Decoy-State BB84 (2D): Defeating Photon-Number-Splitting Attacks

Practical quantum key distribution runs on weak coherent laser pulses rather than true single photons, and the occasional multi-photon pulse opens a loophole: a photon-number-splitting attack lets an eavesdropper skim extra photons from those pulses undetected while blocking the vulnerable single-photon ones. This top-down 2D view animates pulses from Alice to Bob along a lossy fiber, models a live PNS attacker at the channel midpoint, and computes the real decoy-state lower bound on the single-photon yield from the running detection statistics — pan and zoom the beam line and watch the bound stay healthy on an honest channel, then collapse the moment an eavesdropper is present, exactly as it does in a real quantum-communication link.