The Subterranean Snare

Spotting an eavesdropper by the disturbance they leave

Mission 1: Enter the Quantum World. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Right then, recruit, we've laid a trap down a defunct tube line under King's Cross. Here's the catch, and it's the whole lesson: you cannot look at the line and see which qubits Eve has touched. A tampered qubit and a pristine one look identical, because "looking" is the very thing that does the damage. So we do it the only way anyone ever does it, which is statistically. Alice and Bob sacrifice a slice of their stream and compare it out loud over an open line. On a clean channel the sample agrees. With Eve on the line it disagrees about a quarter of the time. Your job is to set the sample size and read the verdict: too small a sample and Eve slips through on luck; too large and you've burned key you could have used. Find the point where you can call it. Chop chop!