The Voodoo Coins Console

Mission 4: Entanglement. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

One thing needs hammering home before Internal Affairs has an aneurysm: instant correlation is not the same thing as sending a message.

You saw it two screens ago on the no-signalling bench. Both coins land in well under a millisecond, Oxford to Mars, while the classical radio signal below is still crawling across the solar system on its twelve-minute commute. That part's real, and it's exactly as spooky as it sounds.

Here's the part that keeps Einstein's ghost calm: neither Alice nor Bob can choose which result they get. Alice can't decide "I'll get Heads" to signal something to Bob, since the outcome is random on both ends, only correlated with each other. No control means no message, means no faster-than-light communication, means relativity survives the encounter completely intact.

Now to the thing that is useful. Flip Eve onto the line and run a batch. Watch the match rate slide off 100% and settle around 75%, not to a coin flip, note, but to exactly three-quarters. Eve guesses the right basis half the time and does no damage at all; the other half she randomises Bob's result. Half of nothing plus half of a coin flip is a 25% error rate, and that number is going to come back in Module 5 as the tripwire the whole protocol is built on.