A Quantum measurement of a Strontium ion

[pinball1970]

TL;DR Summary

Does this mean there is no instant wave function collapse during measurement?

Can any of quantum guys comment.

https://link.aps.org/doi/10.1103/PhysRevLett.124.080401

I cannot post the science alert article where I saw this. Apologies, this put the study in layman's.
@PeterDonis @vanhees71 et al will not need that.

Is this significant?

Could the wave function not instantaneously collapse?

 

[PeterDonis]

Moderator's note: Moved thread to interpretations forum as collapse vs. no collapse is an interpretation question.

 

[vanhees71]

Well, here it's a clear real-world experiment, showing that (of course), there's no instantaneous collapse but it's explainable by the quantum-mechanical time evolution of the system coupled to the "measurement device". Once more a time-resolved measurement of the interacting quantum system has shown that there's no instantaneous collapse!

 

[pinball1970]

Well, here it's a clear real-world experiment, showing that (of course), there's no instantaneous collapse but it's explainable by the quantum-mechanical time evolution of the system coupled to the "measurement device". Once more a time-resolved measurement of the interacting quantum system has shown that there's no instantaneous collapse!

I cannot tell if you are being sarcastic tbh.
If I have pounced on the pop science aspect without understanding the physics I am fine with that.
If you can explain the result and it's significance (if any) that be useful

 

[vanhees71]

No, it was meant sarcastic only in a different sense than you might have understood from my reply. I find it funny that this posting has been moved to the "interpretations section", which I usually try to avoid.

In this case, it's however about a real experiment done to precisely check the claim of some flavors of the Copenhagen interpretation that there's some strange and not well-defined "classical-quantum cut" that puts measurements outside of the usual quantum-mechanical laws describing the dynamics.

There are some experiments around trying to investigate this claim in various directions. One kind of experiments try to extend typical "quantum phenomenology" like "wave-like beahavior" to larger and larger objects to see, whether there's really some fundamental boundary on the system size which prevents from quantum effects to be valid, i.e., that only classical behavior can be observed: So far there's no hint at such a funcamental "quantum-classical cut". With more and more refined techniques to isolate better an better also very large systems from the environment and cool them down to temperatures close to absolute 0 it has been demonstrated that even very large and very heavy molecules show quantum behavior (it's at the level where the de Brolgie wavelength of the molecules reaches the fm range). One recent example is

https://www.nature.com/articles/s41567-019-0663-9

Then there are invstigations about socalled "quantum jumps", i.e., whether there are really instantaneous transitions between states as in the transitions of an atom from an excited energy-eigenstate to a lower one under emission of a photon (that's how Bohr thought about it before the advent of modern quantum theory in his famous Bohr(-Sommerfeld) model of the hydrogen atom). Modern quantum mechanics predicts that such a transition is rather described as a smooth process described by the time-dependent Schrödinger equation. Recently some investigation has been done that with clever measurements you can resolve the time evolution of such a process and even prevent the decay by some manipulations with external sources (like microwaves or laser). One example for this is

https://arxiv.org/abs/1803.00545

What's shown in the paper you quote in #1 is again that for this special case an ideal measurment is realized through the interaction of a system of a trapped ion with photons to measure whether it's in one of three superimposed states. The conclusion at the end of the paper is

We thus quantitatively demonstrated how a measurement
can be implemented in a natural process while preserving
coherence. By taking snapshots of the measurement proc-
ess itself, we also demonstrated that this process is not
instantaneous. A similar conclusion was reached in [15], in
which it was found that quantum jumps evolve coherently
and are not instantaneous.

[15] is the above quoted work by Ninev at all.

 

[PeterDonis]

I find it funny that this posting has been moved to the "interpretations section", which I usually try to avoid.

It was moved because, as noted, "collapse" vs. "no collapse" is an interpretation question. That does not, of course, mean that the experiment itself was not "real". It just means that answering the question, "what does this experiment mean?" is an interpretation question. And that's the question this thread is about.

 

[vanhees71]

Well, I find the answer of the experiment to the question "collapse" vs. "no collapse" pretty clear and leaving little room for interpretation.

 

[PeterDonis]

The arxiv preprint of the paper referenced in the OP is here:

https://arxiv.org/abs/1903.10398

 

[PeterDonis]

here it's a clear real-world experiment, showing that (of course), there's no instantaneous collapse

That's not what the paper itself says. The paper itself says that "measurement" is a non-unitary process. That is a "collapse" claim. A no collapse interpretation such as MWI would disagree with it: in the MWI, all processes, including those we call "measurements", are unitary.

 

[vanhees71]

Of course, the time evolution of an open system is not described by a unitary time evolution. Maybe I'm biased by I read the paper such that everything can completely be explained by the interaction of the 3-level ("qtrit") system with the em. field used to measure it in the specified way and that there's nothing instantaneous. It's clear that a von Neumann measurement involves always a "open-quantum-system treatment" and a clear split between the measured system and the measurement device. This is however not to be confused with an apparent fundamental "quantum-classical cut".

 

[PeterDonis]

I read the paper such that everything can completely be explained by the interaction of the 3-level ("qtrit") system with the em. field used to measure it in the specified way and that there's nothing instantaneous.

Sure, but I could say the same thing about an electron's interaction with a Stern-Gerlach magnet. The "collapse" in collapse interpretations doesn't come when the electron interacts with the magnet; it comes when the electron hits the detector screen and makes a bright spot at a particular place. In the experiment in the paper, the latter corresponds to the fluorescence detection, which is not the "measurement" that is explained by the interaction of the qtrit with the em field. So the "measurement" that you are talking about is not where a collapse interpretation would put the collapse anyway.

