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

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

Demystifier

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

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

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

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/Facu...oc/arthans.pdf

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

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".

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

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

f95toli

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

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)?



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

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.



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

f95toli

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 refering 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

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




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

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

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

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

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