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Quantum measurement is the process of obtaining classical information about a quantum system, radically changing its state. According to quantum mechanics, before measurement the system is in a superposition of possible values of a physical quantity; measurement irreversibly projects it onto one of these values. The probability of the outcome is determined by the squared modulus of the projection of the initial state onto the corresponding eigenstate. A key feature is that the result is probabilistic even with full knowledge of the wave function, and the act of measurement itself influences the system (observer effect).

History

In the 1920s, Werner Heisenberg and Niels Bohr formulated that measurement does not simply reveal pre-existing values but creates them. Erwin Schrödinger devised the paradox of a cat that is simultaneously alive and dead until the box is opened, to illustrate the strangeness of this rule.

How it works

Suppose we have a photon that is simultaneously polarized vertically and horizontally. Measuring polarization means passing it through a polarizing filter. If the photon passes, its polarization becomes strictly vertical, and any information about the horizontal component is lost. The outcome is random, but its probability can be precisely calculated.

💡 In the delayed-choice quantum eraser experiment, it appears that the decision of what to measure now can change the particle's behavior in the past.
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Erwin SchrödingerJohn Stewart BellAlain Aspect
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