Photon level crosstalk between parallel fibers installed in urban area

Area of Science:

• Quantum Information Science
• Telecommunications Engineering
• Optical Fiber Communications

Background:

• Quantum Key Distribution (QKD) relies on secure transmission channels.
• Dark fibers, unused optical fibers, are potential candidates for QKD deployment.
• Understanding channel characteristics is crucial for practical QKD implementation.

Purpose of the Study:

• To estimate the channel characteristics of field-installed dark fibers for QKD at single photon levels.
• To identify dominant loss and noise sources affecting QKD performance in deployed fibers.

Main Methods:

• Characterization of 45 km of telecom single-mode dark fibers in Tokyo.
• Measurement of total losses and analysis of fiber segments (aerial vs. buried).
• Identification of stray light and crosstalk sources, particularly at cable bending points.

Main Results:

• Total fiber losses measured at approximately 14 dB over 45 km.
• Stray light from adjacent public internet fibers identified as a dominant noise source.
• Crosstalk was observed to occur at cable bending points, indicating a vulnerability.

Conclusions:

• Deployed dark fibers exhibit characteristics suitable for QKD, but with significant challenges.
• Crosstalk from public networks can degrade QKD system performance by increasing the bit error rate.
• Potential for information leakage through adjacent fibers necessitates careful deployment and mitigation strategies.