  Quantum key distribution provides secure keys with information-theoretic
security ensured by the principle of quantum mechanics. The continuous-variable
version of quantum key distribution using coherent states offers the advantages
of its compatibility with telecom industry, e.g., using commercial laser and
homodyne detector, is now going through a booming period. In this review
article, we describe the principle of continuous-variable quantum key
distribution system, focus on protocols based on coherent states, whose systems
are gradually moving from proof-of-principle lab demonstrations to in-field
implementations and technological prototypes. We start by reviewing the
theoretical protocols and the current security status of these protocols. Then,
we discuss the system structure, the key module, and the mainstream system
implementations. The advanced progress for future applications are discussed,
including the digital techniques, system on chip and point-to-multipoint
system. Finally, we discuss the practical security of the system and conclude
with promising perspectives in this research field.
