Summary

Using classical communications, any information that you send over a network can be intercepted and read by a malicious agent. This includes meaningful information such as your credit card number, but it also includes any randomly generated key that you use to encrypt the meaningful information. One way to achieve secure message transmission is for both the sender and the receiver to have information without ever transmitting that information over a network. We know of no way to do this with credit card numbers or any other meaningful information. But, using the BB84 algorithm, the sender and receiver can cooperatively create a random key that’s known only to the two of them. This random key is never transmitted along network lines.

The BB84 algorithm depends on one important fact: you can’t clone a qubit. If you get a qubit in some arbitrary state, , you can’t measure the values of and to end up with two qubits in the same state. But what if you...