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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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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Could anyone help me understand this article on the Many Worlds Interpretation? [closed]

https://arxiv.org/pdf/2001.03771.pdf I am struggling to understand this paper because the depth in which the subject is discussed is much greater than my own. The topic is highly interesting to me, so ...
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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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Can sequential measurements on a particle yield different results?

Suppose the spin of a non-interacting quantum particle is measured "spin up" on the $z$-axis. After the measurement the wavefunction of the particle spreads out again and returns to a ...
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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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Faster than light in quantum mechanics? [duplicate]

Suppose there is a moving particle with a continuous and well spread wavefunction. Then it is surely possible for 2 separate measurements of position to have spacelike separation. Even though the ...
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Faster than light information paradox? [closed]

Suppose there is a moving particle with a continuous and well spread wavefunction. Then it is surely possible for 2 separate measurements of position to have spacelike separation. Even though the ...
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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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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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Checking my understanding of 'collapsing into eigenfunctions'?

Hello could someone please check if my understanding of 'collapsing into eigenfunctions' is correct? Say we have an observable, given by a linear self-adjoint operator $A$ and then we have $\{\psi_n\}...
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What time does the particle reach the screen in this thought experiment?

Suppose a particle with a gaussian wavefunction moves to begin with towards a position detector screen. How do we obtain the 'Time of arrival' distribution, when time can't be an observable? Should ...
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Order of measurement of two observables

Let $\hat A$ and $\hat B$ be two operators representing the observables $A$ and $B$, and let $\Psi(t)$ be the state of a quantum system. Let's suppose that we measure $A$ at $t_0$ and just after that ...
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Eigenstate of position operator after collapse of the wave-function [duplicate]

I am studying basic quantum mechanics in undergrad and have hit a wall. I understand that if a measurement is made for position, the wave function collapses into one of the eigenstates of position, i....
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Collapse of the wavefunction after measurement

I learned during my study that when we make a measurement of a wavefunction then the wavefunction will collapse to one of its eigenfunctions with an eigenvalue. It is also mentioned that when we ...
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What happens in the double slit experiment if the screen is far away?

I've started reading a book on Quantum mechanics and started thinking what would happen if we did the double slit experiment with electrons but we moved the screen far far away. My newborn thought is ...
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Collapse of the wave function in non-discrete systems

Let's consider a discrete quantum system such as the harmonic oscillator or the hydrogen atom. According to quantum mechanics, when a measurement of the energy of the system is made the wave function ...
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Identifying Collapse in Wave-function [duplicate]

Is there a machine or instrument which will notify us instantly when the wave-function has collapsed if we have access to only one of the entangled particles? Note that the wave function collapse will ...
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Is there a way to detect the collapse in wave function of entangled particles instantaneously? [closed]

Is there a machine or instrument which will notify us instantly when the wave-function has collapsed if we have access to only one of the entangled particles? Note that the wave function collapse will ...
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A wave or a particle?

Ever since I first read about the quantum eraser experiment, I was wondering: if after the "measurement" the wave function collapses and we can observe a particle - does that particle remain ...
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What are some of the explanations as to why the wave function of electrons collapses when interacting with an observer?

What are some of the explanations as to why the wave function of electrons collapses when interacting with an observer? Is there a specific reason why the wave function collapses, what are some of the ...
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Are coherent states destroyed by measurements?

For a quantum harmonic oscillator in a coherent superposition, what happens if the energy is measured? Would it collapse to an energy eigenstate (a single excitation) corresponding to the result of ...

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