Do the SQUID experiments falsify Bohmian mechanics?

[Maui]

In the experiments done with superconducting rings by Delft and Stony Brook, currents are shown to be moving in both directions simultaneously. Doesn't this falsify the idea that a pilot wave detemines a single evolution of the states?

Can that be explained without the addition of multiple worlds(on top of a guiding wave dynamics)?

 

[Demystifier]

You misunderstood something not only about Bohmian mechanics and multiple worlds, but also about the mentioned experiments.

Anyway, those experiments do not falsify Bohmian mechanics. What they falsify is the hypothesis that quantum mechanics is valid only on the microscopic scale.

 

[Maui]

You misunderstood something not only about Bohmian mechanics and multiple worlds, but also about the mentioned experiments.

I thought the experiment was a classical reiteration of the cat paradox. Could you be more specific what it is i misunderstood?

Anyway, those experiments do not falsify Bohmian mechanics. What they falsify is the hypothesis that quantum mechanics is valid only on the microscopic scale.

Can you elaborate on the relationship between observed superpositions in the experiment and the pilot wave in the BI? How do they stack up in light of the experiment and how does the determinism and single trajectories proposed by the bohmians fit in with currents going both ways at the same time? How does a pilot wave moving both ways imply determinism? Thanks

 

[Demystifier]

I thought the experiment was a classical reiteration of the cat paradox. Could you be more specific what it is i misunderstood?

Can you elaborate on the relationship between observed superpositions in the experiment and the pilot wave in the BI? How do they stack up in light of the experiment and how does the determinism and single trajectories proposed by the bohmians fit in with currents going both ways at the same time? How does a pilot wave moving both ways imply determinism? Thanks

They prepare the system in a superposition of two states, one corresponding to a current in one direction, the other corresponding to a current in the opposite direction. However, the superposition does NOT mean that the actual current goes in both directions. Instead, it means that they do not measure the direction of the current so they cannot tell what its direction, if any, is.

Bohmian mechanics defines the direction of the current even when it is not measured. In this case, depending on details of the superposition, it may mean that there is no current at all, or that the direction of the current varies with time or position. But at any given point in space at any given time, the current is either vanishing or has a single well defined direction.

 

[Maui]

They prepare the system in a superposition of two states, one corresponding to a current in one direction, the other corresponding to a current in the opposite direction. However, the superposition does NOT mean that the actual current goes in both directions. Instead, it means that they do not measure the direction of the current so they cannot tell what its direction, if any, is.

I think the point of the paper is that they measure the flux(which corresponds to currents going both ways), not the current itself.

 

[Demystifier]

I think the point of the paper is that they measure the flux(which corresponds to currents going both ways), not the current itself.

Current, flux ... whatever. If they prepare the state in a superposition of two different fluxes, it means that they don't measure flux at all, so cannot tell what the flux is. If they measure flux, then they don't have a superposition of two different fluxes at the same time.

More generally, you cannot both measure and not measure some observable at a given time. If you do measure it, then the measurement gives one value only. If you don't measure it, then only theory (not the measurement) can tell you something about the value of the observable, provided that you trust your theory. One such self-consistent theory (which you may or may not trust) is the Bohmian theory.

 

[Maui]

Current, flux ... whatever. If they prepare the state in a superposition of two different fluxes, it means that they don't measure flux at all, so cannot tell what the flux is. If they measure flux, then they don't have a superposition of two different fluxes at the same time.

But that's how a future quantum computer is supposed to work(there are technical difficulties, but this experiment shows they could be overcome). I think it's another case of researchers and technichians moving the field forward, while old dogmas are circulating within the unsuspecting academia.

Here is a short summery from Science magazine hosted at the university that carried out the experiement in Netherlands:

http://tnw.tudelft.nl/fileadmin/Faculteit/TNW/Over_de_faculteit/Afdelingen/Quantum_Nanoscience/Research/Research_Groups/Quantum_Transport/News_and_agenda/News/doc/arthans.pdf

Outside of the scope of the example, would the theory of Bohmian mechanics survive a quantum computer? If yes, how?

 

[Demystifier]

But that's how a future quantum computer is supposed to work(there are technical difficulties, but this experiment shows they could be overcome). I think it's another case of researchers and technichians moving the field forward, while old dogmas are circulating within the unsuspecting academia.

Here is a short summery from Science magazine hosted at the university that carried out the experiement in Netherlands:

http://tnw.tudelft.nl/fileadmin/Faculteit/TNW/Over_de_faculteit/Afdelingen/Quantum_Nanoscience/Research/Research_Groups/Quantum_Transport/News_and_agenda/News/doc/arthans.pdf

Journalists often write very inaccurate statements about physics, especially quantum physics. For example, in the paper above they write "... an object can be in two or more states at once", but this is simply wrong. Being in superposition is NOT being in "two or more states at once".