 

[vanhees71]

That's precisly what I say all the time. The SGE is the perfect example, where you can (almost exactly) calculate that the apparent collapse is as well explained by the interaction of the silver atoms with the (inhomogeneous) magnetic field. You can of course call the simple bump of the silver atom on Stern and Gerlachs plates a "collapse", but that's just abuse of language if you ask me. It's as well a usual interaction of the silver atom with that plate. The same here is the fluorescence, which is also just a transition emitting some photon which is described by the dynamics of quantum theory, and these photons carry the information about the occupation or non-occupation of the state $|0 \rangle$ to the experimenter, but nowhere is a collapse involved.

I don't know which specific interpretation that point of view is though. I'd say it's a flavour of Copenhagen without collapse (most close to the minimal statistical interpretation). It's just the least "esoteric" interpretation, which I think is the most scientific way of thinking about the foundations of quantum mechanics, just based on the formalism (with a strictly probabilistic interpretation of the quantum state) and phenomena as observed with real-world measurement devices in the lab.

 

[PeterDonis]

The SGE is the perfect example, where you can (almost exactly) calculate that the apparent collapse is as well explained by the interaction of the silver atoms with the (inhomogeneous) magnetic field.

Huh? That calculation doesn't tell you why a bright spot shows up on the detector at a particular place, which, as I said, is where the "apparent collapse" is.

 

[vanhees71]

No, in quantum theory you cannot predict where each individual particle hits the plate (except you have prepared the spin to have a determined value $\pm \hbar/2$ in direction of the magnetic field). All you can calculate is the probability distribution for registering the particle. In the usual SGE, where you prepare an unpolarized beam you predict that the beam splits in two partial beams. Each single particle has the corresponding probability to end up in one of these partial beams given by Born's rule, and it says that after the particle has run through the SGE the spin component in direction of the magnetic field is (almost perfectly) entangled with the particle's position: i.e., the particles in each partial beam have a determined spin component in direction of the magnetic field. There's no collapse, just spin preparation through the unitary dynamics, which you can even calculate to high precision analytically.

Usually where some Copenhagen interpretation flavors assume a collapse is at the point, where you register the particle, but that's also nothing special but just the interaction of the particle with the detector allowing a position measurement like a photoplate or CCD camera or whatever position resolving detector you might use.

 

[PeterDonis]

in quantum theory you cannot predict where each individual particle hits the plate

Yes, I know that. But you don't appear to be fully considering the implications. See below.

Usually where some Copenhagen interpretation flavors assume a collapse is at the point, where you register the particle

Yes. Do you understand why?

Suppose we prepare an electron in a spin-x up state, and send it through a SG apparatus oriented in the z direction. Unitary dynamics says this electron evolves into a superposition in which its spin is entangled with its momentum. But when we register the electron, that's not what we register. We register a single spot on the detector plate.

Collapse interpretations explain this by saying that there is a non-unitary collapse of the entangled superposition onto one of its terms.

No collapse interpretations explain this by saying that "we" become entangled with the electron, so there is now an entangled superposition that includes us (and the detector plate, and anything else that has interacted with those things).

If you are saying there is no collapse because you advocate the MWI, then you are advocating a particular interpretation; you are not saying there is no collapse, period, because you can't, that claim is interpretation dependent.

If you are saying there is no collapse because you think that a unitary interaction such as "the interaction of the particle with the detector" can somehow turn an entangled superposition into just one of its terms, then you are simply wrong.

I don't see any other options, but if you think there is one, please point it out.

but that's also nothing special but just the interaction of the particle with the detector

Saying that it's "nothing special" doesn't mean you can just wave your hands and ignore collapse interpretations or the reasons for them. The interaction of the particle with the detector, if you insist on it being unitary, can't change an entangled superposition into just one of its terms. See above.

 

[vanhees71]

My point of view is that collapse is neither necessary nor does it do any good to the theory. To the contrary it contradicts the very foundations of local relativistic QFTs (local in the usual sense of "local interactions", implying the microcausality constraint for physical local observables).

Suppose we prepare an electron in a spin-x up state, and send it through a SG apparatus oriented in the z direction. Unitary dynamics says this electron evolves into a superposition in which its spin is entangled with its momentum. But when we register the electron, that's not what we register. We register a single spot on the detector plate.

Unitary dynamics tells you all there is to be told (that's the view of the minimal statistical interpretation), namely the probabilities for finding the particle on the screen with which spin-$z$ component (suppose we measure the position on the screen and the spin-$z$ component). All QM tells us is the state as a function of time given an initial state (which in terms of the real world is given by some preparation procedure, i.e., in your case you have prepared a particle in a spin-x up state and, I suppose, a sufficiently well defined momentum such that the following SGE with the magnetic field in z-direction works). So all you now know is the probability is high to find the particle at one of two specific regions on the screen and that if the particle is found on position 1 it has a definite $\sigma_z=+\hbar/2$ and if it is found on position 2 it has a definite $\sigma_z=-\hbar/2$.

That's all QT describes, and as long as nobody comes up with some hidden-variable theory as successful in describing all reproducible phenomena as QM, there's no more to describe. Of course, as for any theory, you can only say that up to now no counterexample for the validity of QM has been found, and as for any other physical theory it can well be that it is incomplete, but so far there's no known such incompleteness. My conclusion (using Occam's razor) simply is that QM is, according to our present knowledge, all there is.

