What is the relationship between quantum bits and classical measurements?

[Red_CCF]

I've never really understood how electron taking the form of a standing wave in the orbit of a nucleus can have particle characteristics (ex. momentum based on the formula p = h/lambda).

Also I could never grasp with the idea of something being both a particle and a wave at the same time and behave wave properties in some experiments and particle properties in other experiments. For example in the photoelectric effect photons act as particles while in Young's Double Slit photons act as waves only but photons, to my knowledge in the experiments above, exhibit only one characteristic (particle or wave), never both in the SAME experiment. Can someone give me a good explanation of how that actually works?

Thanks for any help that you can provide

 

[ZapperZ]

I've never really understood how electron taking the form of a standing wave in the orbit of a nucleus can have particle characteristics (ex. momentum based on the formula p = h/lambda).

Also I could never grasp with the idea of something being both a particle and a wave at the same time and behave wave properties in some experiments and particle properties in other experiments. For example in the photoelectric effect photons act as particles while in Young's Double Slit photons act as waves only but photons, to my knowledge in the experiments above, exhibit only one characteristic (particle or wave), never both in the SAME experiment. Can someone give me a good explanation of how that actually works?

Thanks for any help that you can provide

In one of the entry in the FAQ thread in the General Physics forum, there is an article on this so-called 'wave-particle duality", or lack thereof. You may want to start there.

In QM, there isn't a "duality" in terms of the formalism/theoretical description. There is only a single description of electrons, photons, etc. In this one, consistent description, all the particle-like and wave-like behavior can be obtained.

Zz.

 

[Naty1]

Hi Red:
One way to think about it is that nobody truly understands quantum stuff; we are classically wired. Another perspective is that what you detect, that is the observation made, is determined by the question you ask...by the experimental apparatus utilized. Hence double split experiment "anamolies". And you can't perform a wave and particle measurement simultaneously...they conflict with each other.

Richard Feynman used to say "shut up and compute" meaning quantum mathematical calculation procedures are agreed upon; we'll never all agree however on what the math means.

When a single photon passes thru a hole and impacts a reflective screen, it will return to it's source when reflected and not measured; if measured, then the "particle" is "deflected" (disturbed) and reflects in a probabilistic way...it usually goes off in a different direction.

When free falling towards a black hole, you "encounter" virtual photons which, carrying no net energy, are harmless and undetectable. When stationary outside a black hole horizon, those same virtual photons now appear as thermal radiation, high energy stuff, and you are fried to a crisp.

The laws of quantum mechanics are very subtle-so subtle that they allow randomness and uncertainty to coexist with conservation of energy and information.

Brian Green's FABRIC OF THE COSMOS and Leonard Susskind's THE BLACK HOLE WAR each have excellent qualitative discussions of quantum effects.

 

[Naty1]

In QM, there isn't a "duality" in terms of the formalism/theoretical description. There is only a single description of electrons, photons, etc.

(I assume this refers to schrodinger formalism? An entirely reasonable observation.
)
The model is great; it's those darn pesky experimental observations! They are, well, rude!

Debate continues after decades over various approaches ... that is, different formalisms, from Heisenberg origins, many worlds via Hugh everett, Bohm's hidden variables and Schrodinger modification via Ghiradi, Rimini and Weber. (and many others)

Still remaining within the Schrodinger based QM (which is I think is the one in the Standard Model?) is the quantum measurement problem, which is what the OP addresses.

Biran Greene makes these points IN FABRIC OF THE COSMOS, PGS 202-216):

Assuming a wavefuntion is merrily skipping along as described by Schrodinger, an experimenter rudely intervens. Suddenly, the Schrodinger equation is cast aside and we switch to a wavefuntion collapse (formalism) ... (an add on to match observations) ...it turns into a spike where a particle is "observed". Decoherence, quantum probability via interference, the very time reversability (time symmetry) of the Schrodinger equation remain elements of the unsolved quantum measurement problem...

 

[ZapperZ]

(I assume this refers to schrodinger formalism? An entirely reasonable observation.
)
The model is great; it's those darn pesky experimental observations! They are, well, rude!

Debate continues after decades over various approaches ... that is, different formalisms, from Heisenberg origins, many worlds via Hugh everett, Bohm's hidden variables and Schrodinger modification via Ghiradi, Rimini and Weber. (and many others)

Still remaining within the Schrodinger based QM (which is I think is the one in the Standard Model?) is the quantum measurement problem, which is what the OP addresses.

