The Bodleian Wave-Function Heist

Watching filter bases line up as photons stream

Mission 5: Quantum Key Distribution. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Put down the sherry, recruit: we’ve intercepted Alice's quantum key stream snaking under the Bodleian, but the Syndicate has been dreadfully cagey about which filter bases she’s employing. Watch the photons pour down the fibre, and when the pace overwhelms you, drop into slow mode to send them one at a time and read the filter Alice and Bob each happen to pick, for the quantum state is as flighty as an undergraduate before finals. Where their bases align you’ll harvest a pristine bit for our encryption key, but where they clash the wave-function collapses into useless classical noise. Flip Eve onto the line and watch her tap scramble the stream and alert every spy from Jericho to Iffley. Precision is the better part of valor, so do try to keep up with the stream before we’re all unceremoniously sacked. Chop chop!