Recently, a groundbreaking achievement in the field of quantum communication has been made by the research team led by Professor Jin Xianmin from Shanghai Jiaotong University. They have successfully carried out the world's first quantum communication experiment in seawater. This experiment demonstrated that photon polarization states and quantum entanglement can retain their quantum properties within seawater. For the first time globally, this experiment confirmed the possibility of underwater quantum communication, marking a significant stride towards creating an integrated underwater and air-sea quantum communication network in the future. The findings were published in the most recent edition of Optics Express and received special editorial recognition.
At present, quantum communication via optical fibers and free-space atmospheric channels has already been validated as feasible. But what about the ocean as a potential quantum channel? Jin Xianmin explained to the Science and Technology Daily journalist that while seawater presents challenges due to higher levels of photon scattering and absorption caused by suspended particles and salinity compared to optical fibers and the atmosphere, it does offer a "blue-green window" where photons experience less loss. This window allows for detection using commercial single-photon detectors, making seawater-based quantum communication theoretically viable. He emphasized, "Without the ocean, the global quantum communication network would remain incomplete."
In the latest experiments, the team selected photon polarization as the medium for information encoding. Through simulations, they found that even under extreme conditions of high loss and scattering, polarization-encoded photons would not undergo quantum bit flipping but might be lost. This means that as long as a small number of single photons survive despite the immense channel losses in seawater, secure keys could still be established.
The current results indicate that underwater quantum communication can reach distances of several hundred meters. Despite the relatively short range, this capability enables secure communication with submarines and sensor networks at depths of up to 100 meters, and even communication with satellites and aircraft from just a few meters below the surface. This represents a significant advancement over previous beliefs that considered seawater an impenetrable barrier, opening new possibilities in military and other sectors.
Jin Xianmin noted that while this marks the initial step towards underwater quantum communication, a lot remains to be done before practical underwater and air-sea integrated quantum communication networks can be realized. However, he added optimistically, "This latest research confirms that the vision of a comprehensive quantum communication network spanning land, sea, and air is within reach."
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