Transmission of photonic polarization states through 55-m water: towards air-to-sea quantum communication

Abstract

Quantum communication has been rapidly developed due to its unconditional security and successfully implemented through optical fibers and free-space air in experiments. To build a complete quantum communication network involving satellites in space and submersibles in ocean, the underwater quantum channel has been investigated in both theory and experiment. However, the question of whether the polarization encoded qubit can survive through a long-distance and high-loss underwater channel, which is considered as the restricted area for satellite-borne radio waves, still remains. Here, we experimentally demonstrate the transmission of blue-green photonic polarization states through 55-m-long water. We prepare six universal quantum states at the single photon level and observe their faithful transmission in a large marine test platform. We obtain complete information of the channel by quantum process tomography. The distance demonstrated in this work reaches a region allowing potential real applications, representing a step further towards air-to-sea quantum communication.