Even in classical physics
5 apples = 3 apples + 2 apples
but 5 apples is NOT "3 apples and 2 apples at once".

Outside of the scope of the example, would the theory of Bohmian mechanics survive a quantum computer? If yes, how?

Certainly yes. See e.g.
http://xxx.lanl.gov/abs/1012.4843
http://xxx.lanl.gov/abs/1205.2563

 

[f95toli]

But that's how a future quantum computer is supposed to work(there are technical difficulties, but this experiment shows they could be overcome). I think it's another case of researchers and technichians moving the field forward, while old dogmas are circulating within the unsuspecting academia.

When measuring qubits you never actually measure a superposition "directly"; what you usually do is to measure what the system is doing "on average" which is what they were doing in these old measurements (modern systems are much more sophisticated), they never directly "observe" the system to be in a superposition of flux states, the data is statistical in nature.
There are other ways of doing it, and more recent measurements tend to use things like qubits coupled to cavities where you can see the superposition of states as an avoided crossing; i.e. exactly what you have in atomic physics.

 

[Maui]

When measuring qubits you never actually measure a superposition "directly"; what you usually do is to measure what the system is doing "on average" which is what they were doing in these old measurements (modern systems are much more sophisticated), they never directly "observe" the system to be in a superposition of flux states, the data is statistical in nature.

The authors claim that their empirical data corresponds to currents flowing both ways, even if they don't measure the system directly. You both are basically disagreeing with their conclusion that they have "observed"(inferred without disturbing the wavefunction) mixed states based on the fact that 2 frequences have been absorbed. Do any of you have a better explanation or a rebuttal(paper or article)?

they never directly "observe" the system to be in a superposition of flux states, the data is statistical in nature.

I don't think the authors claimed that they directly observed a superposition of states. Same as with the twin slit in which you infer after-the-fact a superposition of states.
You can't observe statistics in a double slit experiment, unless you have a different definition of statistics than the common one.

 

[Maui]

Journalists often write very inaccurate statements about physics, especially quantum physics. For example, in the paper above they write "... an object can be in two or more states at once", but this is simply wrong. Being in superposition is NOT being in "two or more states at once".

Well that's what the experimenters were aiming to prove with the setup. I am not sure that "No phenomenon is a real phenomenon until it is an observed phenomenon", i find it too simplistic in light of other evidence.

Even in classical physics
5 apples = 3 apples + 2 apples
but 5 apples is NOT "3 apples and 2 apples at once".

Yes, sure, but classical analogies are not very good for quantum phenomena.

 

[f95toli]

The authors claim that their empirical data corresponds to currents flowing both ways, even if they don't measure the system directly. You both are basically disagreeing with their conclusion that they have "observed"(inferred without disturbing the wavefunction) mixed states based on the fact that 2 frequences have been absorbed. Do any of you have a better explanation or a rebuttal(paper or article)?

No, I am not disagreeing with their concludsion. What I (and Demystifier) am saying is that you can't say anything about interpretations of QM based on this (or other similar )experiments. The results agree with "orthodox" QM, as well as the Bohmian interpretation etc. You can't "directly" observe a system that is in superposition of states, but there are plenty of ways of doing it "indirectly" using statistical methods.

When these results started appearing in 1999 (the first experimenting demonstrating coherent phenomena in a superconducting qubit were done by Nakamura et al) they were interesting from a "philosophical" point of view in that they demonstrated that even "exotic" QM phenomena could be demonstrated on the macrosopic scale which perhaps wasn't obvous to everyone back then.


Btw, I am very familiar with the paper you are referring to; I've worked in this field for several year (although I haven't done anythign related to flux qubits in a few years).

 

[Maui]

No, I am not disagreeing with their concludsion. What I (and Demystifier) am saying is that you can't say anything about interpretations of QM based on this (or other similar )experiments. The results agree with "orthodox" QM, as well as the Bohmian interpretation etc. You can't "directly" observe a system that is in superposition of states, but there are plenty of ways of doing it "indirectly" using statistical methods.

What do you mean by 'statistical' method? Is the double slit done with c60 molecules a statistical method of 'observing' interference by superpositional states?

When these results started appearing in 1999 (the first experimenting demonstrating coherent phenomena in a superconducting qubit were done by Nakamura et al) they were interesting from a "philosophical" point of view in that they demonstrated that even "exotic" QM phenomena could be demonstrated on the macrosopic scale which perhaps wasn't obvous to everyone back then.

I believe this experiement clearly favors certain classes of realistic interpretations(MWI), but imo the Bohmian one fails miserably as the guiding wave isn't supposed to move in two opposite directions at once(if it's really guiding the particle even when nobody observes it). Even inference of such movement shouldn't have been possible, right? The key word is 'opposite', this is not a simple case of explaining a smeared out wavefunction.