Saying that it's "nothing special" doesn't mean you can just wave your hands and ignore collapse interpretations or the reasons for them. The interaction of the particle with the detector, if you insist on it being unitary, can't change an entangled superposition into just one of its terms. See above.

That's not what I'm saying. There has been an interaction between the measurement device (the detector), and unitary time evolution is valid only for the entire system (here particle + detector) not for the particle alone.

Another argument against collapse is that it predicts in this case that you have a silver atom with some well-defined momentum and a precisely determined $\sigma_z$ after this collapse, but that's not true: You have silver atom sticking in the screen at some spot with macroscopic position resolution, and for sure due to interaction with the atoms in the screen it has not a specific $\sigma_z$ component anymore either. So what you describe by this "collapse" is not according to the facts.

The SGE as a preparation procedure for $\sigma_z$ of course works without any registration and apparent "collapse": All you need to know is that with high accuracy you prepare a position-$\sigma_z$ (or momentum-$\sigma_z$ if you wish) entangled state, i.e., you can be sure that $\sigma_z$ has a determiend value of $+\hbar/2$ when you take silver atoms from one of the partial beams (i.e., in the corresponding retion of position (momentum) space) and of $-\hbar/2$ when you take silver atoms from the other partial beam. In no way you can predict, in which partial beam an individual silver atom ends up, and so far there's no HV been found that could tell us this before the particle is registered. There's nothing else than the probabilities!

But, again, we are lost in a (most probably fruitless) discussion on interpretation. Whether or not you need a collapse to make sense of QT seems to be related with specific world views of individuals, which have nothing to do with physics as a natural science. This discussion is a nice example: I think we don't disagree about what we expect to be measured in the lab when doing an SGE but we just differ in the view about how to interpret the phenomena in view of each individual silver atom. I claim there's not more to it than described by QT, and QT tells us that it depends on the preparation of the state, which observables take determined values and which don't, and for those we only know probabilities, and for me all the highly accurate Bell experiments tells me that there is irreducible randomness in nature, i.e., that the indetermined observables are not indetermined due to our ignorance but they objectively are.

This is of course also founded in my personal conviction that locality and causality are necessary conditions for natural sciences to make sense at all. If there'd really be action at a distance and everything being instantaneously causally connected with everything there is in the universe, there'd be no natural laws to be observed but just some incomprehensible chaos with no obvious cause-effect relations whatsoever.

Note that this does not exclude the inseparability described by entanglement between far-distant parts of entangled quantum systems (like biphoton pairs in Bell states, registered at far-distant places showing the corresponding "stronger-than-classical" correlations).

 

[PeterDonis]

My point of view is that collapse is neither necessary nor does it do any good to the theory.

Which is fine as a point of view about interpretations. But you can't just declare by fiat that your preferred interpretation is the only one. Not everyone shares your preferred interpretation.

Also, your point of view as you state it here means your preferred interpretation is the MWI. See below.

There has been an interaction between the measurement device (the detector), and unitary time evolution is valid only for the entire system (here particle + detector) not for the particle alone.

Then your preferred interpretation is the MWI, since that's what unitary time evolution for the entire system gives you. Is that what you intend to say?

 

[vanhees71]

I've never understood the meaning of MWI. That's why I can't say wether or not I agree with that view. All I say is that we can apply QT in the minimal statistical interpretation to real-world experiments and observations. There's not other need for a successful physical theory.

The collapse is in any case contradicting the very foundations of the most successful class of theories ever, i.e., local relativistic theories, upon which the Standard Model is based, and so far no experiment has been able to disprove the Standard Model despite huge effort to do so since it leaves at least the question open, why we are all here though the CP violation of the Standard Model is too weak to explain this very elementary fact ;-)).

 

[PeterDonis]

I've never understood the meaning of MWI.

That is very surprising to me, since when you say collapse adds nothing to the theory and unitary time evolution applies to the entire system, you are stating the MWI.

 

[PeterDonis]

All I say is that we can apply QT in the minimal statistical interpretation to real-world experiments and observations.

And this is fine as a point of view, but it's a different point of view from the one you stated before. The minimal statistical interpretation does not say that unitary time evolution applies to the entire system, and it does not say that there is no collapse. It does not commit itself at all on these points.

 

[vanhees71]

That is very surprising to me, since when you say collapse adds nothing to the theory and unitary time evolution applies to the entire system, you are stating the MWI.

Of course unitary time evolution applies to any closed system. How else do you define the formalism to begin with.

I don't claim to know the meaning of unobservable things like the "entire universe". It's as empty a phrase as is collapse ;-)).

 

[vanhees71]

And this is fine as a point of view, but it's a different point of view from the one you stated before. The minimal statistical interpretation does not say that unitary time evolution applies to the entire system, and it does not say that there is no collapse. It does not commit itself at all on these points.

Since when that? In Ballentines textbook, which is what I'd think defines clearly the meaning of the minimal statistical interpretation, you have without doubt the usual postulates, including the unitarity of time evolution.

Again: Unitary time evolution applies to a closed system. Projecting out parts ("coarse graining") of course leads to non-unitary time evolutions in terms of all kinds of "master equations" of "open quantum systems" like the Lindblad equation etc.

 

[PeterDonis]

In Ballentines textbook, which is what I'd think defines clearly the meaning of the minimal statistical interpretation, you have without doubt the usual postulates, including the unitarity of time evolution.

