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Physics
Quantum Physics
Definition and Rules of Quantum State Observation
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[QUOTE="Strilanc, post: 6008649, member: 96470"] "Measurement" is a high-level abstraction on top of decoherence, which is a continuous phenomena where information about the system gradually makes its way into the environment. When you impose a measurement onto a system, you are attempting to force [I]specific[/I] information to leak out quickly. If multiple parties are all trying to impose different measurements, the most likely outcome is that they all trip over each others' toes. Pragmatically, this would be due to obvious stuff like one party wanting to put a beamsplitter before the photon counter while another party wants there to be no beamsplitter. More abstractly, you could suppose each party is able to add dynamics into the system and the total dynamics is the sum of all the contributions (i.e. they each specify a Hamiltonian and you just add up the Hamiltonians). The resulting process would be well-defined, but probably wouldn't do what anyone wanted. For example, if you take a Hamiltonian for measuring horizontal-vs-vertical polarization and just add it into a Hamiltonian for measuring clockwise-vs-counterclock polarization, you don't get a Hamiltonian that causes both to be measured simultaneously. You get something that measures upleft-vs-upright polarization (or other stuff; it depends on the details). [/QUOTE]
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Definition and Rules of Quantum State Observation
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