Bennett–Brassard-84 Encryption Protocol

Now that we understand changes of basis states, we want to reward you by looking at an actual application of these ideas in QIS. The method is called the BB84 encryption protocol after its inventors Charles Bennett and Gilles Brassard, who developed it—surprise—in 1984. The basic idea is that Alice and I, located some distance apart, want to establish a secret key (a string of $0$s and $1$s) to encrypt messages. Alice needs to send the key to me, but we are worried that someone, perhaps you, Cardy, is eavesdropping on our communications. The process we will use is an example of quantum key distribution (QKD).