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Real-world single-photon sources have a problem: sometimes they accidentally emit two or three photons instead of one. An eavesdropper named Eve can exploit this - she splits off one photon from multi-photon pulses, measures it, and lets the others continue to Bob. Alice and Bob never notice because they still receive photons. This is called a photon number splitting attack. Decoy states fix it. Alice randomly varies the intensity of her pulses - some strong (signal states), some weak (decoy states), some empty (vacuum states). Eve can't tell which is which before measuring. Alice and Bob later compare statistics on decoy states to detect if Eve took any photons. This elegant solution, proposed in 2003, made commercial QKD practical. QNu Labs' Armos implements decoy states, extending secure distance from maybe 50km to over 150km.
Practical single-photon sources aren't perfect - they follow Poisson statistics. Without decoy states, real QKD systems would either have terrible key rates or be insecure. Decoy states let you use inexpensive diode lasers attenuated to single-photon levels instead of exotic true single-photon sources.
Decoy state QKD is why commercial QKD exists. It bridges the gap between theoretical security (assuming perfect single photons) and practical implementation (using affordable imperfect sources). The key rate and distance improvement makes QKD deployable in real metro networks.
Protecting long-distance QKD links over 100km, securing metropolitan quantum networks, defending against sophisticated eavesdropping attacks, enabling cost-effective commercial QKD, government and military quantum communications
https://www.qnulabs.com/quantum-key-distribution | https://www.youtube.com/c/QNuLabs
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