Securely Establish Encryption Keys with the BB84 Quantum Protocol 🔐

BB84 protocol is a quantum key distribution scheme developed by Charles Bennett and Gilles Brassard in 1984. It is the first quantum cryptography protocol. The protocol is provably secure, relying on the quantum property that information gain is only possible at the expense of disturbing the signal if the two states one is trying to distinguish are not orthogonal. It is a method of securely communicating a private key from one party to another for use in one-time pad encryption. So how to establish a random encryption key securely with the Quantum Key Distribution scheme ? Alice creates a random bit of 0 or 1 and then randomly selects one of her two bases (rectilinear or diagonal) to transmit it in. She then prepares a photon polarization state depending both on the bit value and basis. So for example a 0 is encoded in the rectilinear basis (+) as a vertical polarization state, and a 1 is encoded in the diagonal basis (x) as a 135° state. Alice then transmits a single photon in the state specified to Bob, using a quantum channel. This process is then repeated from the random bit stage, with Alice recording the state, basis and time of each photon sent. As Bob does not know the basis the photons were encoded in, all he can do is to select a basis at random to measure in, either rectilinear or diagonal. He does this for each photon he receives, recording the time, measurement basis used and measurement result. After Bob has measured all the photons, he communicates with Alice over the public classical channel. Alice broadcasts the basis each photon was sent in, and Bob the basis each was measured in. They both discard photon measurements where Bob used a different basis, which is half on average, leaving half the bits as a shared key. Quantum key distribution is only used to produce and distribute a key, not to transmit any message data. This key can then be used with the one-time pad cipher with a secret random key. This video was downloaded and edited from Quantum cryptography, animated by Centre for Quantum Technologies @ https://www.youtube.com/watch?v=LaLzshIosDk