 

[f95toli]

What do you mean by 'statistical' method? Is the double slit done with c60 molecules a statistical method of 'observing' interference by superpositional states?

It means that you prepare the system in a given state (in this case this means apply a certain B-field to get the right flux in the inner 3-JJ loop) and then repeat the same measurement many, many times (for the type of experiment Mooij's group were doing maybe 10-20 000 times); if the system is in a state |0> or |1> you always get one result, but if the system was in a superposition of states when the measurement was done you will sometimes get |0> and sometimes |1> (ideally with a 50%-50% probability).
If you plot the measurements in a histogram you will see the superpostion of states as a histogram with two peaks.

The same is true for the C60, you can't do a double-slit experiment with only one measurement, you need enough data to build up the fringes.

 

[Maui]

It means that you prepare the system in a given state (in this case this means apply a certain B-field to get the right flux in the inner 3-JJ loop) and then repeat the same measurement many, many times (for the type of experiment Mooij's group were doing maybe 10-20 000 times); if the system is in a state |0> or |1> you always get one result, but if the system was in a superposition of states when the measurement was done you will sometimes get |0> and sometimes |1> (ideally with a 50%-50% probability).
If you plot the measurements in a histogram you will see the superpostion of states as a histogram with two peaks.

The same is true for the C60, you can't do a double-slit experiment with only one measurement, you need enough data to build up the fringes.

Yes, sure, but i think the experiment does not involve running thounsands of measurements and reconstructing the fringes corresponding to superpositional states so they become visible. The experimenters' conclusion is that the absorbed frequency corresponds to currents flowing both ways(wikipedia has a short summery for those mostly unfamiliar with the effect like me):

"The magnetic flux quantum Φ0 is the quantum of magnetic flux passing through a superconductor...It is a property of a supercurrent (superconducting electrical current) that the magnetic flux passing through any area bounded by such a current is quantized. The quantum of magnetic flux is a physical constant, as it is independent of the underlying material as long as it is a superconductor. Its value is Φ0 = h/(2e) = 2.067833758(46)×10−15 Wb[4]. Here, h is Planck's constant and e is the elementary charge...There are no supercurrents present at the center of the ring, so magnetic fields can pass through. However, the supercurrents at the boundary will arrange themselves so that the total magnetic flux through the ring is quantized in units of Φ0. This is the idea behind SQUIDs, which are the most accurate type of magnetometer available. At sufficiently high field strengths, some of the magnetic field may penetrate the superconductor in the form of thin threads of material that have turned normal. These threads, which are sometimes called fluxons because they carry magnetic flux, are in fact the central regions ("cores") of vortices in the supercurrent. Each fluxon carries an integer number of magnetic flux quanta."

http://en.wikipedia.org/wiki/Magnetic_flux_quantum

 

[Demystifier]

The experimenters' conclusion is that the absorbed frequency corresponds to currents flowing both ways

That means they don't actually measure currents flowing both ways, but measure the absorbed frequency. And I insure you, Bohmian mechanics can also explain why the absorbed frequency takes the value it takes.

 

[Demystifier]

Yes, sure, but classical analogies are not very good for quantum phenomena.

Except in Bohmian interpretation, where classical analogies ARE quite good for quantum phenomena.

 

[Demystifier]

Well that's what the experimenters were aiming to prove with the setup. I am not sure that "No phenomenon is a real phenomenon until it is an observed phenomenon", i find it too simplistic in light of other evidence.

If you do not accept that "No phenomenon is a real phenomenon until it is an observed phenomenon", then you cannot base your conclusions about QM solely on measurements (such as this SQUID experiment). Anyway, what Bohmian mechanics predicts is that any OBSERVED phenomenon will be exactly the same as that in standard or many-world interpretation. Yet, different interpretations do not agree on predictions regarding UN-observed properties. But precisely because they are not observed, the experiments cannot tell us anything about them.

 

[Demystifier]

Journalists often write very inaccurate statements about physics, especially quantum physics. For example, in the paper above they write "... an object can be in two or more states at once", but this is simply wrong. Being in superposition is NOT being in "two or more states at once".

Well that's what the experimenters were aiming to prove with the setup.

No, that is not what the experimenters were aiming to prove. If you think it is, please quote a corresponding statement written by the experimenters themselves in their original scientific paper. (Sentences written by journalists or wikipedia don't count.)

 

[f95toli]

The experimenters' conclusion is that the absorbed frequency corresponds to currents flowing both ways(wikipedia has a short summery for those mostly unfamiliar with the effect like me):

No, that is NOT their conclusion. I don't remember exactly what they wrote in the paper back then, but I've read most of their papers and I have also attended several talks by Hans Mooij and members of his group. I might be wrong, but Mooij has never given me the impression that he is overly interested in philosophy. He is probably -like most experimentalists in the field (including me)- more or less agnostic (or simply does not care) when it comes to interpretations.

