Do the SQUID experiments falsify Bohmian mechanics?

[Maui]

This is getting boring. Where in that quote do they say that the superposition corresponds to the current flowing in both directions at once? No one is disputing the fact that the two states correspond to clockwise and counter-clockwise currents (this is just a "classical" effect, the same is true for a regular SQUID operated in the classical regime), which is all they are saying in that quote.

Clockwise and counter-clockwaise currents flow in opposite directions with the relevant amperes they managed to infer. It seems their inference describes something real, that also goes through both slits and leaves an interference pattern.

By that logic, the superposition of states for a charge qubit would correspond to half an electron on the island. Or, the the superposition of spins for an electron to correspond to it "spinning" (in terms of angular momentum transfer) in both directions at once...etc. etc

As you say, without a measurement it's hard to say what really happens in superpositional states, but some progress is being made and maybe more clues will be available soon. I think quite a few teams are exploring that domain at this time(as it's left to people's imagination and personal interpretations at present). Quite possibly, classical intuition will be relegated to the past.

Without a measurement you can't say anything. This is the whole point of "common" QM. You can't say anything about what the C60s were doing while passing the slits, if you could the interference pattern would disappear!

I think this is more or less correct(though at face value it seems to deny the reality of an outside world) but it appears that the notion that information about a system in superposition cannot be indirectly extracted without causing a 'collpse' of the wavefunction is wrong. I think the situation is changing and the principle will be put in practice in the near future. And it seems more weight will be given to certain classes of interpretations :)

 

[Dadface]

You keep ignoring the fact that the abstract i quoted several times now is from a peer reviewed paper published in Nature:

"Here we present experimental evidence that a superconducting quantum interference device (SQUID) can be put into a superposition of two magnetic-flux states: one corresponding to a few microamperes of current flowing clockwise, the other corresponding to the same amount of current flowing anticlockwise."

http://www.nature.com/nature/journal/v406/n6791/full/406043a0.html




Here are the "journalists" you keep talking about:

-Jonathan R. Friedman, Vijay Patel, W. Chen, S. K. Tolpygo & J. E. Lukens
Department of Physics and Astronomy, The State University of New York, Stony Brook, New York 11794-3800, USA



If you need further reference of indirectly 'observed' quantum behavior(mezoscopic) - just look at the double slit experiment carried out with the C60 molecule. I guess you'd say without a measurement no C60 molecule ever passed the slits, even though an ensemble had been emitted one after one and left an interference pattern on the screen?

I think a major problem we have here is one of interpretation.The quote you refer to is,to me,ambiguous.The authors seem to claim that it is magnetic flux states that are in superposition.Although they state that each flux state corresponds to,eg is compatible with, a current,they do not state that the currents are in superposition.

The quote I gave in post 34 seems to be clearer with Huang and Moore referring directly to the currents being in superposition.One wonders if this is a conclusion based on how they interpreted the findings of the original experiment.

I think what we could do with here is an even bigger dose of clarification.

For what it's worth,I think that the superpositions that have been claimed here and in the past and which I have some familiarity with are,to put it in archaic cockneyese,"a load of old toffee"

 

[Cthugha]

Clockwise and counter-clockwaise currents flow in opposite directions with the relevant amperes they managed to infer. It seems their inference describes something real, that also goes through both slits and leaves an interference pattern.

Does it seem so? I disagree. That is not what they said. They just say that they can prepare a superposition of the two states as mentioned and what these two states are. They do not say that the superposition means that both states are real at the same time. They do not even infer that.

As you say, without a measurement it's hard to say what really happens in superpositional states, but some progress is being made and maybe more clues will be available soon. I think quite a few teams are exploring that domain at this time(as it's left to people's imagination and personal interpretations at present). Quite possibly, classical intuition will be relegated to the past.

I know of no serious team of physicists working on what "really happens in superpositional states" in the manner you understand it because every serious physicist knows that these questions on reality are not accessible by experiment. The only people working on that are crackpots. Or as f95toli correctly said: "What is true is not a question for science."

I think this is more or less correct(though at face value it seems to deny the reality of an outside world) but it appears that the notion that information about a system in superposition cannot be indirectly extracted without causing a 'collpse' of the wavefunction is wrong. I think the situation is changing and the principle will be put in practice in the near future. And it seems more weight will be given to certain classes of interpretations :)

Personally, I do not like the Bohmian interpretation because I do not find it "inspiring" with respect to the special topic I work on. Nevertheless all interpretations predict exactly the same results for experiments in standard physics. Therefore assuming that some interpretations will be ruled out soon, shows some lack of understanding the difference between interpretations and physical theories.

How many agreed with - "the standard interpretation of qm is the complete description of reality"? None.

