Passive implementations of quantum key distribution (QKD) sources are highly desirable as they eliminate side-channels that active modulators might introduce. Up till now, however, only partial solutions have been proposed, in which either the preparation of decoy-states or BB84 states is done passively with linear optical elements, but not both of them simultaneously, which greatly limits the practicality of such passive QKD schemes. Very recently researchers from the Universities of Hong-Kong and Toronto, the multinational technology conglomerate corporation Cisco Inc., and the Quantum Communication Theory Group at the VQCC have overcome this limitation and proposed a fully-passive QKD source with linear optics that eliminates active modulators for both decoy-state choice and BB84 encoding. This allows for highly practical QKD systems that avoid side-channels from the source modulators. Moreover, the passive source designed can create any arbitrary state on a qubit system and is protocol-independent. That is, it can be used for various protocols such as BB84, reference-frame-independent QKD, or the six-state protocol. It can also in principle be combined with e.g. measurement-device-independent QKD, to build a system without side-channels in either detectors or modulators.


V. Zapatero, W. Wang, M. Curty, “A fully passive transmitter for decoy-state quantum key distribution”, Quantum Science and Technology 8, 025014 (2023).