This question states that measuring the spin of an entangled particle causes the collapse of the wavefunction and thus the entanglement is broken.

Then this question states that we don't know what exactly is the cause for the collapse of wavefunctions.

However, what processes are known to collapse the wavefunction, and specifically break entanglement?

So measuring the spin collapses the wavefunction. What else does?

  • Chemical processes?
  • Presence in magnetic field without a screen (similar to Stern-Gerlach experiment)?
  • Irradiation of some form?
  • Heating?


Given comments that the collapse of the wavefunction is still not understood, I'd like to emphasize experimental observations.
Also, given that the collapse of a wavefunction may or may not be an artificial construct, can we focus on what processes have been observed to destroy entanglement?

(From what I understand of current theory, entanglement is only broken by the resolution/collapse of the wavefunction, thus asking "what has been observed to collapse the wavefunction" and "what has been observed to break entanglement" should be questions with the same answers.)

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    $\begingroup$ Any interaction that leads to the entangled system to get entangled with yet more degrees of freedom. Then if you only consider the original degrees of freedom, the system has to be described using a density matrix, it then looks like as if the wavefunction has collapsed breaking the original entanglements. $\endgroup$ – Count Iblis Mar 20 '17 at 22:10
  • $\begingroup$ This is not a question about entanglement; you could ask exactly the same questions about an unentangled state. $\endgroup$ – WillO Mar 20 '17 at 22:12
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    $\begingroup$ Interpretations of quantum mechanics disagree whether collapse of the wave function is a physical process or an artifact of theoretical description. If it is an artifact nothing can "cause" it because it is only a manner of speaking. What is known is what can "prepare" a system to undergoing collapse (whatever it is or isn't), any interaction with environment that causes decoherence. "Environment" can be any system with many degrees of freedom, i.e. "classical". $\endgroup$ – Conifold Mar 20 '17 at 23:15
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    $\begingroup$ As the Wikipedia article on the Measurement Problem makes clear, this is an open problem in quantum mechanics - and indeed this question is pretty much equivalent to the measurement problem. $\endgroup$ – Emilio Pisanty Mar 26 '17 at 20:55

Since you already talk about Stern-Gerlach, I suspect that the focus of your question is more about at which point in existing experimental techniques the collapse occurs, and not about learning existing techniques. In Stern-Gerlach that would be the deflection, not the screen, because this is where the spin value gets to be determined. If I got the question right, then the general answer is "at the point in the experiment where the studied property gets a specific value and superposition ends".

Also: Measuring is interacting with the system under study. There is nothing special putting measurement apart from any other physical process. This means that any interaction of the original system with anything else in the universe will break the wavefunction, preparing a new state.

I think the most concise (and entertaining) answer was the first comment (source) in the first of your links:

Basically, for observations to happen, there has to be an interaction between particles, or as the post put it less/more(?) eloquently, whenever a physicist says "observe", mentally replace it with "hit with sh*t.

  • $\begingroup$ Ok, tell me if I have this right. Superposition is only possible because many particles don't "get hit with sh*t" for a relatively long time, and as soon as they do, their wavefunctions collapse into something pretty finite. And then this wavefunction spreads out again as time goes by I assume? $\endgroup$ – Roman Mar 30 '17 at 7:15
  • $\begingroup$ I think this is a good description of what is usually believed to happen. (Except maybe for the part "superposition is only possible because"; superposition is believed to be a very inherent thing, not something happening due to the absence of interactions.) $\endgroup$ – Helen Mar 30 '17 at 14:38
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    $\begingroup$ As a new member I can't yet add comments to others, so please allow me to write here about alanf's answer: The many worlds interpretation is a highly subjective interpretation of QM. Some caution should be taken that statements like "the wavefunction doesn't collapse" are not made in a way that could make anyone assume that they are considered as belonging to the body of scientific knowledge. $\endgroup$ – Helen Mar 30 '17 at 14:44
  • $\begingroup$ This answer illustrates what David Ellerman calls the "separation fallacy" (arxiv.org/abs/1112.4522): "The separation fallacy mistakes the creation of a tagged or entangled superposition for a measurement. Thus it treats the particle as if it had already been projected or collapsed to an eigenstate at the separation apparatus rather than at the later detectors. But if the detectors were suddenly removed while the particle was in the apparatus, then the superposition would continue to evolve and have distinctive effects (e.g., interference patterns in the two-slit experiment)." $\endgroup$ – Stéphane Rollandin Apr 12 '17 at 22:58
  • $\begingroup$ @StéphaneRollandin: I'm sorry but I don't see how the answer fits the quote. Do you mean that the collapse doesn't happen upon the deflection? (In any case the answer clearly separates the creation of entanglement from the measurement, so I earnestly don't get the meaning of the comment.) $\endgroup$ – Helen Apr 13 '17 at 4:20

The wavefunction doesn't collapse. Rather, when you do a measurement, each of the possible outcomes happen, but they are dynamically isolated from one another by decoherence and act approximately like non-interacting versions of the same object - this is commonly called the many worlds interpretation of quantum mechanics and treated as some controversial optional extra, but it is just a consequence of taking the equations of motion of quantum mechanics seriously as a description of how the world works.

