Quantum Cryptography

Catching an eavesdropper

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• BB84: bits encoded in random bases; keep only the photons where bases match.
• An intercept-resend eavesdropper causes about 25% errors in the kept key.
• The no-cloning theorem makes undetected copying impossible.

Steps

1. Two kinds of measurement
2. How much survives?
3. Enter Eve
4. Eve’s footprint
5. Catching Eve
6. Why can’t Eve copy?