  The two-way quantum clock synchronization has been shown not only providing
femtosecond-level synchronization capability but also security against
symmetric delay attacks, thus becoming a prospective method to compare and
synchronize distant clocks with both enhanced precision and security. In this
letter, a field test of two-way quantum synchronization between a H-maser and a
Rb clock linked by a 7 km-long deployed fiber was 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 stability was improved by more than one-magnitude to 1.9 ps,
even though the number of acquired photon pairs was only 1440 in 30 s due to
the 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 field applications.