Entanglement itself involves system 1 having information about system 2 that can't be accessed except by direct interaction or comparing results of measurements on them (locally inaccessible information):



If system 2 interacts with some other system, system 3, then the locally inaccessible information is now in the joint system of system 2 and system 3. As such, system 2 alone no longer contains the information required to do entanglement type stuff. And if system 3 is the environment, then in practice it is impossible to get that locally inaccessible information back. So systems 1 and 2 are effectively disentangled.

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    $\begingroup$ I tend to agree with this interpretation, but it is an interpretation, and reasonable and knowledgeable people can and do disagree with it. It would be a misrepresentation to claim that the material in this answer is uncontroversial. $\endgroup$ – Emilio Pisanty Mar 30 '17 at 8:54
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    $\begingroup$ How do you get the Born rule? $\endgroup$ – innisfree Mar 30 '17 at 9:28
  • $\begingroup$ @innisfree See arxiv.org/abs/quant-ph/0303050, arxiv.org/abs/1508.02048 and references therein. $\endgroup$ – alanf Mar 30 '17 at 10:37
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    $\begingroup$ The many worlds interpretation is a highly subjective interpretation of QM. Some caution should be taken that statements like "the wavefunction doesn't collapse" are not made in a way that could make anyone assume that they are considered as belonging to the body of scientific knowledge. -Helen (I took the liberty to copy paste the comment) $\endgroup$ – Roman Mar 30 '17 at 16:42
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    $\begingroup$ @alanf All of your statements above are subjective. MWI has not come forward with proposed experiments to distinguish it from other alternatives and Occam's razor cuts it like there's no tomorrow. I didn't even start speaking about the "rationality" of an infinity of actual universes being spun every single moment. $\endgroup$ – Helen Apr 1 '17 at 4:18

Given comments that the collapse of the wavefunction is still not understood, I'd like to emphasize experimental observations.

Imo the term "collapse of the wavefunction" is a misleading term for "interaction" or measurement..

The wavefunction is not measurable,it is a complex-number mathematical function necessary for calculating quantum mechanical probabilities of an interaction happening. It is not an observable balloon that can collapse. Only the complex conjugate square of a wavefunction is observable

Take a simpler mathematical solution , the parabola of a projectile: Is the parabola observable? Only the projectile's motion is observable. If suddenly the projectile changes direction, we will not say that the parabola was broken. We will look for the obstruction in the path of the projectile, i.e an interaction that will change the model function.

So measuring the spin collapses the wavefunction. What else does?

The wave function is a solution of a quantum mechanical differential equation with the boundary conditions of the problem. Any interaction changes the boundary conditions, and measurements are interactions.The measurement will give one point in the probability density distribution which can be measured by repeating the process many times.

This single electron double slit accumulation can give an intuition of how a probability density is connected with individual measurements:


Electron buildup over time

Each electron fired at the two slits has a probability on ending as a point in the screen. Once it hits the screen its wavefunction is no longer controlled by the boundary conditions "electron impinging on two slits with given dimensions". It has been absorbed in the screen raising dots from a large number of ionizing interactions with the molecules of the screen.

Does it have a meaning to ask whether the "wave function collapsed"? A new wavefunction is needed the instant the electron impinges on the first atom of the screen.

The wave function leaves its imprint in the probability distribution shown in the later slides, showing the wave nature of the electron, which is the complex conjugate squared of the wavefunction. For a single electron only a point can be seen.

So all quantum mechanical solutions of specific boundary value problems give wave functions which , once the boundary conditions change, by interactions, the old wave function is not longer valid and a new one has to be calculated with the new boundary conditions.

  • $\begingroup$ By updating boundary condition, are you interpreting the Born rule, $|\psi\rangle \to |n\rangle $ with probability $|a_n|^2$?, as a boundary condition applied at the time of measurement? $\endgroup$ – innisfree Mar 30 '17 at 12:04
  • $\begingroup$ @innisfree correction: I did not see the "at the time of the measurement" next to boundary condition. The boundary conditions define the wavefunction for all the electrons above, for example. The projection is the throw of the dice that gives a specific measurement. The probability is the complex conjugate squared of the wave function, and should be evaluated for that x,y spot. so if the n represents the delta(area) of the screen then yes. $\endgroup$ – anna v Mar 30 '17 at 13:10
  • $\begingroup$ a new psi with new n is needed after impact $\endgroup$ – anna v Mar 30 '17 at 13:17

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