Quantum Cryptography: Public Key Distribution and Coin Tossing

要点

• ▸Encodes key bits in non-orthogonal photon polarization states.
• ▸Any eavesdropper necessarily disturbs the states, revealing their presence in the error rate.
• ▸Security follows from quantum measurement and no-cloning, not from any unproven mathematical assumption.

やさしい解説

Ordinary encryption is safe only because certain math problems are slow to solve; a fast enough computer breaks it. Bennett and Brassard proposed something different: send the key as individual photons tilted at angles the receiver measures. Quantum mechanics forbids copying an unknown state and forces any measurement to disturb it, so an eavesdropper cannot listen in without leaving statistical fingerprints. The two parties simply compare a sample of their results; too many errors means someone was listening, so they discard the key and try again. Security comes from physics, not from hoping the attacker's computer is too slow.

なぜ重要か

BB84 is the foundation of quantum cryptography and is deployed commercially today. It is the mirror image of Shor's result: quantum mechanics breaks classical encryption, but also offers a replacement that no computer — quantum or classical — can crack.