Practical scheme for long distance side-channel-free quantum key distribution with weak coherent states only

We show that a side-channel-free (SCF) source does not have to be an ideal source by introducing the idea of mapping from ideal source. We propose a 3-state no-touch protocol for quantum key distribution (QKD) where Alice and Bob does not modulate any light sent out, the only thing the do is to send (or not send, in sending-or-not protocol). The reference light are from independent Lasers. We show that, the protocol is side-channel-free (i.e., both source side channel free and measurement device independent) and there is no modulation to the weak beams for QKD coding, except for sending or not sending. Calculation shows that one can reach a side-channel-free secure distance over 300 km using only coherent-state source. We use worst-case analysis which takes no limitation to the channel or detection loss for security. Our protocol is immune to all adverse due to side channels such as the photon frequency spectrum, emission time, propagation direction, spatial angular moment, and so on. Numerical simulations show that our scheme can reach a side-channel-free result for quantum key distribution over a distance longer than 200 km given the single-photon-interference misalignment error rate of 30%, and a distance longer than 300 km given the single-photon-interference misalignment error rate of 10%. Our no-touch idea can also apply to phase-coding twin-field QKD protocols. The no-touch idea also applies to twin-field QKD with phase coding.