Unitarity of time evolution of an isolated system that is not being measured. But earlier, you said:

There has been an interaction between the measurement device (the detector), and unitary time evolution is valid only for the entire system (here particle + detector) not for the particle alone.

There you are saying that unitary time evolution applies to the particle + detector system when a measurement is made. That is going beyond the minimal statistical interpretation; it is the MWI.

 

[Lord Jestocost]

My point of view is that collapse is neither necessary nor does it do any good to the theory.

The mathematical formalism of quantum theory has nothing to say regarding the actual outcome of a single measurement event; there is no mechanism which can be proposed for its occurrence, no algorithm for it can be given and no causal description is possible. Thus, in order to relate the mathematical formalism of quantum theory to our – maybe putative - perceived reality (the “actual outcomes” of single measurements), the wave-packet reduction postulate has to put in “by hand” as part and parcel of quantum physics.

To the contrary it contradicts the very foundations of local relativistic QFTs (local in the usual sense of "local interactions", implying the microcausality constraint for physical local observables).

As Peter Mittelstaedt remarks in “Quantum Holism, Superluminality, and Einstein Causality”:

“Finally, we analyze these arguments and show that the micro-causality condition of relativistic quantum field theory excludes entanglement induced superluminal signals but that this condition is justified by the exclusion of superluminal signals. Hence, we are confronted here with a vicious circle, and the question whether there are superluminal EPR-signals cannot be answered in this way.”

 

[vanhees71]

Unitarity of time evolution of an isolated system that is not being measured. But earlier, you said:
There you are saying that unitary time evolution applies to the particle + detector system when a measurement is made. That is going beyond the minimal statistical interpretation; it is the MWI.

No, particle+detector can also be seen as a closed system, and nothing is special with detectors compared to any other lump of matter, and the quantum dynamical laws, i.e., unitary time evolution applies. There's not a single experimental fact known which contradicts this view. If this is MWI, so be it!

 

[vanhees71]

The mathematical formalism of quantum theory has nothing to say regarding the actual outcome of a single measurement event; there is no mechanism which can be proposed for its occurrence, no algorithm for it can be given and no causal description is possible. Thus, in order to relate the mathematical formalism of quantum theory to our – maybe putative - perceived reality (the “actual outcomes” of single measurements), the wave-packet reduction postulate has to put in “by hand” as part and parcel of quantum physics.
As Peter Mittelstaedt remarks in “Quantum Holism, Superluminality, and Einstein Causality”:

“Finally, we analyze these arguments and show that the micro-causality condition of relativistic quantum field theory excludes entanglement induced superluminal signals but that this condition is justified by the exclusion of superluminal signals. Hence, we are confronted here with a vicious circle, and the question whether there are superluminal EPR-signals cannot be answered in this way.”

There's no need for wave-packet reduction if you simply keep true to your own words: There's not more concerning the outcome of measurements than the probabilities, and all we can measure to check QM is to take an ensemble and make statistics to compare it with the predicted probabilities.

I disagree with the bold part of the text. All Bell tests with photons show that QED works perfectly well. The correlations described by entanglement are there from the very beginning due to the preparation of the photons in the entangled state, and this preparation is a local process (e.g., parametric downconversion with a laser interacting with a BBO). The correlations are there as long as there's no interaction destroying it again. Then the photons can be detected at very far distances, showing the correlations. The detection events themselves are again local. There's no necessity to assume a superluminal signal propagation, because measurement event A has no instantaneous cause for the outcome of measurement event B (if you take QED seriously and make the measurment events at A and B space-like separated). The correlations are there because of the preparation in the entangled state at the very beginning of the experiment and not due to superluminal signal propagation from one measurement event to the other.

Also entanglement swapping is not a counterargument against this. We had a long debate some time ago in this forum about this, where I repeatedly argued, using the above point of view, that the observed facts can be understood taking this point of view of the minimal interpretation in application to local relativistic QFT. I know that some people take the point of view of Mittelstaedt, but nobody could come up with a working formulation of relativistic QT which is not a local relativistic QFT, and as long as there is no convincing model that allows spooky actions at a distance and is as successful as the Standard Model, I take the minimally interpreted local relativistic QFT as the correct description. At least there's not a single discrepancy between theory and experiment taking this point of view.

 

[Lord Jestocost]

The correlations described by entanglement are there from the very beginning due to the preparation of the photons in the entangled state, and...

The assumption that correlations described by entanglement are there from the very beginning due to the preparation of photons in an entangled state can merely account for the perfect anti-correlation at equal angles, but it is incompatible with the correlations at unequal angles. It makes no sense to promote “Ballentine’s ensemble interpretation” repeatedly by disguising in a lot of wording that behind this interpretation there lurks a wishful – but not always outspoken - thinking of some kind of hidden variables or something equivalent bearing a different name.

 

[vanhees71]

This is not true. No experiment contradicts the predictions of quantum mechanics in Bell measurements, or can you point us to a peer-reviewed paper, where the predictions in Bell measurements on the polarization states of entangled photons do not follow the predictions of quantum mechanics? AFAIK the prediction of the violation of Bell's inequality have been always fulfilled, and this indeed involves measurements of the single-photon polarizations in non-orthogonal directions. In some of the most accurate experiments the quantum mechanical predictions are fulfilled at amazing levels of significance!

There are no hidden variables in the minimal interpretation of quantum mechanics either. As the name says the minimal interpretation just takes the quantum-theoretical formalism as is without adding additional pieces whatsoever.

