Continuous-variable quantum key distribution with noisy coherent states

An excess noise in coherent-state preparation can prevent secure key distribution through lossy channel. The feasible single-copy and multicopy linear optical methods are proposed to purify the prepared state. The single-copy method always sufficiently reduces the excess noise to obtain the key secure against both the individual and collective attacks even through any lossy channel. To increase the secure key rate, two feasible applications of the multicopy linear optical purification are proposed. As a result, maximal secure key rate achievable through a given lossy channel can be approached.