  Quantum communication has been rapidly developed due to its unconditional
security and successfully implemented through optical fibers and free-space air
in experiment. To build a complete quantum communication network involving
satellites in space and submersibles in ocean, 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-meter-long water. We prepare six universal quantum states at single photon
level and observe their faithful transmission in a large marine test platform.
We obtain the 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.
