  Quantum secure direct communication (QSDC) is a approach of communication to
transmit secret messages base on quantum mechanics. Different from the quantum
key distribution, secret messages can be transmitted directly on quantum
channel with QSDC. Higher channel capacity and noise suppression capabilities
are key to achieving long-distance quantum communication. Here we report a
continuous-variable QSDC scheme based on mask-coding and orbital angular
momentum, in which the mask-coding is employed to protect the security of the
transmitting messages and to suppression the influence of excess noise. And the
combination of orbital angular momentum and the information block transmission
can effectively improve the secrecy capacity. In the 800 information blocks *
1310 bits length 10-km experiment, the results show that the statistical
average of bit error rate has decreased by 36% and reached 0.38% comparing to
existing solutions, and the system excess noise is 0.0184 SNU, and the final
secrecy capacity is 6.319 * 106 bps. Therefore, this scheme reduces error bits
while increasing secrecy capacity, providing a solution for long-distance
large-scale quantum communication.
