One of the most significant developments in physics during the past 20 years is the clear and convincing proof that macroscopic systems, such as a superconductor with billions of electrons in it, can behave quantum mechanically. This conclusion changes the view, which was established in the early days of quantum mechanics, that there is a fundamental difference between the microscopic systems that obey quantum mechanics and the macroscopic world of classical physics. This has important implications for the conceptual foundations of quantum theory, in particular quantum measurement theory, which is frequently described in terms of the separation of the microscopic object and the macroscopic measuring device.
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