Really? Then make that a one now. Although I think "a complete description" would be more appropriate than "the complete description".

 

[Maui]

Does it seem so? I disagree. That is not what they said. They just say that they can prepare a superposition of the two states as mentioned and what these two states are. They do not say that the superposition means that both states are real at the same time. They do not even infer that.

They said their superpositions correspond to n amperes of clockwise and anti-clockwise current. Ampere is a measure of flow.

I know of no serious team of physicists working on what "really happens in superpositional states" in the manner you understand it because every serious physicist knows that these questions on reality are not accessible by experiment. The only people working on that are crackpots. Or as f95toli correctly said: "What is true is not a question for science."

That's not what i said, what i said that certain ongoing research will likely shed additional light on the interpretational issues(e.g. quantum computing)

Personally, I do not like the Bohmian interpretation because I do not find it "inspiring" with respect to the special topic I work on. Nevertheless all interpretations predict exactly the same results for experiments. Therefore assuming that some interpretations will be ruled out soon, shows some lack of understanding the difference between interpretations and physical theories.

No, you misunderstand what i said - different interpretations might predict the same results, but they treat the nature of the wavefunction in a different manner.

Really? Then make that a one now. Although I think "a complete description" would be more appropriate than "the complete description".

No problem with that, this thread has been mostly opinions(some of them served as facts as happens in many other threads here). Though i'd still wonder what it is that causes an interference pattern in a double slit experiement if standard qm were a complete description of reality.

 

[Cthugha]

They said their superpositions correspond to n amperes of clockwise and anti-clockwise current. Ampere is a measure of flow.

No, they do not say that their superposition corresponds to that. They say that "[...]can be put into a superposition of two magnetic-flux states: one corresponding to a few microamperes of current flowing clockwise, the other corresponding to the same amount of current flowing anticlockwise."

It is not the superposition, but the two states from which the superposition is composed of, which correspond to clockwise and anti-clockwise current. Saying that the superposition of two states is just the sum of the properties of its constituents is a giant leap and not necessarily correct. More is different as Anderson said.

By the way in optics a huge part of the confusion beginners typically have comes from a similar intuitive mistake. They consider a two-photon state as just the sum of two single photon states and then run into several counterintuitive traps because that assumption is not justified for entangled states, Fock states or NOON-states.

Therefore I am very careful when it comes to drawing conclusions that seem intuitive like identifying a superposition of two states with simultaneous reality of the two states which form the basis for the superposition. This can go very wrong.

No problem with that, this thread has been mostly opinions(some of them served as facts as happens in many other threads here). Though i'd still wonder what it is that causes an interference pattern in a double slit experiement if standard qm were a complete description of reality.

Yes, I agree. Being more clear on what is opinion and what is fact might help, but it is very complicated as our everyday language works differently.

These "what causes XXX?"-questions do not really lead anywhere as one could always go one step beyond and ask "what causes the cause for XXX?". In standard qm it is the interference of probability amplitudes for indistinguishable events which creates the interference pattern. I can understand that this may not be satisfying from an ontological point of view for some people. However, it is not the necesssity of a physical theory to answer ontological questions to everyone's satisfaction. Being in accordance with experimental results while using a small number of assumption is enough.

 

[Maui]

No, they do not say that their superposition corresponds to that. They say that "[...]can be put into a superposition of two magnetic-flux states: one corresponding to a few microamperes of current flowing clockwise, the other corresponding to the same amount of current flowing anticlockwise."

But the superconducting quantum interference device(macroscopic) was put in two states, that's the cruicial part and the point to be made in the experiment - showing(inferring) truly quantum behavior at the macroscopic scale. The other point is that the inferred quantum behavior corresponds to something 'real' even though it behaves non-classically, and is not caused and brought forward by statistics(we are not living in the Matrix, are we?).

It is not the superposition, but the two states from which the superposition is composed of, which correspond to clockwise and anti-clockwise current. Saying that the superposition of two states is just the sum of the properties of its constituents is a giant leap and not necessarily correct. More is different as Anderson said.

Could be, i am not arguing. But it's beside the main point being made.

Yes, I agree. Being more clear on what is opinion and what is fact might help, but it is very complicated as our everyday language works differently.

These "what causes XXX?"-questions do not really lead anywhere as one could always go one step beyond and ask "what causes the cause for XXX?". In standard qm it is the interference of probability amplitudes for indistinguishable events which creates the interference pattern. I can understand that this may not be satisfying from an ontological point of view for some people. However, it is not the necesssity of a physical theory to answer ontological questions to everyone's satisfaction. Being in accordance with experimental results while using a small number of assumption is enough.

