Towards quantum communications in free-space seawater

Area of Science:

• Quantum Information Science
• Optics and Photonics
• Marine Technology

Background:

• Quantum communication relies on robust quantum channels, previously established in fiber optics and free-space.
• Seawater's potential as a quantum channel for secure communication remained unexplored, limiting global quantum network development.

Purpose of the Study:

• To investigate the feasibility of using seawater as a medium for long-distance quantum communication.
• To experimentally verify the transmission of quantum states through natural seawater.

Main Methods:

• Experiments were conducted using seawater samples from the Yellow Sea over a 36-kilometer range.
• Polarization quantum states of single photons (405 nm) and entangled photon pairs (810 nm) were transmitted.
• Process fidelity for single photons and Bell inequality violation for entangled states were measured.

Main Results:

• Single photons maintained high fidelity (average >98%) after transmission through seawater.
• Entangled photons, despite significant loss, demonstrated a violation of Bell inequality with 33 standard deviations.
• The study confirmed the survival of quantum states during propagation through seawater.

Conclusions:

• Seawater can serve as a viable quantum channel for transmitting quantum states.
• This research is a foundational step towards establishing underwater quantum communication networks.
• The findings pave the way for extending quantum communication globally, including oceanic regions.