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.

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How is the wavefunction collapse related to the application of that operator on a state?

I am following Griffith's Introduction to Quantum Mechanics, and after proving that $$\sigma _A ^2 \sigma _B ^2 \geq \left( \frac{1}{2i} \langle [A,B]\rangle \right)$$ he says You can certainly ...
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Has there been experimental evidence that entangled state collapses simultaneously?

If we observe the observable of one particle among two particles which are entangled each other and separated far away, we can determine the value of the other particle’s same observable from the ...
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Position of the wavefunction's collapse

When a wave function is said to "collapse" to a single point during a measurement, is there uncertainty about the point's position or is it known infinitely precisely?
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Can Einstein's action at a distance be action into the past?

This youtube video is about how it is not measurable or of any consequence what the one way speed of light is, only that a two way trip has an average speed of $c$. An example is moving ...
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Whose state (or wavefunction) changes (or collapses) upon observation: System's or Observer's?

I was learning about Qubit measurement and the basics of Quantum computing. The instructor forced a lot on the following statement : Assume an isolated system S. Let an observer O interact with the ...
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What would one see, if they looked at a system in energy eigenstate?

We know, time evolution of energy eigenstates is extremely simple $\psi(t)=\psi(0)e^{-iEt}$. On the other hand we have symmetry breaking. Here I present an interesting thought experiment, and wonder ...
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What problem is the Many-Worlds Interpretation actually solving? Is it a reframing of the measurement problem?

Before I state my question I want to say I am in no way an expert/professional in this field. I read quite a bit on the subject and I consider myself familiar with the basic concepts but I really want ...
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Testing objective vs. subjective collapse for double slit

Suppose we have double-slit experiment. Before the right slit, we add a particle detector that allows the particle to go through. It is well-known that if we carry this out, then the wave ...
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Do all wavefunction collapses have to be evident in some way?

All wavefunction collapses are events which reveal some kind of classical information or thermodynamically visible 'event'. Is this true? In that case, what is the evidence of a 'not' collapse. A 'not'...
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When does a collapse occur?

For example, in the double slit experiment, we shoot a single photon at two slits and see where it hits on a screen. We measure the time interval between when we sent the signal to emit the photon, ...
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How is the collapse of the wave function explained through electron atom scattering?

I have learned from this answer/comments that people have put much effort into understanding how scattering of electrons on atoms could explain where, for example on a screen behind a double-slit ...
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Does the collapse of the wave function happen immediately everywhere?

It is usually taught that when we measure some measurable value the wave function collapses immediately everywhere. This idea sounds like a simplification of some more complicated mechanism. Are ...
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How sharply does the wavefunction collapse upon measuring position? [duplicate]

It cannot be a Dirac delta as this is not a continuous normalisable function. Further to the answer written by John Rennie How does wave function collapse when I measure position? We may assume that ...
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Measurement accuracy in quantum mechanics?

Here's some classical background to this thought experiment. Let us first consider a classical scale ruler (called A) of a continuous classical variable. Rulers must be discretised. Let's compare this ...
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What state does a system collapse to after measuring a degenerate eigenvalue?

$\newcommand{\ket}[1]{|#1\rangle}$ Let $\hat A$ be some observable, and $\ket n$ and $\ket m$ two degenerate eigenstates with eigenvalue $a$, such that $$\hat A \ket n=a\ket n,$$ $$\hat A \ket m=a\ket ...
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Is there an equivalence between the creation of a wavefunction and the collapse of a wavefunction?

I am thinking here of how creation vs. absorption compare for a photon. A single photon may be emitted when an electron in an atom returns down to the ground state from an excited state. In a ...
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Wave function and speed of light

When a photon is generated, it travels at the speed of c in the form of propagating electromagnetic wave until the photon interacts with something else to have its energy absorbed or converted. Is ...
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Why does the wavefunction of a particle spread out after a measurement?

Quantum mechanics states that the wave packet of a particle "spreads-out" in position again after a measurement on this particle has been made. Is this spreading or "dispersion" ...
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Is there a limit of wave function splitting?

It is possible to split the wave function (for example using a semi mirror like in Wheeler delayed choice experiment). I wonder what happens if we split it many times. My "though experiment" ...
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Interpretation of measurements and Eigenstates for continuous variables [duplicate]

I find myself (probably like many others) somewhat unclear on the implications of the postulate of quantum mechanics that "measurements of a value leave the system in an eigenstate". The ...
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Relating the Minev et al experiment to Brun QTT survey

I have been studying from many angles the groundbreaking/now renowned Minev et al experiment: To catch and reverse a quantum jump mid-flight [1]. I have many questions on this complex/intricate setup ...
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How precisely can particle position be measured in a laboratory?

If we have any given particle, such as a photon or an electron (it doesn't really matter what for the sake of the question), how precisely can modern physics devices measure their position? ...
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Is the recent Nature paper by Minev et al. evidence of new physics?

Ok the title might be slightly clickbaity. A better one might be "Why isn't the recent Nature paper by Minev et al. evidence of new physics?", but I didn't want to make too many assumptions ...
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Is there any prevalent philosophy regarding how many “initial worlds” exist in Everettian style “Many Worlds“ theories?

If I understand the general concept correctly, in the context of the Everettian “Many-Worlds interpretation” of QM, as opposed to seeing the wavefunction of some quantum process collapse and result in ...
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Why is decoherence considered to be a 'trick' used for practical purposes?

As I understand, decoherence consists of making the observation that when we trace out the environment from the overall density matrix of the system, we get von Neumann's projection postulate state ...
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Possibility of staying within nucleus for a electron according to Uncertainty Principle

Academic Problem: According to Uncertainty Principle, show that electron can't stay within nucleus. That's a general problem indeed. Anyway, we know that uncertainty of position in this case can't ...
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Theory and practice of repeated measurements of nuclear spin

Could someone please confirm or correct my understanding of the below scenario? It is possible to isolate a single beryllium ion in a Penning trap. It is possible to repeatedly measure the spin of ...
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What happens to phase after wavefunction collapse?

