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Does wave function collapse have a speed





yagnyavalkya
What is the speed at which wave function collapses
Is it instantaneous or equal to C
kelseymh
yagnyavalkya wrote:
What is the speed at which wave function collapses
Is it instantaneous or equal to C


"Wave function collapse" is a somewhat antiquated term which doesn't properly reflect our understanding of quantum processes.

The underlying process is called decoherence, and involves a series of interactions of an "isolated" quantum system with the larger environment. The end result of these random interactions is a randomization of phase information in the quantum system, driving it toward classical (additive probabilities) behaviour.

The rate of decoherence was first measured in 1996 using rubidium atoms in a superposition state, and observing how long it took for the superposition to "disappear" (i.e., for the two atoms to become independently classical-looking).
PatrickLeung
Suppose that a quantum pure state is prepared, then upon measurement of that state, the "wave function collapses", which means the quantum state turns into a definite classical state. The speed of the "collapse" is then related to how quickly the quantum information turns into classical information. Using a sequence of weak quantum measurements, the state can take a long time to collapse. The speed should be related to how much information is extracted during a sequence of periods of time of measurements.
kelseymh
PatrickLeung wrote:
Suppose that a quantum pure state is prepared, then upon measurement of that state, the "wave function collapses", which means the quantum state turns into a definite classical state. The speed of the "collapse" is then related to how quickly the quantum information turns into classical information. Using a sequence of weak quantum measurements, the state can take a long time to collapse. The speed should be related to how much information is extracted during a sequence of periods of time of measurements.


You're talking about the quantum "Zeno effect." You are quite right that by properly intervening in a quantum system, a normally short-lived state can be made to persist for quite a long (even macroscopic) time.

However, the original poster was asking about "natural" state collapse. That is, if a system is prepared in a pure quantum state without a classical analogue (such as a superposition of macroscopically separated spatial states, or a superposition of spin up and spin down), is there a measurable or inferrable time delay when a measurement triggers the state to select a classically observable eigenstate?

The 1996 experiment I cited in my previous comment was an attempt to address that question. By preparing a superposition state, and then measuring the full denisty matrix at different time slices after preparation, the experimenters were able to map the temporal evolution of the system from a "quantum" to a "classical" form, driven by environmental decoherence.
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