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 Schrödinger 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.