 

[Lord Jestocost]

The so-called “minimal statistical interpretation” is nothing else than the “instrumentalist minimal interpretation”. As Cord Friebe et al. put it in “The Philosophy of Quantum Physics”:

“If one tries to proceed systematically, then it is expedient to begin with an interpretation upon which everyone can agree, that is with an instrumentalist minimal interpretation. In such an interpretation, Hermitian operators represent macroscopic measurement apparatus, and their eigenvalues indicate the measurement outcomes which can be observed, while inner products give the probabilities of obtaining particular measured values. With such a formulation, quantum mechanics remains stuck in the macroscopic world and avoids any sort of ontological statement about the (microscopic) quantum-physical system itself.”

You repeatedly remove yourself from this interpretation by ontological remarks like “The correlations described by entanglement are there from the very beginning due to the preparation of the photons in the entangled state…..”, which points to the fact that you confuse the “minimal statistical interpretation” with some flavor of an “ensemble interpretation”. It is a deep misconceptions in case one reads the pre-measurement situation of a collective of identically prepared quantum systems in a statistical way. The preparation (our knowledge at this moment) allows us merely to represent each individual system by its appropriate wave function which expresses the potentially possible for that system. As remarked by Sir Arthur Stanley Eddington: “The quantum physicist does not fill the atom with gadgets for directing its future behaviour, as the classical physicist would have done; he fills it with gadgets determining the odds on its future behaviour. He studies the art of the bookmaker not of the trainer.”

All these discussions about quantum foundations reveal always the same problem: A lot of Physicists without any background in philosophy are not conscious about the implicit assumptions they are making when discussing quantum foundations.

 

[vanhees71]

I don't understand what you mean. In a way I'm arguing within Friebe's minimal interpretation (whether it's epistemic or ontic I don't care the least in this connection). Of course instead of hermitian he should have written self-adjoint, but that's a mathemtical detail.

Again, all I'm saying is that an (idealized) polarization-entangled two-photon state is described by a pure state $\hat{\rho}=|\psi \rangle \langle \psi|$ with the ket something like (I take the singlet state as one of the possible Bell states as an example):
$$|\psi \rangle = N [\hat{a}^{\dagger}(\vec{p}_1,1) \hat{a}^{\dagger}(\vec{p}_2,-1) - \hat{a}^{\dagger}(\vec{p}_1,-1) \hat{a}^{\dagger}(\vec{p}_2,1)]|\Omega \rangle,$$
where $\hat{a}^{\dagger}(\vec{p},\lambda)$ is the creation operator of a photon with momentum $\vec{p}$ and helicity $\lambda \in \{1,-1\}$, and $|\Omega \rangle$ is the vacuum state.

This state contains all information QT (in this case QED) provides about the state (a preparation procedure for a polarization-entangled photon pair, e.g., realized via parametric down conversion by shining a laser on a BBO crystal), and this information is probabilistic, including the strong correlations described by such an entangled state. So these correlations are there due to the preparation procedure and is not caused by the outcome of any measurement on the system. I don't see, where this violates the minimalism of the interpretation: All there is are the statistical properties for the outcomes of measurements described by such a state.

Taking the corresponding partial traces reveals that the single-photon polarizations are maximally random, i.e., described by the state $\hat{\rho}_j=\frac{1}{2} (|\vec{p}_j,1 \rangle \langle \vec{p}_j,1|+ |\vec{p}_j,-1 \rangle \langle \vec{p}_j,-1 |)$, $j \in \{1,2\}$.

That means that both observers at the places measuring their respective photon, defined by the momentum $\vec{p}_1$ (Alice) and $\vec{p}_2$ (Bob) and measured by placing A's and B's detectors in the corresponding direction far away from the BBO, where the photons have been created, just measure completely unpolarized photons.

Only if A and B take accurate measurement protocols (including time stamps such that one knows which two photons belonged to each individual entangled pair) they can observe the correlations between their measurements. If both measure the polarization in the same (or perpendicular) directions they get 100% correlations: if A measures H in the choosen direction of a linear-polarizer experiment then B necessarily finds V and vice versa. It doesn't matter, who detects her/his photon first (or at time-like separated measurement events) or at the same time (or at space-like separated mesurement events). I think so far I state experimental facts without any interpretation in mind.

My interpretation now is that I take the entire formalism of QED seriously and I assume that the measurement of the single-photon polarization at A's and B's place are governed by the laws of QED. Then I must conclude:

The 100% correlation cannot be caused by A's outcome due to a causal connection of A's measurement device with B's, if the measurement events are space-like separated (due to the locality and microcausality of QED). Thus the 100% correlation must be due to the preparation of the photon pair in the said entangled state, i.e., the correlation is there from the very beginning, i.e., at creation of the photon pair by the parametric down-conversion. That's also what the formalism tells me anyway.

The same holds also for other measurements, i.e., when A and B do not direct their polarization filters in the same direction (or perpendicular to each other). One has also done tests of the Bell inequality, where such non-parallel orientation of polarization filters have to be done, and all these tests give (in recent years with amazing statistical significance!) precisely what QED predicts and at the same significance contradicts local HV models due to the established violation of Bell's inequalities. For details of the argument (using spin instead of photon polarization, but that's of course unimportant here too), see, e.g., J. J. Sakurai, Modern Quantum Mechanics.

 

[Lord Jestocost]

The correlations described by entanglement are there from the very beginning due to the preparation of the photons in the entangled state, and...

