BB84

Two distinct theorems do the work here. No-cloning (Wootters & Zurek, 1982; Dieks, 1982): no operation can produce a perfect copy of an arbitrary unknown quantum state. It is a statement about copying being impossible, not about disturbance — it is why Eve cannot duplicate the photon and let the original pass through untouched. Measurement disturbance: measuring in the wrong basis collapses the state and randomizes the outcome. This is what lets Alice and Bob detect Eve. No-cloning removes Eve's copy-and-forward option; measurement disturbance exposes the attack this demo simulates. This panel models the simplest full attack, intercept-resend: measure each photon and send Bob a replacement. But Eve does not know Alice's basis. If she guesses wrong: • She collapses the photon to her basis • She resends in her measured state • Bob, using Alice's correct basis, gets a random result 50% of the time • Full interception produces a ~25% QBER — the expected disturbance for intercept-resend in the ideal four-state model A general Eve has subtler strategies: entangling probes, storing quantum states until the bases are announced, collective measurements across many signals, or attacks on the hardware itself. BB84's security does not rest on intercept-resend being Eve's best move — it rests on a security proof that bounds Eve's information from the observed error rate under an explicit model of the devices. Physics supplies the constraints that proof uses; it does not by itself secure the equipment around the channel.