Operation Bodleian

A noisy channel flips bits at random; send each one several times and the receiver out-votes the errors

Mission 0: The Information Problem. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Put down the Earl Grey, recruit. We’ve tapped a stolen Echo Network data stream snaking right under the Bodleian floorboards, but the line is absolutely filthy with noise. Every so often the channel flips a bit at random, turning a 1 into a 0 with nobody any the wiser, and a single corrupted bit can rot a whole letter. Drag the flip-probability slider up and watch the corruption bloom red in the noise band. Then try the trick the boffins swear by: instead of sending each bit just once, send it three or five times over. Switch the coding from raw to 3× or 5× and the receiver simply takes a majority vote, out-voting any lone flip before it reaches the far end. Watch the decoded error rate sink below the raw one. Let's see if you can work out why repetition beats noise, or if you're about to make a total fiasco of this heist. Chop chop!