Passive continuous variable quantum key distribution

Passive quantum key distribution (QKD) has been proposed for discrete variable protocols to eliminate side channels in the source. Unfortunately, the key rate of passive DV-QKD protocols suffers from sifting loss and additional quantum errors. In this work, we propose the general framework of passive continuous variable quantum key distribution. Rather surprisingly, we find that the passive source is a perfect candidate for the discrete-modulated continuous variable quantum key distribution (DMCV QKD) protocol. With the phase space remapping scheme, we show that passive DMCV QKD offers the same key rate as its active counterpart. Considering the important advantage of removing side channels that have plagued the active ones, passive DMCV QKD is a promising alternative. In addition, our protocol makes the system much simpler by allowing modulator-free quantum key distribution. Finally, we experimentally characterize the passive DMCV QKD source, thus showing its practicality.