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The quantum Zeno effect (also known as the Turing paradox) is a situation in which an unstable particle, if observed continuously, will never decay.〔 〕 One can "freeze" the evolution of the system by measuring it frequently enough in its known initial state. The meaning of the term has since expanded, leading to a more technical definition in which time evolution can be suppressed not only by measurement: the quantum Zeno effect is the suppression of unitary time evolution caused by quantum decoherence in quantum systems provided by a variety of sources: measurement, interactions with the environment, stochastic fields, and so on.〔 〕 As an outgrowth of study of the quantum Zeno effect, it has become clear that applying a series of sufficiently strong and fast pulses with appropriate symmetry can also ''decouple'' a system from its decohering environment.〔 〕 The name comes from Zeno's arrow paradox which states that, since an arrow in flight is not seen to move during any single instant, it cannot possibly be moving at all.〔The idea depends on the ''instant of time'', a kind of freeze-motion idea that the arrow is "strobed" at each instant and is seemingly stationary, so how can it move in a succession of stationary events?〕 The comparison with Zeno's paradox is due to a 1977 paper by George Sudarshan and Baidyanath Misra.〔 The first rigorous and general derivation of this effect was presented in 1974 by Degasperis ''et al.'' 〔 〕 It had previously been described by Alan Turing in 1954:〔 〕 resulting in the earlier name ''Turing paradox''. The idea is contained in the early work by John von Neumann, sometimes called the ''reduction postulate''.〔 See also ); ; 〕 It was shown that the quantum Zeno effect of a single system is equivalent to the indetermination of the quantum state of a single system.〔 〕 According to the reduction postulate, each measurement causes the wavefunction to "collapse" to a pure eigenstate of the measurement basis. In the context of this effect, an "observation" can simply be the absorption of a particle, without an observer in any conventional sense. However, there is controversy over the interpretation of the effect, sometimes referred to as the "measurement problem" in traversing the interface between microscopic and macroscopic.〔 〕〔 〕 Another crucial problem related to the effect is strictly connected to the time-energy indeterminacy relation. If one wants to make the measurement process more and more frequent, one has to correspondingly decrease the time duration of the measurement itself. But the request that the measurement last only a very short time implies that the energy spread of the state on which reduction occurs becomes increasingly large. However, the deviations from the exponential decay law for small times, is crucially related to the inverse of the energy spread so that the region in which the deviations are appreciable shrinks when one makes the measurement process duration shorter and shorter. An explicit evaluation of these two competing requests shows that it is inappropriate, without taking into account this basic fact, to deal with the actual occurrence and emergence of Zeno's effect.〔 〕 Closely related (and sometimes not distinguished from the quantum Zeno effect) is the ''watchdog effect'', in which the time evolution of a system is affected by its continuous coupling to the environment. 〔 〕 〔 〕〔 〕 ==Description== Unstable quantum systems are predicted to exhibit a short time deviation from the exponential decay law.〔 〕〔 〕 This universal phenomenon has led to the prediction that frequent measurements during this nonexponential period could inhibit decay of the system, one form of the quantum Zeno effect. Subsequently, it was predicted that an ''enhancement'' of decay due to frequent measurements could be observed under somewhat more general conditions, leading to the so-called anti-Zeno effect.〔This definition is a paraphrase of the introductory remarks in Raizen ''et al.'' 's paper cited later in this article.〕 In quantum mechanics, the interaction mentioned is called "measurement" because its result can be interpreted in terms of classical mechanics. Frequent measurement prohibits the transition. It can be a transition of a particle from one half-space to another (which could be used for atomic mirror in an atomic nanoscope〔 〕) as in the time of arrival problem,〔 〕〔 〕 a transition of a photon in a waveguide from one mode to another, and it can be a transition of an atom from one quantum state to another. It can be a transition from the subspace without decoherent loss of a qubit to a state with a qubit lost in a quantum computer.〔 〕〔 〕 In this sense, for the qubit correction, it is sufficient to determine whether the decoherence has already occurred or not. All these can be considered as applications of the Zeno effect.〔 〕 By its nature, the effect appears only in systems with distinguishable quantum states, and hence is inapplicable to classical phenomena and macroscopic bodies. 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 Zeno effect」の詳細全文を読む スポンサード リンク
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