  In the practical continuous-variable quantum key distribution (CV-QKD)
system, the postprocessing process, particularly the error correction part,
significantly impacts the system performance. Multi-edge type low-density
parity-check (MET-LDPC) codes are suitable for CV-QKD systems because of their
Shannon-limit-approaching performance at a low signal-to-noise ratio (SNR).
However, the process of designing a low-rate MET-LDPC code with good
performance is extremely complicated. Thus, we introduce Raptor-like LDPC
(RL-LDPC) codes into the CV-QKD system, exhibiting both the rate compatible
property of the Raptor code and capacity-approaching performance of MET-LDPC
codes. Moreover, this technique can significantly reduce the cost of
constructing a new matrix. We design the RL-LDPC matrix with a code rate of
0.02 and easily and effectively adjust this rate from 0.016 to 0.034.
Simulation results show that we can achieve more than 98% reconciliation
efficiency in a range of code rate variation using only one RL-LDPC code that
can support high-speed decoding with an SNR less than -16.45 dB. This code
allows the system to maintain a high key extraction rate under various SNRs,
paving the way for practical applications of CV-QKD systems with different
transmission distances.
