# Questions tagged [wavefunction-collapse]

Wavefunction collapse amounts to the apparent reduction of a wavefunction consisting of a superposition of several eigenstates to a single eigenstate (by "observation"). It underlies measurement in quantum mechanics and connects the wave function with classical observables, in a thermodynamically irreversible interaction with a classical environment, normally disfavoring future QM interference.

261 questions
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### What happens when we measure the particle *before* the double slit?

Like the titles states, if we measure the particle before the double slit, like right after it gets shot out, do we see an interference pattern or clump pattern on the screen after the slits?
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### Does measuring operator $A$ on state $|\psi\rangle$ leave the system in $A |\psi\rangle$?

Given an observable $A$ and a system in the state represented by the vector $|\psi \rangle$, after the measurement is the system in the state $A |\psi \rangle$?
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### Measurement of position of particle

Say a particle was found to be at $C$. If I measure the position of particle immediately, its given in Griffith's book that it will still be at $C$. In the macroscopic world, isnt this true only if ...
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### Does collapse of wave function to a momentum eigenstate violate speed of light restriction?

Let's consider a free system where the Hamiltonian is $\hat{p}^2/2m$. At time $t=0$, we start with a state at position $x$. An instantaneous time $\delta t$ later, where $\delta t\rightarrow 0$, we ...
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### Collapse of wave function

Suppose a quantum system is initially at a state $\psi_0$ and that a measurement of an observable $f$ is performed. Immediately after the measurement, the system will be in a state that is an ...
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### Can you detect when wave function collapse occurs?

Not literally when as in the moment it occurs, but if you're given a series of particles can you differentiate between the particles which have been collapsed and those that have not? If you have a ...
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### Does a Polarizing Beam Splitter Cause Wave Function Collapse?

For a single photon, its polarization can be a superposition of two orthogonal bases (e.g. horizontal or vertical). However, as I understand, once it has been measured, it must collapse into one of ...
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### Collapse of Wavefunction after space-like Seperated Measurements

Suppose I have a pair of entangled particles, each one goes in a different direction. Now I measure the momentum of one particle and the position of the other at the same time. Because the ...
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### Double slit experiment; evidence of wavefunction collapse

This video shows the change of a photon's interference pattern in real time of the Young's single and double slit experiment. In this video it is claimed that by adding a detector to view how the ...
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### Is wavefunction collapse “global”?

I have the feeling that the premises of this question may not be coherent (so to speak), but here goes: Suppose we have a system $X$ in a quantum superposition between states $0$ and $1$, say, with ...
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### Why superpositions? [closed]

I've seen a lot of stuff on superpositions, namely the double slit experiment. And every video I watch, it tells me the same thing: It's amazing that when these particles are being observed they ...
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### What the wave function looks of a particle in the infinite square well looks like after collapse for measurements of position and energy

Consider a particle in a the infinite square well from x=0 to x=L. At t=to, I make a measurement of position and get x=L/2. What is the resulting wave function at t=to? My understanding, from reading, ...
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### Is a photon always in a state of superposition while traveling through space?

In the double-slit experiment, we emit a photon that is in a state of superposition (wave form) which travels through both slits to interfere with itself. When we measure which slit it went through, ...
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### Has this experiment really demonstrated wave-function collapse?

My question is: why did the following experiment claim that it had demonstrated the wave-function collapse? Experimental proof of nonlocal wavefunction collapse for a single particle using homodyne ...
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### The nature of measurement

Does the measurement of the particle change it's physical state? Or does it only seem to do that? Ex. if a particle was measured before the slits, would we see an interference pattern, or a particle ...
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Two observers – A & B - conduct a single double slit experiment and watch the same detector screen for the appearance of an interference pattern. A separate detector records which slit each ...
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### Time reversal on superposition: I think [duplicate]

Imagine I have a box, and in it, I have a photon in a superposition of state |1> and |0>. I look into the box and register that the photon is in state |1>. Now, if I have ALL information in the ...