Note that I am not saying that there is anything wrong with "current flowing both ways" intepretation (I use it quite frequently when I explain how flux qubits work), but it is NOT what the experimental data shows.

 

[Dadface]

I'm not following this.I thought that at any particular time there are different possibilities one being that there is a current flowing in one direction and a second being that there is a current flowing in the opposite direction.If a superposition is not the same current flowing in both directions (which seems impossible and daft and to me anyway) then what is it?
Reference to a well written article or paper will be appreciated.

 

[JK423]

Dadface,
The answer is simple and no reference to an article is needed: Nobody knows what superposition means!

 

[Demystifier]

I'm not following this.I thought that at any particular time there are different possibilities one being that there is a current flowing in one direction and a second being that there is a current flowing in the opposite direction.If a superposition is not the same current flowing in both directions (which seems impossible and daft and to me anyway) then what is it?
Reference to a well written article or paper will be appreciated.

Let me give you an analogy. Consider states |up> and |down> describing spin 1/2 states with spin-up and spin-down in the z-direction. What is the superposition
|up> +|down> ?

As should be well known to anyone who studied quantum mechanics, it is NOT a particle with both up and down spin in the z-direction. Instead, it is a particle with spin up in the x-direction.

Of course, this property of spin is difficult to visualize, but we are used not to worry about that because we are used to think that classical intuition does not work at the microscopic level. But currents in opposite directions are macroscopic things for which we are used to think that classical intuition should work well. So what goes wrong? The wrong is assumption that classical intuition is always suitable at the macro level. This experiment clearly shows that it is not.

But if you still insist on having an intuitive classical-like understanding of quantum phenomena, then I cannot advice you anything else but to study Bohmian interpretation, something about I mentioned in posts above, especially in post #4.

 

[f95toli]

I'm not following this.I thought that at any particular time there are different possibilities one being that there is a current flowing in one direction and a second being that there is a current flowing in the opposite direction.If a superposition is not the same current flowing in both directions (which seems impossible and daft and to me anyway) then what is it?
Reference to a well written article or paper will be appreciated.

You can't get a definite answer for "what it is" from experiments, this is why you have interpretations.
Note that in the past few years people have started doing similar experiments with micro-mechanical resonators (microscopic "trampoline" and other geometries), we are now able to cool these in into a QM ground state and put them into a superposition. It is only a question of time before someone manages to do Rabi-type experiments with a mechanical system. Note also that these systems are BIG, you can easily see them in a normal optical microscope.
My point is that we will just have to get used to the fact that "weird" quantum effects can also be observed at the macroscopic scale.

 

[ZapperZ]

Unless I misread this entire thread, I think there are two different directions here that are being discussed. I believe that f95toli brings out the typical, "standard" interpretation of these experiments, i.e. we DO have a simultaneous current going in both directions in the SQUID experiments. This is consistent with the papers themselves in what is being interpreted from the concept of superposition and the infamous Schrodinger Cat states.

Demystifier, on the other hand, pushes for the more radical Bohmian interpretation of what superposition actually means in QM. So I'm afraid, Dadface, you are being pulled in different interpretational directions.

The one missing citation here that should be pointed out is the Leggett paper that dealt with this in painful detail (Leggett J. Phys.: Condens. Matt 14, 415 (2002)), and his subsequent summary in Leggett Science 307, 871 (2005). While the main focus of these two papers were on the progress in increasing the scale where such quantum superposition can be observed, he made clear indication on why he considers both the Bohmian interpretation and Everett-Wheeler "many worlds" interpretation are "... little more than verbal window dressing of the basic paradox..."

However, this does answer the original question, that the SQUID experiments out of Delft and Stony Brook DO NOT falsify Bohmian mechanics.

Zz.

 

[Demystifier]

I believe that f95toli brings out the typical, "standard" interpretation of these experiments, i.e. we DO have a simultaneous current going in both directions in the SQUID experiments.

Even though many "standard physicists" have a tendency to say what you are saying here, I insist that standard interpretation of QM is not saying it. Instead, standard interpretation of QM is saying that

1) In the SQUID experiments we have a SUPERPOSITION of two different eigenstates of the observable described by the current operator.
2) A superposition of eigenstates of the current operator is NOT an eigenstate of the current operator.
3) When a system is not in an eigenstate of an observable, then the value of that observable IS NOT SPECIFIED.

Thus, standard QM does not say that current goes in both directions. Instead, it says that the direction of current is not specified. And please, just because it is said by me, don't conclude that the above has anything to do with Bohmian QM, because it don't. It is standard QM, not Bohmian QM, that says that values of observables are not specified in states which are not eigenstates of the observable.

