  Continuous-variable quantum key distribution (CVQKD) is considered to be an
alternative to classical cryptography for secure communication. However, its
transmission distance is restricted to metropolitan areas, given that it is
affected by the channel excess noise and losses. In this paper, we present a
scheme for implementing long-distance CVQKD using separable Gaussian states.
This tunable QKD protocol requires separable Gaussian states, which are
squeezed and displaced, along with the assistance of classical communication
and available linear optics compoments. This protocol originates from the
entanglement of one mode and the auxiliary mode used for distribution, which is
first destroyed by local correlated noises and restored subsequently by the
interference of the auxillary mode with the second distant separable correlated
mode. The displacement matrix is organized by two six-dimensional vectors and
is finally fixed by the separability of the tripartite system. The separability
between the ancilla and Alice and Bob's system mitigates the enemy's
eavesdropping, leading to tolerating higher excess noise and achieving longer
transmission distance.
