  Consider a situation in which a quantum system is secretly prepared in a
state chosen from the known set of states. We present a principle that gives a
definite distinction between the operations that preserve the states of the
system and those that disturb the states. The principle is derived by
alternately applying a fundamental property of classical signals and a
fundamental property of quantum ones. The principle can be cast into a simple
form by using a decomposition of the relevant Hilbert space, which is uniquely
determined by the set of possible states. The decomposition implies the
classification of the degrees of freedom of the system into three parts
depending on how they store the information on the initially chosen state: one
storing it classically, one storing it nonclassically, and the other one
storing no information. Then the principle states that the nonclassical part is
inaccessible and the classical part is read-only if we are to preserve the
state of the system. From this principle, many types of no-cloning,
no-broadcasting, and no-imprinting conditions can easily be derived in general
forms including mixed states. It also gives a unified view on how various
schemes of quantum cryptography work. The principle helps to derive optimum
amount of resources (bits, qubits, and ebits) required in data compression or
in quantum teleportation of mixed-state ensembles.
