Books of quantum mechanics are very ambigous in this aspect. A classical apparatus that collapse wavefunction of a quantum system must "quasieliminate" the non-diagonal elements of density matrix, or have a quasiclassical wavefunction of the form(adsbygoogle = window.adsbygoogle || []).push({});

[itex]ae^{iS\hbar^{-1}}[/itex] where a is a real slowly changing in space and time and S is the classical action in the trayectory between 2 points of the wavefunction. But ¿ how much slow this change of a, and rapid oscillating S ?. There is qualitative but no quantitative answers in both forms of watch a classical system, in particular a measurement apparatus

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# What grade of decoherence to be a classical aparatus

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