Information-theoretically secure quantum access network (QAN) provides a multiple-user structure for large-scale key distribution. However, active reference-frame compensation and clock synchronization require extra devices and signals, increasing the complexity of the system and time/resource consumption. In this work, a fully passive received QAN comprising both free-space and fiber channels is demonstrated. Based on an improved reference-frame-independent quantum key distribution (RFI-QKD) protocol and qubit-synchronization method, Bob only passively receives quantum signals from all Alices with various wavelengths and laser repetition rates. Extra classical signals for reference frame calibration and clock synchronization are not required, so that the QAN can be asynchronous. Furthermore, RFI-QKD with a 7 km free-space channel in an urban environment is first fully demonstrated with a positive key rate, and the maximal distance over a 250 km fiber channel for four-state RFI-QKD is also achieved. The experimental results show that our fully passive received QAN is robust, cost-effective, and flexible, paving its application in the future quantum network.
Fan et al. (Mon,) studied this question.