Now that we have seen the power and simplicity of the one-time pad, we are left with a single question: how do we generate these pads in a convenient manner, such that Eve cannot gain access to them, and such that we never have to use a pad repeatedly? In this lecture, Tim Coopmans will explain a simple quantum protocol, called BB84 (hinted at in the quizzes for Module 4), which allows Alice and Bob to obtain correlated random bits, without ever having to disclose the value of those bits.

Main takeaways

• BB84 is a simple, entanglement-free quantum key distribution protocol, which allows Alice to generate and transmit random numbers such that Bob can successfully share a large portion of them, but Eve cannot intercept them.
• This scheme uses the fact that consecutive measurements in different bases produce random outcomes in order to allow Alice and Bob to detect Eve’s presence.
• At long distances, where quantum repeaters become necessary, we use an alternate quantum key distribution protocol called E91, invented by Artur Ekert in 1991.