The continuous-variable quantum key distribution (CVQKD) has been demonstrated to be available in practical secure quantum cryptography. However, its performance is restricted strongly by the channel excess noise and the reconciliation efficiency. In this paper, we present a quantum key distribution (QKD) protocol by encoding the secret keys on the random choices of two measurement bases: the conjugate quadratures
X and
P. The employed encoding method can dramatically weaken the effects of channel excess noise and reconciliation efficiency on the performance of the QKD protocol. Subsequently, the proposed scheme exhibits the capability to tolerate much higher excess noise and enables us to reach a much longer secure transmission distance even at lower reconciliation efficiency. The proposal can work alternatively to strengthen significantly the performance of the known Gaussian-modulated CVQKD protocol and serve as a multiplier for practical secure quantum cryptography with continuous variables.
According to the basic principle of the density evolution for the irregular low-density paritycheck(LDPC)codes,the parity check matrix His optimized and a better performance one is obtained by combining the Gaussian approximation algorithm and the particle swarm optimization(PSO)algorithm.Applied to the data reconciliation of continuous variables coherent states based quantum key distribution,the algorithm gets a lower SNRthreshold and improves reconciliation efficiency so that the communication distance of quantum key distribution is extended.When the SNRof quantum channel is above 4.9dB and the number of packets of continuous variables is 2×105,the simulation results of the algorithm written by C language show that the reliable data reconciliation can be realized.Meanwhile,the reconciliation efficiency can achieve 92.06%and the maximum transmission distance can be extended up to 35.5km.