 

[Demystifier]

we DO have a simultaneous current going in both directions in the SQUID experiments.

both the Bohmian interpretation and Everett-Wheeler "many worlds" interpretation are "... little more than verbal window dressing of the basic paradox..."

The first quote above is an excellent example of a "verbal window dressing of the basic paradox". And that's exactly why we need something more than that, even if it is "only little more". In other words, it is better to use a speculative but logically consistent interpretation (e.g. Bohmian, many-world, or even a precise version of standard interpretation as in post #26) than to use a logically inconsistent one (as in the first quote above).

 

[Maui]

No, that is NOT their conclusion. I don't remember exactly what they wrote in the paper back then, but I've read most of their papers and I have also attended several talks by Hans Mooij and members of his group. I might be wrong, but Mooij has never given me the impression that he is overly interested in philosophy. He is probably -like most experimentalists in the field (including me)- more or less agnostic (or simply does not care) when it comes to interpretations.

Note that I am not saying that there is anything wrong with "current flowing both ways" intepretation (I use it quite frequently when I explain how flux qubits work), but it is NOT what the experimental data shows.

Here i am quoting the authors themselves which say the opposite(check the link below for full reference):

"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/pdf/406043a0.pdf
I must have read at least half a dozen times of experiments where superpostions have been 'observed' indirectly. Never thought i'd see such fierce opposition for something i thought was commonly accepted among the specialsts and especially experimentlists(and grounded in experimental evidence). As to what it means or would mean or whether it would be logically inconsistent(whatever that means in a quantum mechanical setting), well that'd be a whole different topic.

 

[Maui]

Well that's what the experimenters were aiming to prove with the setup.

No, that is not what the experimenters were aiming to prove. If you think it is, please quote a corresponding statement written by the experimenters themselves in their original scientific paper. (Sentences written by journalists or wikipedia don't count.)

"Here we present experimental evidence that a superconducting
quantum interference device (SQUID) can be put into a superposition
of two magnetic-¯ux states: one corresponding to a few
microamperes of current ¯owing clockwise, the other corresponding
to the same amount of current ¯owing anticlockwise."
http://www.nature.com/nature/journal/v406/n6791/pdf/406043a0.pdfTo me, this sounds like "aiming to prove that currents can be inferred to be flowing both ways at once"

I didn't read what it is that is happening in your opinion in this experiment - only what you think is NOT happening. Are you implying that their experimental evidence indicates that it's as if currents are flowing in opposite directions, but in reality they are not, unless an observation is carried out? Wouldn't that be too conspirational?BTW, i am trying to understand if the following experiement is as valid as the one being discussed:

"First quantum effects seen in visible object"

"Does Schrödinger's cat really exist? You bet. The first ever quantum superposition in an object visible to the naked eye has been observed.

Aaron O'Connell and colleagues at the University of California, Santa Barbara, did not actually produce a cat that was dead and alive at the same time, as Erwin Schrödinger proposed in a notorious thought experiment 75 years ago. But they did show that a tiny resonating strip of metal – only 60 micrometres long, but big enough to be seen without a microscope – can both oscillate and not oscillate at the same time."

http://www.newscientist.com/article/dn18669-first-quantum-effects-seen-in-visible-object.html

The author's name - Aaron O'Connell was honored with the "Breakthrough of the year 2010" by Science magazine for constructing the first quantum machine:

The first quantum machine was created on August 4, 2009 by Aaron D. O'Connell while pursuing his Ph.D. under the direction of Andrew N. Cleland and John M. Martinis at the University of California, Santa Barbara. O'Connell and his colleagues coupled together a mechanical resonator, similar to a tiny springboard, and a qubit, a device that can be in a superposition of two quantum states at the same time. They were able to make the resonator vibrate a small amount and a large amount simultaneously—an effect which would be impossible in classical physics. The mechanical resonator was just large enough to see with the naked eye—about as long as the width of a human hair.[4] The groundbreaking work was subsequently published in the journal Nature in March 2010.[5] The journal Science declared the creation of the first quantum machine to be the "Breakthrough of the Year" of 2010.[6]"

http://en.wikipedia.org/wiki/Quantum_machineAre theoretical physicists lagging behind the experiemntalists and researchers?

 

[f95toli]

"
To me, this sounds like "aiming to prove that currents can be inferred to be flowing both ways at once"

No, it does not. The two states of a flux qubit corresponds to current flowing in opposite directions (call them |L> and |R>), this is not controversial and no one is doubting that. However, it does not follow that a superposition of these states correspond to currents flowing on both directions at once.

Note that this is exactly analogues to spin, electrons can have spin up and down; but can also be put into a superposition of these states.

