The Eavesdropper’s Prelims

Auditing a link for tampering

Mission 7: Building the Quantum Internet. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Right then, pay attention, recruit; we are running an audit on a quantum link that the Echo Network's operative, Eve, has been rather clumsily tampering with, and it's high time we marked her script.

You are the proctor here, so mark it the way a proctor does: on the aggregate, not on a single answer. Alice broadcasts her filter basis over the open line and never the bit, which means there is nothing in the public traffic for you to catch her with. What Alice and Bob do instead is spend some of their own key on the question. They sacrifice a random sample, read those bits out loud, and count how often they disagree.

Set the sample size and read the verdict. You already know the number Eve leaves behind from Module 5: intercept-resend puts the error rate at roughly one in four. Your task is to decide how much key you are prepared to burn to be sure of seeing it, and to notice what happens at the small end, where a lucky Eve gets a clean bill of health and an unlucky stretch of honest fibre gets accused.

Trust the physics, but trust it statistically. Chop chop!