The Port Meadow Pulse

Catching photons in the right basis

Mission 2: The Art of Quantum Messages. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Swap that glass of Pimm's for a terminal, recruit; the Echo Network is firing a stream of encrypted espionage photons straight through the dark fibre running under Port Meadow, some on the straight ($+$) basis, some on the diagonal ($\times$). Our experimental catcher is nothing more than a polarising filter, and the entire art of reading a photon comes down to the ANGLE you set that filter to. Pick a Filter angle and a Photon angle from the chips and fire a batch. The fraction that survive is cos²(α − θ), governed purely by the gap between the two angles: line the filter up with the photon and nearly all of them sail through; cross it by 90 degrees and every last one shatters into random static. That is the catch that makes quantum interception so unforgiving. Set the wrong angle and you do not merely miss the bit, you destroy the state outright and smear your fingerprints all over the survival rate. Walk the filter round the dial, watch the pass rate lock onto the cos² prediction, and mind you do not tip off their counter-espionage boffins. Chop chop!