The Sifting Scandal

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

Put down the Pimm's, old sport, and pay attention to this cryptographic catastrophe unfolding on your terminal. We're sifting a quantum key stream out of the Clarendon Lab, and the whole business hangs on a step so mundane that people forget it is happening.

Drop the bench into Slow mode and take it one photon at a time. Alice picks a basis and a bit; Bob, who has been told nothing, picks a basis of his own. When the two bases happen to agree, Bob's bit is Alice's bit, guaranteed. When they disagree, Bob's result is a coin flip that means nothing to either of them.

So afterwards they get on an open line, with Eve entirely welcome to listen, and read out only which basis each of them used, never the bit. Every photon where the bases agree is kept. Every photon where they don't is thrown away. Watch the sent counter climb and the sifted key counter climb at almost exactly half the rate, because two people guessing independently between two bases agree half the time.

That is the scandal, such as it is: half your painstakingly transmitted key goes straight in the bin, in public, and the protocol is stronger for it. Now put the rate back up and let it run.