You don't get it. The above statement is an ontological statement about an entangled system, and such a statement doesn't belong to the “instrumentalist minimal interpretation”.

 

[PeterDonis]

If this is MWI, so be it!

If you're going to take that position, that's fine, but then you need to stop claiming that you are using the "minimal statistical interpretation", since you're not. You can't have it both ways.

In a way I'm arguing within Friebe's minimal interpretation

No, you're not. At least, not if you're claiming that the particle + detector system evolves unitarily. You need to make up your mind what interpretation you are using.

 

[Lord Jestocost]

This forum is labeled “Quantum Interpretations and Foundations”. It would thus be helpful when the participants in discussions within this forum would clearly define and denote which kind of “interpretation” they follow. To my mind, this could save long and fruitless debates.

 

[vanhees71]

If you're going to take that position, that's fine, but then you need to stop claiming that you are using the "minimal statistical interpretation", since you're not. You can't have it both ways.
No, you're not. At least, not if you're claiming that the particle + detector system evolves unitarily. You need to make up your mind what interpretation you are using.

It is new to me that only because one part of a closed system is a "detector" it's not subject to unitary time evolution anymore. What makes a detector distinct from a usual piece of matter that you don't call a detector? Also a detector consists of the known particles of the standard model following the universal physical laws described by it.

Of course you are right when you say that a measurement involves also some irreversibility, i.e., you have to somehow "store the measurement result" to make it readable to us macroscopic human beings, and here you have to use some macroscopic description, involving coarse graining to effective macroscopic variables, leading to description in terms of non-unitary time evolution (which doesn't make sense to begin with for, e.g., describing something like macroscopic fluid flow in terms of fluid dynamics). All this doesn't contradict of course the validity of unitary time evolution for closed on the fundamental microscopic level. To the contrary, it's one of the most important principles guaranteeing the consistency of our macroscopic description, including the "arrow of time" in the sense of Boltzmann's H-theorem.

 

[PeterDonis]

It is new to me that only because one part of a closed system is a "detector" it's not subject to unitary time evolution anymore.

Again, if you want to claim that the particle + detector system is subject to unitary time evolution, that's fine--but then you need to admit that you are using the MWI, not the "minimal statistical interpretation". The latter does not claim that the particle + detector system is subject to unitary time evolution; it leaves that question open.

 

[vanhees71]

No, you make a very far-reaching claim, namely that unitary time evolution is invalid only because some piece of matter is used by human beings as a detector for measurement. I think this is not true, and I don't know any physicist who claims this. It may be part of some Copenhagen interpretation flavors, where a fundamental quantum-classical cut is assumed (I think Bohr himself never claimed it in this strong sense, I'm not sure about Heisenberg, who might have been more inclined to think about it in this way, because he's the one inventing this idea of a cut), but it's for sure not in the orthodox interpretation of textbooks and not in the minimal interpretation.

Also there are some decades of research about open quantum systems and decoherence and all that between Bohr and today, and this shows that the classical behavior of macroscopic systems has nothing to do with a fundamental cut but with a coarse grained view on macroscopic systems in terms of the relevant macroscopic observables, which tend to behave classical in almost all circumstances (there are of course some famous exceptions like BECs, superfluidity and superconductivity, where you have quantum-coherent behavior for macroscopic observables, but that's well understood either within standard QT without any fundamental quantum-classical cut). Also more and more experimental investigations indicate that there seems indeed to be no such cut, given that with proper preparations you can observe quantum behavior of very large (mesoscopic?) systems like in a recent experiment involving very large molecules with de Broglie wavelengths in the order of fm (nuclear scale!).

So far, nothing in the physical laws indicates the validity of a fundamental quantum-classical cut, let alone the necessity of "extra rules" for "detectors" or general "measurement devices", but I think I better give up on this topic once more. It's anyway hopeless to communicate about these vague philosophical issues in a coherent way. I'm only wondering, how one can avoid always falling in this trap when discussing very uncontroversial quantum physics in this forum. Maybe after all you were right in shifting this question about a (real-world not gedanken!) experiment to the Foundations Forum. This should have been warning enough for me :-((((.

 

[Lord Jestocost]

It's anyway hopeless to communicate about these vague philosophical issues in a coherent way.

@vanhees71
Nobody forces you to participate in such discussions!

 

[vanhees71]

No, indeed, but I answered a question which on the first indication for me had nothing to do with this quantum-philosophy business and immediately the discussion went into these philosophical issues, but you are right. I should have simply ignored the thread from then on. It was again a lack of self-disciplin on my side. I'm sorry.

 

[PeterDonis]

you make a very far-reaching claim

I haven't made any claims at all. I have only been pointing out the implications of your claims.

I think this is not true, and I don't know any physicist who claims this.

Any physicist who does not support the MWI is (at least implicitly) denying that unitary time evolution is always valid, because if you assume that unitary time evolution is always valid, the MWI is what you get.

I'm only wondering, how one can avoid always falling in this trap when discussing very uncontroversial quantum physics in this forum.

When we split off this foundations and interpretations forum from the main quantum physics forum, we were quite clear about the ground rules for what counts as "uncontroversial quantum physics" in the main quantum physics forum. For reference, the guidelines for the main QM forum are here:

https://www.physicsforums.com/threads/guidelines-for-quantum-physics-forum.978328/

There is also a link there to the Insights article where the "minimal" interpretation of QM that is to be used for discussion in the main QM forum is described. The way to avoid falling into a "trap" in the main QM forum is to stick to that minimal interpretation.

