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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Quantum nondemolition measurement ： ウィキペディア英語版 Quantum nondemolition measurement Quantum nondemolition (QND) measurement is a special type of measurement of a quantum system in which the uncertainty of the measured observable does not increase from its measured value during the subsequent normal evolution of the system. This necessarily requires that the measurement process preserve the physical integrity of the measured system, and moreover places requirements on the relationship between the measured observable and the self-Hamiltonian of the system. In a sense, QND measurements are the "most classical" and least disturbing type of measurement in quantum mechanics. Most devices capable of detecting a single particle and measuring its position strongly modify the particle's state in the measurement process, e.g. photons are destroyed when striking a screen. Less dramatically, the measurement may simply perturb the particle in an unpredictable way; a second measurement, no matter how quickly after the first, is then not guaranteed to find the particle in the same location. Even for ideal, "first-kind" projective measurements in which the particle is in the measured eigenstate immediately after the measurement, the subsequent free evolution of the particle will cause uncertainty in position to quickly grow. In contrast, a ''momentum'' (rather than position) measurement of a free particle can be QND because the momentum distribution is preserved by the particle's self-Hamiltonian ''p''2/2''m''. Note that the term "nondemolition" does not imply that the wave function fails to collapse. QND measurements are extremely difficult to carry out experimentally. Much of the investigation into QND measurements was motivated by the desire to avoid the standard quantum limit in the experimental detection of gravitational waves. The general theory of QND measurements was laid out by Braginsky, Vorontsov, and Thorne〔 〕 following much theoretical work by Braginsky, Caves, Drever, Hollenhorts, Khalili, Sandberg, Thorne, Unruh, Vorontsov, and Zimmerman. ==Technical definition== Let $A$ be an observable for some system $\backslash mathcal$ with self-Hamiltonian $H\_$ is measured by an apparatus $\backslash mathcal$ which is coupled to $\backslash mathcal$ through interactions Hamiltonian $H\_\}$ evolves freely according to $H\_$ and $\backslash mathcal$ into the approximate form :$\backslash vert\; \backslash psi\; \backslash rangle\; \backslash approx\; \backslash sum\_i\; \backslash vert\; A\_i\; \backslash rangle\_\backslash mathcal\; \backslash vert\; R\_i\; \backslash rangle\_\backslash mathcal$ where $\backslash vert\; A\_i\; \backslash rangle\_\backslash mathcal$ are the eigenvectors of $A$ corresponding to the possible outcomes of the measurement, and $\backslash vert\; R\_i\; \backslash rangle\_\backslash mathcal$ are the corresponding states of the apparatus which record them. Allow time-dependence to denote the Heisenberg picture observables: :$A(t)\; =\; e^\}.$ A sequence of measurements of $A$ are said to be QND measurements if and only if〔 :$()\; =\; 0$ for any $t\_n$ and $t\_m$ when measurements are made. If this property holds for ''any'' choice of $t\_n$ and $t\_m$, then $A$ is said to be a ''continuous QND variable''. If this only holds for certain discrete times, then $A$ is said to be a ''strobescopic QND variable''. 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「Quantum nondemolition measurement」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.