That's right, i was asking for an account of the C60 double slit experiement(also done recently) that also highlights the same issue - we have emission of electrons, we have interference on the screen(it makes little sense to conclude nothing went through the slits). Without the experimentalists and those who don't know what is in principle impossible by the standard interpretation and what the general perceptions among theoretical physicists are, physics would still be in the 1920's.

Here is one more case(a copy of a paper published by Science) of Schroedinger's cat-like quantum state, done on a single atom level(nothing novel, just another case of indirectly obsered quantum behavior).

http://www.quantumsciencephilippines.com/seminar/seminar-topics/SchrodingerCatAtom.pdf

Most of their papers are peer-reviewed so it's probably worth of attention, even just for the principle.

 

[Demystifier]

Personally, I do not like the Bohmian interpretation because I do not find it "inspiring" with respect to the special topic I work on.

May I ask what topic do you work on?

 

[Cthugha]

But the superconducting quantum interference device(macroscopic) was put in two states, that's the cruicial part and the point to be made in the experiment - showing(inferring) truly quantum behavior at the macroscopic scale.
[...]
Could be, i am not arguing. But it's beside the main point being made.

I disagree. It is the main point (as also the others here tried to tell you) that putting something (i) into a superposition state is not the same as putting something (ii) into two states at once. This is ok as a loose and unprecise usage of language, but it is strictly speaking not correct. The authors claim only (i), not (ii).

That's right, i was asking for an account of the C60 double slit experiement(also done recently) that also highlights the same issue - we have emission of electrons, we have interference on the screen(it makes little sense to conclude nothing went through the slits).

It makes little sense to conclude anything without an experiment showing direct eveidence what is happening. As that does not seem possible, a conclusion on what is happening "in between" seems pointless without having testable predictions. You can of course create an interpretation instead of a conclusion.

May I ask what topic do you work on?

Hmm, I try not to post or link to my own stuff here to avoid being too easily identifyable using google. So let me instead link to a link of one of my papers which ZapperZ posted a while ago in the newsworthy papers section when I was still pretty new to these forums.

https://www.physicsforums.com/showpost.php?p=2275080&postcount=85

Basically, I am working on the border between optics and semiconductor physics with a special interest in photon statistics and collective phenomena. As to the "inspiring" part I find it pretty neat that in standard qm you can treat (in rough terms) photon bunching, stimulated emission and stimulated scattering on the one hand and the exclusion principle and antibunching on the other hand on pretty much the same footing as there is just a sign changing between bosons and fermions leading to constructive or destructive interference of the probability amplitudes for events leading to species of the same type ending up in indistinguishable states. Also the "more is different" aspect of collective phenomena comes out quite clear.

Anyway I am aware that you can also treat these topics well in BM. Some of the approaches are just not my cup of tea. For example it is interesting and makes sense that degeneracy pressure can be attributed to a quantum force in BM, but it seems odd to consider the reverse process for bosons causing photon bunching. While it is consistent to treat them similarly ( and I am sure that are more sophisticated takes on the topic in BM which may be more elegant), a quantum force pushing massless photons together is just not intuitive for me.

 

[Maui]

I disagree. It is the main point (as also the others here tried to tell you) that putting something (i) into a superposition state is not the same as putting something (ii) into two states at once. This is ok as a loose and unprecise usage of language, but it is strictly speaking not correct. The authors claim only (i), not (ii).

I respect your opinion and the ones previously expressed in the thread(i am mostly learning and trying to separate the physics from philosophy, it's really hard, as everyone is pushing their own ideas and viewpoints).
If i am wrong, so be it.

If we disregard their inference that their flux states correspond to n-Amperes of clockwise and anti-clockwise currents(whatever that means according to your interpretation of the experiment, namely probabilities producing interference), we are still left with the dazzling inadequacy of addressing the C60 issue, namely -- emission - slits - interference pattern on the absorbtion plate. We can't just turn our heads and pretend this obvious problem doesn't exist, for as soon as you get back to it, it's still there. As i stated earlier even more trouble would be coming for this viewpoint from the area of quantum computing.

It makes little sense to conclude anything without an experiment showing direct eveidence what is happening. As that does not seem possible, a conclusion on what is happening "in between" seems pointless without having testable predictions. You can of course create an interpretation instead of a conclusion.

