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First quantization : ウィキペディア英語版
First quantization

A first quantization of a physical system is a semi-classical treatment of quantum mechanics, in which particles or physical objects are treated using quantum wave functions but the surrounding environment (for example a potential well or a bulk electromagnetic field or gravitational field) is treated classically. First quantization is appropriate for studying a single quantum-mechanical system being controlled by a laboratory apparatus that is itself large enough that classical mechanics is applicable to most of the apparatus.
==Theoretical background==

The starting point is the notion of quantum states and the observables of the system under consideration. Quantum theory postulates that all quantum states are represented by state vectors in a Hilbert space, and that all observables are represented by Hermitian operators acting on that space. Parallel state vectors represent the same physical state, and therefore one mostly deals with normalized state vectors. Any given Hermitan operator \hat has a number of eigenstates |\psi_\alpha\rangle that are left invariant by the action of the operator up to a real scale factor \alpha, i. e., \hat|\psi_\alpha\rangle=\alpha|\psi_\alpha\rangle. The scale factors are denoted the eigenvalues of the operator. It is a fundamental theorem of Hilbert space theory that the set of all eigenvectors of any given Hermitian operator forms a complete basis set of the Hilbert space.
In general the eigenstates |\psi_\alpha\rangle and |\psi_\beta\rangle of two different Hermitian operators \hat and \hat are not the same. By measurement of the type \hat the quantum state can be prepared to be in an eigenstate |\psi_\beta\rangle. This state can also be expressed as a superposition of eigenstates |\psi_\alpha\rangle as |\psi_\beta\rangle=\sum_\alpha|\psi_\alpha\rangle C_. If one measures the dynamical variable associated with the operator \hat in this state, one cannot in general predict the outcome with certainty. It is only described in probabilistic terms. The probability of having any given |\psi_\alpha\rangle as the outcome is given as the absolute square |C_|^2 of the associated expansion coefficient. This non-causal element of quantum theory is also known as the wave function collapse. However, between collapse events the time evolution of quantum states is perfectly deterministic.
The time evolution of a state vector |\psi (t)\rangle is governed by the central operator in quantum mechanics, the Hamiltonian \hat (the operator associated with the total energy of the system), through Schrödinger's equation:
i \hbar \frac|\psi (t)\rangle = \hat H |\psi (t)\rangle
Each state vector |\psi\rangle is associated with an adjoint state vector (|\psi\rangle)^\dagger = \langle \psi | and can form inner products, "bra(c)kets" \langle \psi |\phi\rangle between adjoint "bra" states \langle \psi| and "ket" states |\phi\rangle. The standard geometrical terminology is used; e.g. the norm squared of |\psi\rangle is given by \langle \psi |\psi\rangle and |\psi\rangle and |\phi\rangle are said to be orthogonal if \langle \psi |\phi\rangle = 0. If is an orthonormal basis of the Hilbert space, the above-mentioned expansion coefficient C_ is found forming inner products: C_=\langle \psi_\alpha |\psi_\beta\rangle. A further connection between the direct and the adjoint Hilbert space is given by the relation \langle \psi |\phi\rangle = \langle \phi |\psi\rangle^
*, which also leads to the definition of adjoint operators. For a given operator \hat the adjoint operator \hat^\dagger is defined by demanding \langle \psi |\hat|\phi\rangle = \langle \phi |\hat^\dagger|\psi\rangle^
* for any |\psi\rangle and |\phi\rangle.

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「First quantization」の詳細全文を読む



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