Operation BB84: Full Dress Rehearsal

Mission 5: Quantum Key Distribution. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Right then, recruit, enough watching over my shoulder. You've caught photons, sifted a key, and spotted a spy. Now do the whole rotten job yourself, start to finish, no safety net.

You're Alice. For every photon, you pick the filter. Bob will guess his own, independently, same as always, and you won't find out how he did until the public sifting call. Then comes the bit that separates a real spy service from a Bodleian reading group: a security check, sacrificing a slice of the key to see if anyone's been listening.

Flip the switch at the top to invite Eve onto the line before you start, or leave her off and enjoy an honest run. Either way, do the full protocol properly this time: filters, sifting, checking, verdict. No skipping to the answer.