Consider a situation in which a quantum system is secretly prepared in a state chosen from the known set of n states. We present a principle that gives a definite distinction between the operations that preserve the marginal states of the system and those that do not. Many types of no-cloning and no-imprinting conditions for mixed states and entangled states can easily be derived from this principle. The principle also gives a unified view on how various schemes of quantum cryptography work.

Topics
Quantum mechanical systems and processes, Quantum information, Cloning
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© 2001 American Institute of Physics.
2001
American Institute of Physics
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