Decoherence, einselection, and the quantum origins of the classical
Wojciech H. Zurek
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Every machine on this site that does something useful with quantum phase has to answer the same question first: why does phase survive here and not there. This review is the standard answer. Zurek's argument is that a quantum system is never really alone, that its environment is continuously measuring some of its properties whether anyone intends it or not, and that this monitoring destroys interference between exactly the states the environment can tell apart. What is left is a small, stable family of states, which Zurek calls pointer states, selected by their resilience the way a species is selected by fitness. He gives that selection a name, environment-induced superselection, and shows it is not the same thing as friction or noise: relaxation is the environment pushing the system about, while decoherence is the system leaving an imprint on the environment. The review also works through how big and how fast the effect is, why chaotic macroscopic bodies would otherwise be smeared across their own orbits, and how the redundancy of records in the environment is what makes a fact look objective.
Pourquoi cela compte iciThe coherence engineering this site teaches, from superconductors to the coherent matter-wave beam, lives or dies on how long a phase relationship survives contact with the world. This is the review that turns that into a calculable quantity rather than a worry, and it supplies the vocabulary, pointer states and environment-induced superselection, that the quantum-phase material on the site uses. It also makes the constructive point behind every dilution refrigerator: decoherence is caused by what the system tells the environment, so coherence is engineered by controlling what the system is allowed to say.
Ce qu'il affirme
01Decoherence and einselection are two views of one process. Decoherence is the destruction of coherence between the states the environment can distinguish; einselection is its consequence, the effective exclusion of all but a small set of pointer states from a much larger Hilbert space. Pointer states are distinguished by resilience: they survive monitoring, and the review describes this in explicitly Darwinian terms, with the pointer states as the fit ones that leave descendants.Section I B, Decoherence and einselection
Settled physics02Decoherence is not relaxation and not classical noise, and the review states the distinction sharply: relaxation and noise are caused by the environment perturbing the system, whereas decoherence and einselection are caused by the system perturbing the environment. The effect has no classical analogue, because only in quantum mechanics does acquiring information inevitably alter the state.Section I B, Decoherence and einselection
Settled physics03The decoherence rate for a superposition of two positions goes as the relaxation rate multiplied by the square of the separation divided by the thermal de Broglie wavelength. For a one-gram mass at room temperature separated by one centimetre this predicts decoherence about ten to the fortieth times faster than relaxation, and even the cosmic microwave background is enough to decohere objects as small as dust grains.Section V, equation 5.38 and the paragraph following it
Settled physics04The review is careful about the limits of that estimate: the simple high-temperature master equation it comes from is often not valid, and the decoherence rate cannot be faster than the inverse of the spectral cutoff of the environment.Section V, paragraph following equation 5.38
Settled physics05A worked macroscopic case shows why the question cannot be confined to the microscopic. Hyperion, the chaotically tumbling moon of Saturn, has an orbital period of 21 days and a Lyapunov time of about 42 days, and a generous estimate of its action divided by Planck's constant is ten to the seventy-seventh. Because only the logarithm of that number enters, isolated unitary evolution would put Hyperion into a superposition of orientations differing by a full turn after about twenty years.Section III, equations 3.6 to 3.8
Settled physics06What to watch, in the review's own assessment: decoherence and einselection are the only account of classicality that requires no modification of quantum theory, and yet the review rejects the label new orthodoxy, noting that many conceptual and technical questions remain open, including what counts as a system, and that the explanation had with very few exceptions not reached the textbooks.Section IX, Concluding remarks
What to watch
La porte d'entrée
https://doi.org/10.1103/RevModPhys.75.715SOURCE READ IN FULL, TEXT NOT REPRODUCED. The full fifty-three-page review was read on 2026-09-11 from the arXiv record quant-ph/0105127, version 3, which is the version carrying the Reviews of Modern Physics 75, 715 journal reference. The arXiv record declares no open licence: what it carries is arXiv's own non-exclusive licence to distribute, which permits arXiv to serve the file and grants a reader nothing further, so it is not a grant to republish and no sentence of the review is carried here. The summary and the claims below are written in this site's own words from the review read in full, and each locator names the section or the numbered equation the statement comes from. The one place where the review's own coined term is unavoidable is einselection, which is its name for the phenomenon and is used here as a name.
Comment le citer
Wojciech H. Zurek (2003) Decoherence, einselection, and the quantum origins of the classical. doi:10.1103/RevModPhys.75.715
Sa place dans le programme
Le vide comme fluide quantiqueContrôle de la gravité et supraconducteurs