Recently, a team led by Professor Jin Xianmin at Shanghai Jiaotong University achieved a significant breakthrough by successfully conducting the world's first underwater quantum communication experiment in seawater. This groundbreaking experiment demonstrated that both the polarization state of photons and quantum entanglement could retain their essential quantum properties within marine environments. For the first time globally, this achievement confirmed the viability of underwater quantum communication, marking an important milestone towards building 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 were highlighted by the journal’s editors.
At present, quantum communication via optical fibers and free-space atmospheric channels has already been validated. But what about using the ocean itself as a quantum channel? As Jin Xianmin explained to the Science and Technology Daily reporter, while factors such as suspended particles and salinity do cause substantial scattering and photon loss in seawater—far exceeding those encountered in optical fibers or the atmosphere—the ocean also presents a "blue-green window" where photons experience lower levels of attenuation. This allows for detection using commercially available single-photon detectors, thus making underwater quantum communication theoretically plausible. “Without incorporating the ocean,†Jin emphasized, “the global quantum communication network would remain incomplete.â€
In their latest experiments, the researchers selected photon polarization as the medium for information encoding. Through simulations, they found that even under conditions of extreme loss and scattering, polarization-encoded photons would only diminish in number rather than flip quantum bits. Consequently, so long as a minimal number of single photons persist, secure keys can still be established despite the vast losses inherent to seawater.
The current results indicate that underwater quantum communication can cover distances up to several hundred meters. While the range remains limited, it is sufficient to facilitate secure communications with submarines and sensor networks operating at depths of around 100 meters—or even just a few meters beneath the surface. This capability extends beyond previous assumptions, which viewed the ocean as a "no-go zone," offering new possibilities for satellite and aircraft encrypted communication. Hence, it holds immense potential applications in military and other domains.
Professor Jin Xianmin noted that this marks merely the initial phase in realizing underwater quantum communication. There remains a considerable journey ahead before practical integration between underwater and air-sea quantum networks becomes feasible. Nevertheless, he concluded optimistically, “This latest research underscores that the vision of a comprehensive terrestrial, aerial, and maritime quantum communication infrastructure is not only attainable but increasingly foreseeable.â€
[Image description: A photo showing the experimental setup for Shanghai's pioneering underwater quantum communication test.]
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