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In physics, physical information refers generally to the information that is contained in a physical system. Its usage in quantum mechanics (i.e. quantum information) is important, for example in the concept of quantum entanglement to describe effectively direct or causal relationships between apparently distinct or spatially separated particles. ''Information'' itself may be loosely defined as "that which can distinguish one thing from another". The information embodied by a thing can thus be said to be the identity of the particular thing itself, that is, all of its properties, all that makes it distinct from other (real or potential) things. It is a complete description of the thing, but in a sense that is divorced from any particular language. When clarifying the subject of information, care should be taken to distinguish between the following specific cases: * The phrase instance of information refers to the specific instantiation of information (identity, form, essence) that is associated with the being of ''a particular example'' of a thing. (This allows for the reference to separate instances of information that happen to share identical patterns.) * A holder of information is a variable or mutable instance that can have different forms at different times (or in different situations). * A piece of information is a particular fact about a thing's identity or properties, i.e., a portion of its instance. * A pattern of information (or ''form'') is the pattern or content of an instance or piece of information. Many separate pieces of information may share the same form. We can say that those pieces are ''perfectly correlated'' or say that they are ''copies'' of each other, as in copies of a book. * An embodiment of information is the thing whose essence is a given instance of information. * A representation of information is an encoding of some pattern of information within some other pattern or instance. * An interpretation of information is a decoding of a pattern of information as being a representation of another specific pattern or fact. * A subject of information is the thing that is identified or described by a given instance or piece of information. (Most generally, a thing that is a subject of information could be either abstract or concrete; either mathematical or physical.) * An amount of information is a quantification of ''how large'' a given instance, piece, or pattern of information is, or how much of a given system's information content (its instance) has a given attribute, such as being known or unknown. Amounts of information are most naturally characterized in logarithmic units. The above usages are clearly all conceptually distinct from each other. However, many people insist on overloading the word "information" (by itself) to denote (or connote) several of these concepts simultaneously. (Since this may lead to confusion, this article uses more detailed phrases, such as those shown in bold above, whenever the intended meaning is not made clear by the context.) ==Classical versus quantum information== The instance of information that is contained in a physical system is generally considered to specify that system's "true" ''state''. (A realist would assert that a physical system ''always'' has a true state of some sort—whether classical or quantum—even though, in many practical situations, the system's true state may be largely unknown.) When discussing the information that is contained in physical systems according to modern quantum physics, we must distinguish between classical information and quantum information. Quantum information specifies the complete quantum state vector (or equivalently, wavefunction) of a system, whereas classical information, roughly speaking, only picks out a definite (pure) quantum state if we are already given a prespecified set of distinguishable (orthogonal) quantum states to choose from; such a set forms a basis for the vector space of all the possible pure quantum states (see pure state). Quantum information could thus be expressed by providing (1) a choice of a basis such that the actual quantum state is equal to one of the basis vectors, together with (2) the classical information specifying which of these basis vectors is the actual one. (However, the quantum information by itself does not include a specification of the basis, indeed, an uncountable number of different bases will include any given state vector.) Note that the amount of classical information in a quantum system gives the maximum amount of information that can actually be measured and extracted from that quantum system for use by external classical (decoherent) systems, since only basis states are operationally distinguishable from each other. The impossibility of differentiating between non-orthogonal states is a fundamental principle of quantum mechanics, equivalent to Heisenberg's uncertainty principle. Because of its more general utility, the remainder of this article will deal primarily with classical information, although quantum information theory does also have some potential applications (quantum computing, quantum cryptography, quantum teleportation) that are currently being actively explored by both theorists and experimentalists.〔Michael A. Nielsen and Isaac L. Chuang, ''Quantum Computation and Quantum Information'', Cambridge University Press, 2000.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「physical information」の詳細全文を読む スポンサード リンク
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