Another example: back when Mooij&co published their papers on flux qubits, the main compentor was the charge qubit. This is based on a small superconducting island with 0 or 1 extra charge on it (each charge can be one electron); |0> corresponds to no electron, |1> corresponds to 1 electron on the island. Hence, following this line of reasoning one could say that a superposition of these states correponds to "0 AND 1 extra electron on the island at the same time".

Modern superconducting qubits (transmons etc) are hybrids of charge and flux qubits and use more complicated "hybrid" flux-charge" states

All of these examples can be interpreted according to orthodox QM, Bohmian, MWI or whatever intepretation you like; they will ALL agree with experimental data.

 

[f95toli]

Are theoretical physicists lagging behind the experiemntalists and researchers?

No, because O'Connell &co did not actually observe their MEMS resonator while it was in a superposition of states; whenever a measurement is done, it always gives the result that the resonator is either of these states. From these measurement you can then infer that it was in superposition, but this is still an "indirect" statistical measurement.
Btw, their measurement scheme was more or less identical to the way you read out a flux qubit (it is possible to read out a MEMS resonator using a superconducting qubit as the "sensor").

Never thought i'd see such fierce opposition for something i thought was commonly accepted among the specialsts and especially experimentlists(and grounded in experimental evidence). As to what it means or would mean or whether it would be logically inconsistent(whatever that means in a quantum mechanical setting), well that'd be a whole different topic.

But "what it means" is what we are arguing about! Your claim was that these experiments favour one interpretation over the others, but what we are saying is that this is not the case. If you don't care about what it means then there is no need to worry about interpretations in the first place.

 

[Maui]

No, because O'Connell &co did not actually observe their MEMS resonator while it was in a superposition of states; whenever a measurement is done, it always gives the result that the resonator is either of these states. From these measurement you can then infer that it was in superposition, but this is still an "indirect" statistical measurement.

No question that it's an indirect statistical inference, the question is - is it valid? There is more than one that have been performed, so the experimental evidence is strong.

But "what it means" is what we are arguing about! Your claim was that these experiments favour one interpretation over the others, but what we are saying is that this is not the case. If you don't care about what it means then there is no need to worry about interpretations in the first place.

I had in mind what it meant for reality, not what it physically or observationally might mean. Superpositions are inobservable(unmeasureable) directly, but so is the Sun's surface temperature. We can only infer from other supporting circumstantial evidence about them. At any rate, the other option sounds too conspirational - currents that conspire to act as if they were flowing both ways whenever valid inference were in principle possible.

 

[f95toli]

No question that it's an indirect statistical inference, the question is - is it valid? There is more than one that have been performed, so the experimental evidence is strong.

Yes, it is valid. There are lots of groups working on similar experiments (even I have been involved in some similar work), The group O'Connell worked in was just the first to observer this. The result was not in any way surprising, everyone was just waiting for somone to do it first. There is nothing "surprising" about the result since it agrees with basic QM predictions, you just have to get used to the idea of QM effects at the macroscopic scale.

I had in mind what it meant for reality, not what it physically or observationally might mean. Superpositions are inobservable(unmeasureable) directly, but so is the Sun's surface temperature. We can only infer from other supporting circumstantial evidence about them. At any rate, the other option sounds too conspirational - currents that conspire to act as if they were flowing both ways whenever valid inference were in principle possible.

"What is real" is a very complicated question in QM, and one which can't be answered by any known experiment. Again, this is what interpretations is all about.
The problem here is that "currents flowing both ways" make no sense whatsoever accoriding to classical physics, and it is not strictly speaking what the math is saying either. Note that a flux qubit can be put into ANY state on the Bloch sphere,you also have to take the phase into account, "currents flowing both ways" only works as a classical "explanation" for some of the experiments you can do. I am not sure how you would do it for e.g. a Ramsey type experiment.

 

[Dadface]

Thanks to those who replied to my previous question.There seems to be some differences of opinion here.The quote referred to by Maui (as written in the letters to Nature) certainly seems to suggest that the currents flow in opposite directions but also seems to suggest that the superposition is that of the flux states only.

Trying to get more clarification I continued searching and came across the following:

"the most striking experimental results to date being the observation of a superposition between clockwise and counter-clockwise currents in a superconducting quantum interference device."This is part of a sentence written in the introduction of a paper authored by Y.P.Huang and M.G Moore(see paper title below)

"Creation,detection and decoherence of macroscopic quantum superposition states in double-well Bose-Einstein condensates"

Published by PHYSICAL REVIEW A,023606

This paper was published in 2006 and the letter to Nature published in 2000

 

[yoda jedi]

"What is real" is a very complicated question in QM, and one which can't be answered by any known experiment.

beyond QM.


.

 

[Demystifier]

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

That only confirms what I (and f95toli) am telling you repeatedly, but you fail to digest.

