to 1.95 ps, even though the acquired photon pairs was only 1440 in 30 s due to the fairly low sampling rate of the utilized coincidence measurement system. Such implementation demonstrates the high practicability of two-way quantum clock synchronization method for promoting the performance of field applications.