The interference pattern is direct evidence of something passing the slits after emission from the device(this is a minimalist, almost no assumption statement)

 

[f95toli]

I think a major problem we have here is one of interpretation.The quote you refer to is,to me,ambiguous.The authors seem to claim that it is magnetic flux states that are in superposition.Although they state that each flux state corresponds to,eg is compatible with, a current,they do not state that the currents are in superposition.

this is not really ambiguous. That the persistent current in the loop correspond to a certain flux follows from "classical" SQUID physics (flux quantization). you can find more about this in any standard textbook on superconductivity (say Tinkham's book).
The 3 junction SQUID used as a qubit is not in any way fundamentally different from a normal 2 junction SQUID. If you calculate the potential energy of the system you will find that the addition of an "extra" small junction gives rise to a double well potential, with the two wells corresponding to clockwise and counter-clockwise persistent currents flowing in the loop, respectively. If you then plug this potential energy into the Schrodinger equation you get what is more or less a complete description of the system (minus the effect of decoherence), anyone who has done say a first course in QM at university can do this.

I am going to be lazy and just refer to the following paper on the arXiv where you can find a brief summary and some references, just ignore the bit about the resonator. I am sure there are better references, but I can't think of one now.

http://arxiv.org/abs/0704.0727

 

[Dadface]

this is not really ambiguous. That the persistent current in the loop correspond to a certain flux follows from "classical" SQUID physics (flux quantization). you can find more about this in any standard textbook on superconductivity (say Tinkham's book).
The 3 junction SQUID used as a qubit is not in any way fundamentally different from a normal 2 junction SQUID. If you calculate the potential energy of the system you will find that the addition of an "extra" small junction gives rise to a double well potential, with the two wells corresponding to clockwise and counter-clockwise persistent currents flowing in the loop, respectively. If you then plug this potential energy into the Schrodinger equation you get what is more or less a complete description of the system (minus the effect of decoherence), anyone who has done say a first course in QM at university can do this.

I am going to be lazy and just refer to the following paper on the arXiv where you can find a brief summary and some references, just ignore the bit about the resonator. I am sure there are better references, but I can't think of one now.

http://arxiv.org/abs/0704.0727

Thank you for the reply but I would be grateful if you could clarify something.I'm fine with the correspondence between flux and current and with the superposition of two flux states. The ambiguity I referred to was that the sentence referred to earlier can be interpreted as the currents being in superposition as well as the flux states.
All I am trying to find out is whether there are any theoretical physicists who work in QM and who believe that there can be events such as an electron flowing in two opposite directions at the same time, or a bar which is vibrating at the same time when it is not vibrating,or a cat which is simultaneously dead and alive,and so on.I don't need details.Thank you.

 

[Demystifier]

All I am trying to find out is whether there are any theoretical physicists who work in QM and who believe that there can be events such as an electron flowing in two opposite directions at the same time, or a bar which is vibrating at the same time when it is not vibrating,or a cat which is simultaneously dead and alive,and so on.I don't need details.

I believe I know a lot about different interpretations of QM, and yet I think I never seen a serious proposal for an interpretation that would claim exactly that.

Yet, there is an interpretation that claims something similar - the many world interpretation. But even in that interpretation, both possibilities exist at once only after the measurement (or more precisely, after the decoherence) - that is, when the observer can see explicitly that only one of the two alternative possibilities has been realized.

 

[Demystifier]

Basically, I am working on the border between optics and semiconductor physics with a special interest in photon statistics and collective phenomena. As to the "inspiring" part I find it pretty neat that in standard qm you can treat (in rough terms) photon bunching, stimulated emission and stimulated scattering on the one hand and the exclusion principle and antibunching on the other hand on pretty much the same footing as there is just a sign changing between bosons and fermions leading to constructive or destructive interference of the probability amplitudes for events leading to species of the same type ending up in indistinguishable states. Also the "more is different" aspect of collective phenomena comes out quite clear.

Anyway I am aware that you can also treat these topics well in BM. Some of the approaches are just not my cup of tea. For example it is interesting and makes sense that degeneracy pressure can be attributed to a quantum force in BM, but it seems odd to consider the reverse process for bosons causing photon bunching. While it is consistent to treat them similarly ( and I am sure that are more sophisticated takes on the topic in BM which may be more elegant), a quantum force pushing massless photons together is just not intuitive for me.

Thanks! Basically, I agree with you: Any quantum phenomenon can be described by BM, but not all phenomena look equally intuitive with the Bohmian view.

Anyway, a recent book
http://www.panstanford.com/books/9789814316392.html
demonstrates that Bohmian mechanics may be useful in many branches of physics, even for practical people who are not interested in interpretations of QM.

 

[Dadface]

I believe I know a lot about different interpretations of QM, and yet I think I never seen a serious proposal for an interpretation that would claim exactly that.

Yet, there is an interpretation that claims something similar - the many world interpretation. But even in that interpretation, both possibilities exist at once only after the measurement (or more precisely, after the decoherence) - that is, when the observer can see explicitly that only one of the two alternative possibilities has been realized.

Thank you Demystitifier.It's interesting stuff.