Twin-Field QKD: Beating the Repeaterless Bound

Twin-field quantum key distribution routes each side's weak coherent pulses only halfway across the link, to an untrusted central node where single-photon interference — not direct transmission — generates the shared key. Because the surviving signal only has to cross half the fibre, the achievable key rate scales with the square root of the channel loss instead of the loss itself, letting it out-run ordinary BB84 past a crossover distance you can watch shift live as you drag the distance, brightness, phase-visibility and dark-count sliders. The 3D view streams real pulses from Alice and Bob into the central beamsplitter and flashes a detector every time the analytic click probability fires, while the side panel plots both protocols' rate-vs-distance curves so the sqrt(η) advantage over η is visible directly, not just asserted.