Xerox or Zeroes?

Classical copying vs. quantum copying

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

Step lively, recruit: we are live at the High Quantum Court in Geneva, and the Echo Syndicate is moving aggressively to strike our hard-earned evidence from the record. You've been handed a highly sensitive stack of stolen dossiers, and the entire trial rests on your intellectual sorting speed. Standard classical paperwork is robust enough to handle duplication. Xerox it, hand it to the jury, and nobody can tell the copy from the original.

But for heaven's sake, do not hand the delicate quantum anomalies to their copier expecting a clean duplicate. The machine won't refuse, and it won't explode. That's the trap. It will run, cheerfully, and hand back two qubits that are welded to each other rather than two copies of anything.

The bench in front of you is that copier, wired open so counsel can watch it work. Set it to |0⟩,|1⟩ only and feed it the two states it was built for: perfect fidelity, flawless copies, no complaints. Now switch it to Universal and sweep the input state around the sphere. Watch the fidelity readout refuse to climb past 0.833, wherever you point it. That flat ceiling is the exhibit. Chop chop!