Maybe after all you were right in shifting this question about a (real-world not gedanken!) experiment to the Foundations Forum.

The criterion for a thread belonging in this forum instead of the main QM forum is not whether or not an experiment being discussed is "real". It is interpretation. The OP of this thread asked explicitly about "instant wave function collapse", which makes it an interpretation discussion since that goes beyond the minimal interpretation referred to above.

 

[Killtech]

Sure, but I could say the same thing about an electron's interaction with a Stern-Gerlach magnet. The "collapse" in collapse interpretations doesn't come when the electron interacts with the magnet; it comes when the electron hits the detector screen and makes a bright spot at a particular place.

Has this been experimentally verified or is this just a speculation on your part? A simple detection and counting of results itself is not able to distinguish between measuring a superposition of states and a stochastic distribution of states whenever the effective distributions are identical. So this has to be tested in a more sophisticated way than the Stern-Gerlach setup. But given a separation into two beams those could be brought back together to cause interference - if the interaction with the magnet did not already collapse the state. Otherwise no interference of the beams will be observed.

purely on an intuitive level i would have assumed that an interaction with a magnetic field is significantly more intrusive to a state then for example the interaction with an atomic grid of a mirror. i mean classically shooting a small permanent magnet in zero gravity through a big Stern-Gerlach device will render its original state highly unstable and it will have to decide one way or the other for every time it passes one of the magnetic fields. so if it had any means to get rid of the excess energy from its residual angular momentum (initial and from potential energy when entering the B field away from the stable equilibrium configuration) it should classically start to collapse into one of the two stable equilibriums in a dampened oscillation - while its remains in the field. Sure classic analogs don't usually count for much but i don't exactly see how a quantum state would be immune to this type of interaction.

 

[PeterDonis]

Has this been experimentally verified

Has what been experimentally verified? Remember I was talking about interpretations. All interpretations agree on the predicted and observed experimental results; the difference is in how to explain why those predicted and observed results are observed.

given a separation into two beams those could be brought back together to cause interference

Yes, and this has been observed, with photon polarization if not electron spin: the simplest example is a Mach-Zehnder interferometer. I don't know if the analogous experiment has been run with electrons, but I don't think anyone is in doubt about what result would be obtained.

 

[Killtech]

Has what been experimentally verified? Remember I was talking about interpretations. All interpretations agree on the predicted and observed experimental results; the difference is in how to explain why those predicted and observed results are observed.

an experimental yes or no question has nothing to do with interpretation. your assumption however makes for a different experimental expectation in the extended setup i described later. so in term of observing the interference it makes a big difference whether the wave function leaves the Stern-Gerlach device in a collapsed state or a superposition state. Its further time evolution before the actual measurement will simply be a distinguishably different one. I would generally consider any experimentally testable statement not an interpretation issue.

However, yeah. Where a measurement/collapse (or however one likes to call & describe the mechanic) happens is not well defined in QM... so i guess this leaves it up for interpretation of a different type exactly because it should be experimentally decidable. As in: Mach-Zahnder obviously shows that it never applies to a photon-mirror interaction.

Yes, and this has been observed, with photon polarization if not electron spin: the simplest example is a Mach-Zehnder interferometer. I don't know if the analogous experiment has been run with electrons, but I don't think anyone is in doubt about what result would be obtained.

But the Mach-Zahnder interferometer is a very different setup and i cannot find a reason why a photons interaction with the atomic grid (unless it's temporarily absorbed) would cause any instability promoting anything like a collapse. But for a spinning magnet the state it is hardly imaginable why it should remain stable. The circumstance that one should be able replicate Stern Gerlach using macroscopic permanent magnets in zero-g with random initial angular momentum if they were given enough travel time to lose their excess energy (e.g. via EM-radiation from their oscillation) should put that photon comparison at least under scrutiny.

 

[PeterDonis]

the Mach-Zahnder interferometer is a very different setup

Not as far as this discussion is concerned; as far as this discussion is concerned, it is the same as sending, say, a spin-x up an electron through one Stern-Gerlach device oriented in the z direction, then taking the two output beams and sending them back through a second Stern-Gerlach device that recombines them. In both cases you have a qubit whose spatial path gets split into two parts and then recombined, and you need to take interference into account to correctly predict what happens at the recombination.

or a spinning magnet the state it is hardly imaginable why it should remain stable

An electron is not a "spinning magnet". The spin-x up state (or indeed any eigenstate of one of the spin operators) of an electron is also an eigenstate of the free particle Hamiltonian, so it will be unchanged by time evolution. That is the same property that you correctly ascribe to the photon states in a Mach-Zehnder interferometer.

 

[PeterDonis]

using macroscopic permanent magnets in zero-g with random initial angular momentum

I don't know where you are getting that from since it's nothing like what I specified.

 

[Killtech]

An electron is not a "spinning magnet". The spin-x up state (or indeed any eigenstate of one of the spin operators) of an electron is also an eigenstate of the free particle Hamiltonian, so it will be unchanged by time evolution. That is the same property that you correctly ascribe to the photon states in a Mach-Zehnder interferometer.

yeah, except in simple QM the electron cannot radiate off energy because its coupling to the EM-field goes only one way. that makes the state artificially stable. same for classics: if the magnet goes through vacuum the only way to lose energy is via radiation. if you disable that process you suddenly make an unstable state into a stable periodic one. so you'd need QED to allow it to lose excess energy via a photon emission just like the classical magnet would do. for the atomic levels this doesn't need measurement to happen but should occur naturally.

