The Port Meadow Purloining

How wiretapping a data line actually works

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

Listen up, recruit. We're stepping directly into the dastardly loafers of the Echo Network to see exactly how they attempt to breach our secure dark fibre line running beneath Port Meadow.

Here is the entire technique, and it is embarrassingly simple. A classical line carries a voltage. To read it, you do not have to take the voltage away. You skim a fraction of it, amplify what you skimmed, and let the rest travel on. The original signal arrives at the far end at very slightly reduced power, which is indistinguishable from a slightly longer cable, a slightly colder night, or a slightly older connector. There is no missing bit. There is no altered bit. There is nothing for Bob to find, because nothing happened to his data.

That is the whole of it. No exotic hardware, no clever mathematics, no risk. Classical information can be copied perfectly and silently, and the copy is as good as the original, which means the Echo Network can sit on our line for a decade and the only way we would ever know is if someone eventually used what they stole.

Hold onto that, because it is the exact thing that stops being true the moment we start sending qubits. Chop chop!