E91: A Quantum Key from Entangled Photons

In BB84, Alice prepares the photons herself. In E91, proposed by Artur Ekert in 1991, photons are born in entangled pairs at a shared source. Each photon on its own is random, but the two in a pair are linked. Alice and Bob check that the link is really quantum with a Bell test, and if it passes, nobody could have read the key on the way.

The game has four stages:

1. Pair source. A warm-up, then starting the source of entangled pairs.
2. Announcing angles. Which pairs go into the key and which into the test.
3. Bell test. Played as a guessing game: the best classical strategy wins 75% of the time, entangled pairs reach about 85%.
4. Secret key. Inversion, error correction and compression.

If you haven't seen BB84 yet, it is worth playing first: E91 builds on it.

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).