Every earlier chapter treated measurement’s fragility as a problem. BB84 turns it into the whole point: a shared secret key that an eavesdropper cannot read without leaving fingerprints all over it.
Alice sends Bob a stream of qubits, each prepared in one of two random “languages” — think of them as two pairs of polarized glasses. Bob, not knowing which she used, guesses a pair of glasses for each. Afterward they compare only which glasses they used (never the bits) and throw away the rounds where they disagreed. The survivors are their secret key.
Now the magic: an eavesdropper has to measure to learn anything — but measuring through the wrong glasses scrambles the qubit. So if anyone listened in, the key Alice and Bob share will no longer match on a chunk of their bits. Compare a handful, see errors, and you know you were watched.
Imagine mailing letters under a wax seal that cracks the instant anyone opens them. You don't hide the contents — you just agree to bin any letter that arrives cracked. An eavesdropper can still steam one open, but the broken seal gives them away. BB84's "seal" is the rule that measuring a qubit the wrong way scrambles it: tampering can't help but show.
Each round shows Alice’s basis (+ or ×) and bit, and Bob’s. Green = bases matched, bits agree (a good key bit). Red = bases matched but bits disagree — impossible unless someone tampered. Toggle Eve and watch the error rate jump toward 25%.
Look only at the sifted bits — the ones where Alice and Bob used the same basis. With no eavesdropper those always agree. Eve, intercepting and resending, must guess a basis blind:
Alice and Bob sacrifice a random handful of key bits and compare them out loud. A clean sample → keep the rest as a secret key. A sample peppered with mismatches → throw it all away; someone was listening. Eve cannot do better — no-cloning forbids her from copying the qubit to measure later.
“BB84 sends my message over the quantum channel.” It doesn’t send a message at all. It distributes a shared stream of random bits — a key. The actual message travels later over an ordinary channel, scrambled with that key. Quantum mechanics secures the key exchange, nothing more.
And it doesn’t prevent eavesdropping — Eve is free to listen. What it guarantees is that she can’t do so undetected. Spotting her costs only a few sacrificed bits; the moment errors appear, Alice and Bob simply discard the key and try again.