The no-cloning theorem is a fundamental principle in quantum mechanics that states it is impossible to create an identical copy (or "clone") of an arbitrary unknown quantum state. This theorem is significant because it highlights a key difference between classical information and quantum information. In classical physics, if you have a piece of information, you can make copies of it easily.
Articles by others on the same topic
No deterministic quantum operation can make two perfect copies of every unknown pure state. For a proposed unitary operator acting with a fixed blank and environment, preservation of inner products gives , where is the input overlap and is the overlap of final environment states. Nonorthogonal distinct states have , contradicting this equality. The no-cloning theorem for two pure states is its elementary two-state form.