  Long-distance quantum channels capable of transferring quantum states
faithfully for unconditionally secure quantum communication have been so far
confirmed feasible in both fiber and free-space air. However, it remains
unclear whether seawater, which covers more than 70% of the earth, can also be
utilized, leaving global quantum communication incomplete. Here we
experimentally demonstrate that polarization quantum states including general
qubits and entangled states can well survive after travelling through seawater.
We performed experiments in a 3.3-meter-long tube filled with seawater samples
collected in a range of 36 kilometers in Yellow sea, which conforms to Jerlov
water type I. For single photons at 405 nm in blue-green window, we obtained
average process fidelity above 98%. For entangled photons at 810 nm, even with
high loss, we observe violation of Bell inequality with 33 standard deviations.
This work confirms feasibility of seawater quantum channel, representing the
first step towards underwater quantum communication.
