Operation Xerox Paradox?

The no-cloning proof, in action

Mission 3: The No-Cloning Mystery. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Right, look alive, old sport. The sneaky blighters of the Echo Network have planted a rather vulgar "Blind Copier" right under the floorboards of the Bodleian Library to clone all our outgoing data. Naturally, we intend to let them.

Their machine does not measure anything, which is the clever part and also their undoing. It simply applies a copy operation and hopes. Feed it a plain classical dossier, a definite 0 or a definite 1, and it performs beautifully; that is what it was built and tested on, and the bench in front of you will confirm it if you set the copier to |0⟩,|1⟩ only.

So let those fragile quantum states drop straight into the enemy's slot. Their copier will process them without complaint, and that's what makes this so beautifully humiliating. It will produce two qubits, hand them proudly to counsel, and only under cross-examination will it emerge that the two are entangled with each other and neither is a copy of anything at all. No explosion, no alarm, just a machine confidently doing the wrong arithmetic in public.

Switch the bench to Universal and sweep the input around the sphere yourself. The best forgery physics permits scores 5/6 on fidelity, that is 83.3%, and not a whisker more, whatever state you feed it. Nature doesn't ban copying. She caps it, and the flatness of that ceiling is the whole case for the prosecution.