BB84: Quantum Key Distribution, Step by Step

BB84 is the first quantum key distribution protocol, proposed by Charles Bennett and Gilles Brassard in 1984. Alice sends single photons to Bob in randomly chosen bases, they compare the bases in public, and anyone listening in leaves a trace in the error rate.

In this game you run the protocol yourself, in four stages:

1. Quantum channel. Alice sends qubits, Bob measures them.
2. Sifting. Alice and Bob compare bases and keep the bits where they match.
3. QBER check. They give up part of the key to look for signs of Eve.
4. Secret key. Error correction and compression turn what is left into the final key.

The model is simplified, but every stage follows the real protocol.

I started this project after giving a talk on quantum communications and realising I couldn't explain BB84 even to myself. This game is how I finally sorted it out.

Part of Ludus Lab: small browser games about cryptography and quantum key distribution, each with a short article. Theory and the other games: https://ludus-lab.com. Languages: English, Spanish, Chinese and Russian (buttons in the top-right corner).