To me, this sounds like "aiming to prove that currents can be inferred to be flowing both ways at once"

Maybe it sounds so to you, but as I repeated several times here, this is neither what they say nor what they mean.

Please read my post #26, especially point 3).

If you still don't understand it, repeat point 3) until you get used to it.
After that, "superposition" should no longer sound to you as "both at once". If it still does, come here again for a new prescription.

 

[Maui]

That only confirms what I (and f95toli) am telling you repeatedly, but you fail to digest.

You keep repeating what standard quantum mechanics and the what the formalism says happens. Yet, those who actually run the experiements say, as is visible in the exact quotes, that you are wrong. Superposition of states have been indirectly observed at least several times(whether through putting the system in the ground state or measuring flux states) and the motivation to start the thread was that, to my knowledge, superpositions don't occur in the Bohmian Interpretation.

Maybe it sounds so to you, but as I repeated several times here, this is neither what they say nor what they mean.

You are entitled to your own opinion, whether right or wrong is a totally different issue.

 

[Maui]

Please read my post #26, especially point 3).

If you still don't understand it, repeat point 3) until you get used to it.
After that, "superposition" should no longer sound to you as "both at once". If it still does, come here again for a new prescription.

So if the experiment doesn't agree with the standard interpretation, the experiment is wrong, not the theory. Where did you get the weird idea that the standard interpretation is the complete and final Truth?

How different are "...corresponds to n Amperes of currents flowing both ways at once" and "...n Amperes of currents are flowing both ways at once"? Or "500 cubic meters of water displaced by a vessel, corresponds to 200 tonnes of actual weight of the ship" and " the ship is 200 tonnes of actual weight"? Does qm conspire to act as if there are currents flowing both directions? Who says what is "logically inconsistent" in Nature, as you replied earlier to ZapperZ? You?

BTW, arrogance isn't a nice way of making an argument. If anything, it degrades its value(even if it had such).

 

[Demystifier]

Maui, I admit, I was somewhat arrogant and I apologize for that. It was caused by my frustration steming from impression that you ignored what I was telling you. Now I see that you didn't ignore it. You simply did not understood what I am telling you, which may very well be my fault.

But still, point 3) in post #26 is, in my opinion, crucial for correct understanding of standard QM.

You seem to think that the SQUID experiment falsifies not only the Bohmian interpretation, but also the standard interpretation. As you can see, nobody in this forum shares that opinion with you.

 

[Demystifier]

So if the experiment doesn't agree with the standard interpretation, the experiment is wrong, not the theory. Where did you get the weird idea that the standard interpretation is the complete and final Truth?

I am saying that experiment DOES agree with standard interpretation (as well as with Bohmian interpretation).

How different are "...corresponds to n Amperes of currents flowing both ways at once" and "...n Amperes of currents are flowing both ways at once"?

That's irrelevant, because neither of those two statements can be found in the original scientific paper. In particular, they do not use the words "at once".

Does qm conspire to act as if there are currents flowing both directions?

It may look so, but only to someone who does not understand the meaning of superposition in QM (either standard, Bohmian, or many-worldian).

Who says what is "logically inconsistent" in Nature, as you replied earlier to ZapperZ? You?

Logical consistency (or inconsistency) is not a property of nature. It is a property of human language. So if ZapperZ uses a logically inconsistent language in one sentence (despite the fact that otherwise he understands QM very well), it may be useful for him to draw his attention to it. I also sometimes use logically inconsistent language, and I am grateful when somebody warns me for that.

 

[Dadface]

Even though many "standard physicists" have a tendency to say what you are saying here, I insist that standard interpretation of QM is not saying it. Instead, standard interpretation of QM is saying that

1) In the SQUID experiments we have a SUPERPOSITION of two different eigenstates of the observable described by the current operator.
2) A superposition of eigenstates of the current operator is NOT an eigenstate of the current operator.
3) When a system is not in an eigenstate of an observable, then the value of that observable IS NOT SPECIFIED.

Thus, standard QM does not say that current goes in both directions. Instead, it says that the direction of current is not specified. And please, just because it is said by me, don't conclude that the above has anything to do with Bohmian QM, because it don't. It is standard QM, not Bohmian QM, that says that values of observables are not specified in states which are not eigenstates of the observable.

If it is two eigenstates that are in superposition does it mean that there are two possible observational outcomes (clockwise or anticlockwise current direction) which are in superposition,an actual direction being pinned down only when an observation is made?

 

[Demystifier]

Superposition of states have been indirectly observed at least several times(whether through putting the system in the ground state or measuring flux states) and the motivation to start the thread was that, to my knowledge, superpositions don't occur in the Bohmian Interpretation.

EVERY state is a superposition of some other states. A state which is not a superposition does not exist. Therefore, superpositions are observed in ALL experiments on QM. Likewise, ALL wave functions in Bohmian interpretation are superpositions.