As a QM analogue: Zeeman effect where the higher energy eigenstates aren't perfectly stable. if you are in a magnet field a state that isn't oriented along that field has a different energy value and should at some point fall down to the lower pure state, of which there are only two, no? okay, it is a fair point to ask how long that would take on average.

Therefore i would have expected QM follow a similar time evolution as the (indeed different but somewhat similar) classical process.

 

[PeterDonis]

in simple QM the electron cannot radiate off energy because its coupling to the EM-field goes only one way. that makes the state artificially stable

Sure, and in "simple QM" photons propagating through air don't interact with it, that makes the state artificially stable. So yes, both of these models are approximations; but in practice they are quite useful ones for real experiments.

As a QM analogue: Zeeman effect where the higher energy eigenstates aren't perfectly stable.

This is a bad analogy because the higher energy states in this case decay very quickly. Free electrons or photons propagating through an experimentalist's lab don't.

if you are in a magnet field a state that isn't oriented along that field has a different energy state and should at some point fall down to the lower pure state

What "magnet field" are we talking about in the case of a free electron propagating through a lab?

i would have expected QM follow a similar time evolution as the (indeed different but somewhat similar) classical process

Your naive expectation here fails to take crucial factors into account. See above.

 

[Killtech]

This is a bad analogy because the higher energy states in this case decay very quickly. Free electrons or photons propagating through an experimentalist's lab don't.
What "magnet field" are we talking about in the case of a free electron propagating through a lab?

When an electron enters the Stern-Gerlach magnet its Schrödinger equation changes from a simple free one to one within the presence of a magnetic field. indeed it still remains in an unbound state but this is still different from an entirely free electron traveling the lab experiencing no exterior force at all. EDIT: actually shouldn't be an classical orbit trajectory in a magnetic field be considered a bound state?? and anyway, sorry i meant an atom. doesn't make sense to put an electron through Stern-Gerlach. your mention of the electron just confused me.

And for the Atom: if it travels through a Stern Gerlach device you have for a short time an atom in a magnetic field (and Stern Gerlach is historically performed with atoms). Among other effects shouldn't the Zeeman effect be present? The B field won't be exactly static but on the atoms scale it might be close enough.

 

[PeterDonis]

When an electron enters the Stern-Gerlach magnet its Schrödinger equation changes from a simple free one to one within the presence of a magnetic field.

Yes, and we already know what that does to the electron: it splits its wave function into two components, one corresponding to an "up" result and the other corresponding to a "down" result, with the two components moving in different directions. Or, to put it another way, it entangles the electron's spin degree of freedom in the direction of the device's alignment, with the electron's momentum degree of freedom in the same direction.

But once the electron exits the device, it's a free electron again. And so there's nothing in principle preventing us from re-combining the two output beams by directing them into a second Stern-Gerlach device oriented in the same direction.

shouldn't be an classical orbit trajectory in a magnetic field be considered a bound state?

No.

sorry i meant an atom. doesn't make sense to put an electron through Stern-Gerlach

Sure, it does. The fact that it's very hard to do experimentally doesn't mean it doesn't make sense.

Among other effects shouldn't the Zeeman effect be present?

The Zeeman effect doesn't change anything I said above. It will slightly shift the energy of the unpaired electron in the atom (a silver atom in the most common case), but it won't affect its spin or the entanglement of the spin and momentum degrees of freedom by the interaction with the device.

 

[Killtech]

The Zeeman effect doesn't change anything I said above. It will slightly shift the energy of the unpaired electron in the atom (a silver atom in the most common case), but it won't affect its spin or the entanglement of the spin and momentum degrees of freedom by the interaction with the device.

Okay, my understanding was this: an atom enters a Stern-Gerlach device. in it's magnetic field it's energy levels change due to the Zeeman effect. So initially its wave function should be in some superposition of the different energy levels: specifically of energy levels now different due to the Zeeman effect. If a higher energy state would be sufficiently short lived then the corresponding electron would decay to a lower level losing a very small amount of energy. since it remains bound this should effect the entire atom.

now even if the energy amount is minimal turning the atom into another orientation doesn't take much energy either. i mean to classically change the orientation of a body in space takes a net 0 energy if final angular momentum is the same as the initial (acceleration to start turning takes energy but in order to stop that energy can be taken back theoretically). in a magnetic field the different orientations have slightly different potential energy so it would take the potential energy difference in ideal circumstance to make the orientation change.

So in the end i would expect the lower Zeeman energy state to also directly correspond to a pure spin up/down state of the entire atom: both are the energetically most convenient configurations after all. Also the electrons total angular momentum in the lower Zeeman level should align with the magnetic field and should differ from higher energy states (which due to being a basis must also have components in other less energy optimal directions) to minimize energy and therefore change $J$ and $J_z$ of the entire atom. as such the emission of energy from the higher Zeeman level would imply a mechanism that does just what the wave function collapse does - but already on entering the device long before hitting the detector. and also completely regardless of any interpretation stuff.

But fair enough, i haven't checked what the energy difference is between a pure spin up/down state and the spin state orthogonal to the magnetic field and how that corresponds to the energy between the Zeeman levels.

 

[PeterDonis]

my understanding was this

I don't know where you are getting all this from; it's nothing like the standard description of a Stern-Gerlach device. Do you have a reference? Or is it just speculation on your part?