Quantum communication provides a powerful tool for communication with information-theoretic security and high private capacity. However, until now, almost all studies on underwater quantum communication have been based on the polarization or orbital angular momentum of the single-photon state. Because quantum entanglement is an important resource in quantum communication, it is an interesting but still open question as to whether quantum entanglement still exists after interaction with water and underwater objects. In this study, we use the air-water-metal three-layer model as an example and investigate the dynamics of the output entangled state when both modes are sent through the water. Our results show that strong entanglement exists even after interaction with shallow water, and underwater quantum communication with two-mode quantum entanglement is thus feasible in typical quantum communication scenarios.
Quantum Illumination is a powerful tool for detecting a weakly-reflective target embedded in noise environment. Up to now, all quantum illumination techniques have been investigated in free-space scenario. Thus, it is still an open and interesting question whether quantum illumination could be applied to underwater target detection. Here, by using quantum entanglement generated by spontaneous parametric down conversion and joint quantum measurement of returned signal mode and idler mode, we construct a model of underwater target detection with quantum illumination technique. Further, we apply a set of typical experimental parameter and show that a 10-dB enhancement in the detection of a target embedded in 30-meter water can be observed. This work shows the potential applicability of quantum information technology in future underwater target detection.
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