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In quantum mechanics, the measurement problem is the problem of definite outcomes: quantum systems have superpositions but quantum measurements only give one definite result. [ 1 ] [ 2 ] The wave function in quantum mechanics evolves deterministically according to the Schrödinger equation as a linear superposition of different states.
The orthodox interpretation, or Copenhagen interpretation of quantum mechanics, posits a wave function collapse every time an observer performs a measurement. One thus faces the problem of defining what an “observer” and a “measurement” are. Another issue of quantum mechanics is that it forecasts superpositions of macroscopic objects ...
Quantum state tomography is a process by which, given a set of data representing the results of quantum measurements, a quantum state consistent with those measurement results is computed. [50] It is named by analogy with tomography , the reconstruction of three-dimensional images from slices taken through them, as in a CT scan .
In all collapse models, the noise effect must prevent quantum mechanical linearity and unitarity and thus cannot be described within quantum-mechanics. [ 21 ] : 423 Because the noise responsible for the collapse induces Brownian motion on each constituent of a physical system, energy is not conserved.
This difference is called the measurement problem of quantum mechanics. To predict measurement outcomes from quantum solutions, the orthodox interpretation of quantum theory postulates wave function collapse and uses the Born rule to compute the probable outcomes. [9]
This makes it possible to construct a local hidden-variable theory that reproduces the predictions of quantum mechanics, for which a few toy models have been proposed. [2] [3] [4] In addition to being deterministic, superdeterministic models also postulate correlations between the state that is measured and the measurement setting.
Incompatibility of quantum measurements is a crucial concept of quantum information, addressing whether two or more quantum measurements can be performed on a quantum system simultaneously. [1] It highlights the unique and non-classical behavior of quantum systems. [ 2 ]
Topics that deal with the problems and paradoxes of quantum measurement and wave function collapse, and, more generally, with the interpretation of quantum mechanics. Also, types of quantum measurements and "measurement theory".