Biran Greene makes these points IN FABRIC OF THE COSMOS, PGS 202-216):

Assuming a wavefuntion is merrily skipping along as described by Schrodinger, an experimenter rudely intervens. Suddenly, the Schrodinger equation is cast aside and we switch to a wavefuntion collapse (formalism) ... (an add on to match observations) ...it turns into a spike where a particle is "observed". Decoherence, quantum probability via interference, the very time reversability (time symmetry) of the Schrodinger equation remain elements of the unsolved quantum measurement problem...

I have no idea what you are saying here.

As an experimentalist, I tend to pay more attention to experiment to verify the validity of a theory. So how could they be "rude" to me when I hold them in such high regards?

Anyone doubted the fact that one can also describe, say, wavelike behavior using standard photon approach needs to look at the Marcella paper that I've cited here numerous times. This clearly shows that, without having to switch gears from "particle" to "wave", we can clearly derive all those so-called wave behavior (diffraction, interferences, etc.) using the same, SINGLE, formalism.

As I've mentioned earlier, where's the "duality" here?

Zz.

 

[Red_CCF]

Thanks for the replies. I didn't realize that there was an FAQ thread, I'll definitely read it

Hi Red:
One way to think about it is that nobody truly understands quantum stuff; we are classically wired. Another perspective is that what you detect, that is the observation made, is determined by the question you ask...by the experimental apparatus utilized. Hence double split experiment "anamolies". And you can't perform a wave and particle measurement simultaneously...they conflict with each other.

Can you go into more detail on how they conflict with each other?

I have no idea what you are saying here.

As an experimentalist, I tend to pay more attention to experiment to verify the validity of a theory. So how could they be "rude" to me when I hold them in such high regards?

Anyone doubted the fact that one can also describe, say, wavelike behavior using standard photon approach needs to look at the Marcella paper that I've cited here numerous times. This clearly shows that, without having to switch gears from "particle" to "wave", we can clearly derive all those so-called wave behavior (diffraction, interferences, etc.) using the same, SINGLE, formalism.

As I've mentioned earlier, where's the "duality" here?

Zz.

This is quite new to me. I've always been taught in my QM class that any substance is both a wave and a particle. Can you go further into this? Thanks

 

[Naty1]

Zapper:

Where can I find the marcella paper?? I don't recall reading it. Thanks.

As I've mentioned earlier, where's the "duality" here?

In the results: no simultaneous wave and particle observations. one or the other.

I have no idea what you are saying here.

If you believe there is no quantum measurement issue in physics today, so be it. Many,many others disagree.

Wavefunction collapse is an artifice added on so that some sort of explanation of observational, experimental results is possible. It has nothing to do with Schrodinger formalism...it's an add on, a patch, to make things more sensible. It's like a lot of physics, say the standard model, where all sorts of stuff is patched together to explain as best we can what's going on because we really don't understand the interrelationships. Another example would be inflation being patched on to the big bang model to account for the cosmos we observe.



Red:

Can you go into more detail on how they conflict with each other?

No I can't (wish I could) but I believe it is a quantum "imponderable". Maybe Zapper or someone else can explain why you can't measure particle and wave simultaneously.

But I think of it (perhaps not correctly) as analogous to Heisenberg uncertainty where the measurement of certain parameters (like position and momentum) "conflict". I don't "understand" that but take it as valid theory. In that situation, I think the underlying math is that Heisenberg matrixes for those parameter don't commute, are non commutative...I'd love to know how Heisenberg figured THAT out!

I'd like to believe, as some do, "It's what the math tells us". End of story.

But a fundamental difficulty is that there is far more math than fits our cosmos; so sorting out the pieces of math that do fit is essential. That's why experimental results are so important. If all the math always fit, we could dispense with actual results! Even Einstein faced that problem for about ten years as he tried to pick the tensor formulation for gravity that fit our world. He considered others and rejected them because they did not fit what Einstein either thought or already knew were experimental observations. Also, some believe that physics doesn't go to to next level of detail explanation: if the math fits, that's the end of the story. I'd like to get to an "understanding" which I took to be reflected in your post.