Suppose an initial quantum state $\psi = a_1\phi_1 + a_2\phi_2 + ... + a_n\phi_n$, where $\phi_i$ is the eigenfunction with eigenvalue $\lambda_i$ of some measurement operator. Post-measurement, we ...
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Can Bell's Inequality test results derived from the Bell state wavefunction based on QM?

Suppose that I have two beams of entangled photons described by the Bell state wavefunction $$|\Psi\rangle=\frac{|HH\rangle+|VV\rangle}{\sqrt2}.$$ I direct each entangled beam through a $45^\circ$ ...
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Is the Copenhagen interpretation still the most widely accepted position?

In my undergraduate Quantum Mechanics textbook (Griffiths), of the Copenhagen interpretation it says "Among physicists it has always been the most widely accepted position". I'm currently ...
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Is there anything analogous to wavefunction collapse in Quantum Field theory? [duplicate]

I know that QFT contains quantum mechanics, but the fields do not have a similar probabilistic interpretation as the wavefunction in Quantum mechanics. From my not-so-deep knowledge of QFT, I expext a ...
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How does wave function collapsing fit into QFT?

I followed a basic course on QFT and know some basic concepts but I focussed mainly on calculations and understanding the Feynman rules e.g.. However, sometimes I have the idea that there a quite some ...
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Why is wave function collapse mysterious?

There are lots of questions and answers on this site about wave function collapse (for example, How does a Wavefunction collapse?, Why does a wavefunction collapse when observation takes place?, How ...
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Why are only some eigenstates observed upon collapse?

If we have a system in a superposition of states, we expect that observation would collapse the wave-function to one state. But many eigenstates can be expressed as a superposition of other ...
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A thought experiment in Quantum Mechanics

Suppose you have a particle in deep space, and two observers, such that none of these particles interact with the other two unless either observer decide to. Then if one observer makes a measurement ...
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Is information conserved in quantum mechanics (after wave function collapse)?

I have heard in popular science that there is a law of "conservation of information." Some times this is described as: for any event that happens, there is enough information to reconstruct ...
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Measuring a superposition state of identical particles

Suppose that the state of a system of two identcial particles (say, two photons) is given by the following: $$\frac{1}{\sqrt{}2}(|\psi_{1}\rangle|\psi_{2}\rangle + |\psi_{2}\rangle|\psi_{1}\rangle) \, ...
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Why does particle measurement cause quantum wavefunctions to collapse

When we attempt to measure a certain property of a particle, how and why does its wave function collapse? I've tried to find answers on my own, but they've been far too complicated for me to ...
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Detecting collapse using single slit

A collapsed state should act like a particle and a non-collapsed like a wave. Is this statement true? So if you had a single slit and detectors behind it off to the sides... The collapsed state should ...
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1answer
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Does an entangled pair remain entangled after the first measurement?

Consider an entangled pair described by the wavefunction $$\lvert1,0\rangle = \frac{1}{\sqrt{2}}(\lvert\uparrow_1\downarrow_2\rangle-\lvert\downarrow_1\uparrow_2\rangle)$$ in in the $S_z$-basis. If ...
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Measuring momentum

I'm new here, possibly my apologies for misplacing. After a measurement in quantum mechanics, the wavefunction collapses to an eigenstate corresponding to the outcome of the measurement. Thus if we ...
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Given a collapsed state, can we derive the prior shape of the wavefunction?

More or less the title. Assume that we have found a box containing a completely isolated system of particles. We do not know for how long this system has been allowed to evolve. We do know what ...
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Why “diagonal” polarization doesn't decohere?

It is often said, that qubit should be maintained under very low temperature and kept highly isolated not to decohere. But we can represent $|0\rangle$ with vertical polarization of light and $|1\...
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Wigner's friend experiment - How is there an apparent paradox?

Wigner's friend thought experiment mentions that there is an apparent paradox - 'when exactly did the collapse occur?'. Whether it occurred when the friend made the measurement, or when Wigner asked ...
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Are combined fermion wavefunctions still antisymmetric after wavefunction collapse?

If we have two electrons in a state $|\psi\rangle=\frac{1}{\sqrt2}[|\uparrow\downarrow\rangle+|\downarrow\uparrow\rangle]$ and we measure the spin of the first electron to be up, does the wavefunction ...
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Is a brain neuron the only physical object in existence which cannot be in quantum superposition (firing / not firing)? [closed]

Is a brain neuron (actually even only my own neuron, not anybody else's) the only physical object in existence which cannot be in superposition (firing / not firing)? We never experience any ...
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Is there an experiment that verifies the quantum state collapse is a physical process or a process of reality?

Is there an experiment that verifies the quantum state collapse is a physical process or a process of reality? Or the quantum state collapse is just interpretation, but is not a physical process?
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Has it been practically proven that quantum superposition exists ? If yes, how does it even work?

I was wondering if quantum particles do actually exists in two different states simultaneously and if it has been proven they do indeed exists in a superposition of states. How has it been figured ...
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How do we prepare the electron state $|s=1/2,s_z=+1/2\rangle$ in the laboratory?

If we are interested in preparing the electron state $|+\rangle=|s=1/2,s_z=+1/2\rangle$ in the laboratory, the obvious thing is to apply a magnetic field along the positive $z$-axis (${\vec B}=B\hat{z}...
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Are humans special in that they collapse wave functions?

First of all, I don't really believe that humans are special. So I know the answer must be that they are not. But the way quantum mechanics is described is that all particles exist as clouds when ...

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