Twin-Field QKD in 2D: The Loss-Budget View

This is a 2D companion to the 3D twin-field QKD scene, built around the same exact analytic model but a genuinely different visual language: instead of an orbiting camera over a rendered fibre scene, the link between Alice, the untrusted central node and Bob is drawn as a flat, draggable, zoomable schematic with real pulses streaming toward the beamsplitter and detectors that flash on a probabilistic click, exactly as the analytic click probability dictates. Two extra chart panels beneath the diagram make the underlying reasoning visible rather than just animated: a log-scale transmittance plot showing η_arm(L) = √η_total(L) decaying at half the rate (in dB) of η_total(L), and a loss-budget bar chart translating that same relationship into the decibels each protocol actually has to spend. The sidebar keeps the same four sliders and the live rate-vs-distance chart as the 3D version, so every number you see here is computed from the identical formulas — only the picture changed.