Free-space quantum secure direct communication based on decoherence-free space

Abstract

We propose a free-space quantum secure direct communication protocol by using hyper-encoded single photons. The communication parties can securely exchange private information in a deterministic way. In this protocol, single photons are encoded simultaneously by both the polarization and spatial-mode degrees of freedom that constitute a decoherence-free space. The detrimental effect introduced by either reference-frame misalignment or practical channel rotational noise is passively suppressed. Finally, the security of our protocol against classical individual attacks is proved by quantum mutual information.