Abstract: Until true quantum repeaters with quantum memories mature globally, trusted relay nodes represent the sole engineering method to scale QKD across continents. This paper establishes rigorous mathematical security bounds and hardware enforcement mechanisms for trusted node clusters.
We demonstrate that for an N-node relay chain where adjacent links use independent quantum one-time pads, the mutual information between an eavesdropper controlling N-1 communication channels and the end-to-end secret key is strictly bounded by zero, provided intermediate node hardware security remains uncompromised.
Optical single-photon avalanche diodes and SNSPDs generate distinctive thermal signatures during avalanche breakdown. We specify dual-shielded Faraday enclosures with sound-dampening baffles that eliminate acoustic and electromagnetic side-channel leakage across node perimeters.
Operating across 14 cascaded trusted nodes over a commercial 1,540 km optical trunk line, the node architecture achieved continuous secret key throughput of 14.2 kbps while maintaining link availability exceeding 99.98% over continuous six-month operational runs.