What Is a Secret?

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

What is a secret, really? Whether it’s the launch codes for a satellite or a scandalous rumor whispered in a dark corner of an Oxford pub, information is simply the resolution of uncertainty. Think of it like a high-stakes game of 20 Questions: if I tell you "The meeting is in the Magdalen College gardens," I haven’t handed you physical grass; I have simply eliminated every other coordinate on Earth.

In the classical world (I mean the world of your smartphone and standard fiber-optic cables) we shrink all that glorious complexity down to a painfully boring, microscopic switch: The Bit.