7 km free-space time-bin quantum key distribution

Area of Science:

• Quantum communication
• Free-space optics
• Quantum cryptography

Background:

• Satellite-based quantum key distribution (QKD) is crucial for global secure networks.
• Polarization encoding has been used, but time-bin encoding offers advantages like simplified designs and motion insensitivity.

Purpose of the Study:

• To develop and demonstrate a time-bin QKD system suitable for satellite applications.
• To validate the robustness of time-bin encoding under conditions simulating satellite-to-ground links.

Main Methods:

• Developed a 625-MHz QKD light source using a Sagnac-interferometer modulation scheme.
• Demonstrated time-bin QKD over a 7 km urban terrestrial free-space channel.
• Simulated polarization-basis-rotation using a half-wave plate to test robustness.

Main Results:

• Achieved a low quantum bit error rate (QBER) of 0.87%.
• Obtained a secure key rate of 134 bps@51.4 dB.
• The system operated effectively through atmospheric conditions exceeding typical satellite-to-ground link losses.

Conclusions:

• Time-bin encoding is a robust and compelling alternative for satellite QKD.
• The study lays the technical foundation for future satellite quantum communication networks.