You should distinguish superpositions in general from superpositions of macroscopically distinct states. The former is impossible NOT to have, while only the later are difficult to achieve in practice.

 

[Demystifier]

If it is two eigenstates that are in superposition does it mean that there are two possible observational outcomes (clockwise or anticlockwise current direction) which are in superposition,an actual direction being pinned down only when an observation is made?

Yes, exactly, with the emphasis on the word ONLY, and specification that it must be an observation of the current itself, not of some other related quantity.

Now please, can you explain that to Maui? (Because my methods do not work for him.)

 

[Maui]

Yes, exactly, with the emphasis on the word ONLY, and specification that it must be an observation of the current itself, not of some other related quantity.

Now please, can you explain that to Maui? (Because my methods do not work for him.)

Your methods don't work with reality as well, as qm in its standard minimalist form is certainly not wrong but is incomplete. It doesn't account for the wave-like phenomenon that goes through both slits at the same time in a double slit experiment, that we infer to be a superposition of photon or electron states. As you say - your viewpoint is logically inconsistent.

In post 29, i gave you a few references of similar experiments and the authors wording word for word(see the bolded part). You are of course free to stick to your scroll of interpretational truths, which at least in these experiments, appear to belong in the past.

You seem to think that the SQUID experiment falsifies not only the Bohmian interpretation, but also the standard interpretation. As you can see, nobody in this forum shares that opinion with you.

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

That's irrelevant, because neither of those two statements can be found in the original scientific paper. In particular, they do not use the words "at once".

They used "simultaneously" and "at the same time"(see bolded the part in post 29), which is the same as saying "at once". If you don't agree, i will provide a dictionary reference. I would say that your whole paragraph is irrelevant.

Yes, exactly, with the emphasis on the word ONLY, and specification that it must be an observation of the current itself, not of some other related quantity.

Yes, with no measurement, nothing goes through both slits and the interference pattern is just God's way of saying "the standard minimalist interpretation is the truth"

 

[f95toli]

Your methods don't work with reality as well, as qm in its standard minimalist form is certainly not wrong but is incomplete. It doesn't account for the wave-like phenomenon that goes through both slits at the same time in a double slit experiment, that we infer to be a superposition of photon or electron states. As you say - your viewpoint is logically inconsistent.

Standard QM work very well and can predict the outcome of experiments with incredibly good precision. Whether or not one considers it to be "incomplete" is to some extent up to ones idea of what science should be about, Demystifier and others worry about interpretations etc. because they think science should describe "reality" (whatever that means), others(including me) don't agree or simply do not care.

In post 29, i gave you a few references of similar experiments and the authors wording word for word(see the bolded part). You are of course free to stick to your scroll of interpretational truths, which at least in these experiments, appear to belong in the past.

I don't think those were the word used in the actual article (and if it was, it was wrong). many people (including me) use simplified explanations when we explain what we work on; but in discussions like this we have to be more precise.

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

So what? I don't even see that as a problem. I am an experimentalist, as long as I can use QM to design and predict the outcome of my experiments I am happy.

Yes, with no measurement, nothing goes through both slits and the interference pattern is just God's way of saying "the standard minimalist interpretation is the truth"

[/quote]
Possibly. We can't know for sure. What is true is not a question for science.

 

[Demystifier]

In post 29, i gave you a few references of similar experiments and the authors wording word for word(see the bolded part).

I've told you explicitly in post #19 (before your post #29) that statements by journalists (New Scientist is written by journalists) and wikipedia do not count, which is why these bolded parts do not count.

I will no longer try to reply to any of your other objections, because my replies apparently don't have any positive effect on you.

 

[Maui]

I've told you explicitly in post #19 (before your post #29) that statements by journalists (New Scientist is written by journalists) and wikipedia do not count, which is why these bolded parts do not count.

I will no longer try to reply to any of your other objections, because my replies apparently don't have any positive effect on you.

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?

 

[Maui]

You should distinguish superpositions in general from superpositions of macroscopically distinct states. The former is impossible NOT to have, while only the later are difficult to achieve in practice.

So here you agree that macroscopically distinct states have been indirectly 'observed'? If so, what are you objecting to, considering you say 'observing' quantum macroscopic behavior is technically possible? If it's possible, you don't necessarily need an observation to extract information about the system and 'collapse' it to an eigenstate of the wavefunction.

 

[Demystifier]

To quote myself:

I will no longer try to reply to any of your other objections, because my replies apparently don't have any positive effect on you.

 

[f95toli]

"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

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.

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

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

And they were writing for a popular magazine, they used a common way of explaining how qubits works; but it is not an accurate description. I am somewhat familiar with the work of Luken's group; and I am 100% sure that they know that.

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?

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!