# A security proof of continuous-variable QKD using three coherent states. (arXiv:1709.01758v2 [quant-ph] UPDATED)

We introduce a ternary quantum key distribution (QKD) protocol and asymptotic

security proof based on three coherent states and homodyne detection. Previous

work had considered the binary case of two coherent states and here we

nontrivially extend this to three. Our motivation is to leverage the practical

benefits of both discrete and continuous (Gaussian) encoding schemes creating a

best-of-both-worlds approach; namely, the postprocessing of discrete encodings

and the hardware benefits of continuous ones. We present a thorough and

detailed security proof in the limit of infinite signal states which allows us

to lower bound the secret key rate. We calculate this is in the context of

collective eavesdropping attacks and reverse reconciliation postprocessing.

Finally, we compare the ternary coherent state protocol to other well-known QKD

schemes (and fundamental repeaterless limits) in terms of secret key rates and

loss.