What is the no-cloning theorem?
The rule that an unknown quantum state can't be copied.
You can copy a file as often as you like. You can’t do the same with an unknown qubit, because no sequence of operations takes a qubit and makes an exact independent copy of it while leaving the original intact. It was proved in 1982.
This is why quantum computers can’t simply back up their qubits, and why error correction has to work indirectly. It also makes some quantum communication secure: an eavesdropper can’t copy the qubits in transit without disturbing them.
Related
- MeasurementReading a qubit. It returns 0 or 1 at random, with chances set by the squared amplitudes, and ends the blend.
- Quantum error correctionMethods that protect quantum information by spreading it over many qubits and repeatedly checking for errors.
- EntanglementA link between qubits in which their measurement results are correlated in a way no prearranged plan could produce.