  More than Mbps secret key rate was demonstrated for continuous-variable
quantum key distribution (CV-QKD) systems, but real-time postprocessing is not
allowed, which is restricted by the throughput of the error correction decoding
in postprocessing. In this paper, a high-throughput FPGA-based quasi-cyclic
LDPC decoder is proposed and implemented to support Mbps real-time secret key
rate generation for CV-QKD for the first time. A residual bit error correction
algorithm is used to solve the problem of high frame errors rate (FER) caused
by the limited precision of the decoder. Specifically, real-time high-speed
decoding for CV-QKD systems with typical code rates 0.2 and 0.1 is implemented
on a commercial FPGA, and two throughputs of 360.92Mbps and 194.65Mbps are
achieved, respectively, which can support 17.97 Mbps and 2.48 Mbps real-time
generation of secret key rates under typical transmission distances of 25km and
50km, correspondingly. The proposed method paves the way for high-rate
real-time CV-QKD deployment in secure metropolitan area network.
