Quantum purification for coherent states and its application

The inevitable decoherence of quantum states leaks information to the environments and reduces the practical performance of quantum-information protocols. Quantum purification of coherent states provides an easy-to-implement approach to decouple from the environments utilizing the correlation of several copies. However, repeated transmission of quantum states reduces the randomness of the collectivity and generates security vulnerabilities. In this paper, we propose a modified purification scheme and apply it to the continuous-variable quantum secure direct communication (CV-QSDC) protocol based on coherent states. The designed purification scheme improves the security capacity of communication systems through the effective suppression of excess noises, accompanied by the maintenance of source-side stochasticity. We conduct a proof-of-principle experiment of the purification scheme in the CV-QSDC system. The appearance of transmitted quantum states in phase space and two-dimensional Gaussian distributions have high goodness of fit at two copies. The security capability of this system is improved by 43.6% as the excess noise is reduced to 0.58 times the original noise by purification.