 

[Red_CCF]

Zapper:

No I can't (wish I could) but I believe it is a quantum "imponderable". Maybe Zapper or someone else can explain why you can't measure particle and wave simultaneously.

But I think of it (perhaps not correctly) as analogous to Heisenberg uncertainty where the measurement of certain parameters (like position and momentum) "conflict". I don't "understand" that but take it as valid theory. In that situation, I think the underlying math is that Heisenberg matrixes for those parameter don't commute, are non commutative...I'd love to know how Heisenberg figured THAT out!

I'd like to believe, as some do, "It's what the math tells us". End of story.

But a fundamental difficulty is that there is far more math than fits our cosmos; so sorting out the pieces of math that do fit is essential. That's why experimental results are so important. If all the math always fit, we could dispense with actual results! Even Einstein faced that problem for about ten years as he tried to pick the tensor formulation for gravity that fit our world. He considered others and rejected them because they did not fit what Einstein either thought or already knew were experimental observations. Also, some believe that physics doesn't go to to next level of detail explanation: if the math fits, that's the end of the story. I'd like to get to an "understanding" which I took to be reflected in your post.

But then can you explain, if a something can only exhibit the property of a wave OR a particle at one time but not both at the same time, how does it "know" which state to perform as in experiments like Young's Double Slit where it photons only exhibit wave properties? How does the photon "know" that it must act like a wave in some experiments but particle in other experiments or is it simply the way we made the measurements in experiments that only considers the measurement of the properties of one state?

 

[RUTA]

But then can you explain, if a something can only exhibit the property of a wave OR a particle at one time but not both at the same time, how does it "know" which state to perform as in experiments like Young's Double Slit where it photons only exhibit wave properties? How does the photon "know" that it must act like a wave in some experiments but particle in other experiments or is it simply the way we made the measurements in experiments that only considers the measurement of the properties of one state?

There is another option -- the photon is "no thing." Detector clicks are not the result of a "click-causing particle" traveling from the source through the experimental arrangement and impinging on the detector. Rather, clicks are a subset of the detector resulting from its placement in the experimental configuration. The only "things" which exist are the beam splitters, diffraction gratings, source, detector, mirrors, etc. Not many people subscribe to this view, but those who do aren't all crackpots. Here is a nice quote from a couple Nobel Laureates and their colleague (Ulfbeck):

“Indeed, atoms and particles as things are phantasms (things imagined).”

A. Bohr, O. Ulfbeck & B. Mottelson, “The Principle Underlying Quantum Mechanics,” Found. Phys. 34, Mar 2004, p. 405.

 

[Red_CCF]

There is another option -- the photon is "no thing." Detector clicks are not the result of a "click-causing particle" traveling from the source through the experimental arrangement and impinging on the detector. Rather, clicks are a subset of the detector resulting from its placement in the experimental configuration. The only "things" which exist are the beam splitters, diffraction gratings, source, detector, mirrors, etc. Not many people subscribe to this view, but those who do aren't all crackpots. Here is a nice quote from a couple Nobel Laureates and their colleague (Ulfbeck):

“Indeed, atoms and particles as things are phantasms (things imagined).”

A. Bohr, O. Ulfbeck & B. Mottelson, “The Principle Underlying Quantum Mechanics,” Found. Phys. 34, Mar 2004, p. 405.

Yea I can see how this option is kind of radical. Everyone was brought up using classical physics which deals with the physical world.

 

[kote]

Yea I can see how this option is kind of radical. Everyone was brought up using classical physics which deals with the physical world.

It's not radical at all when you really understand the alternatives. The classical model has been falsified. It is useful at a macro level and may be the only way that we can visualize or comprehend the physical world, but it's wrong.

The wave-like properties of an object and the particle-like properties cannot logically persistently exist. The underlying substance, depending on the type of interaction it is undergoing, will manifest either wave or particle properties for the duration of that interaction. All we can measure, all we can see, is the effect substances have during interactions. We see what their properties are as they are interacting with other substances, but not what they are between interactions. Neither particle nor wave properties are basic persistent properties of matter. QM experiments prove that. We have no knowledge of any basic underlying structure, so we're forced to use these incorrect concepts in the meantime.

 

[jtbell]

Where can I find the marcella paper??

Try the "Search this forum" link at the top of the list of threads in this forum. Searching for simply the word "Marcella" should turn up a reference or link to the paper.

 

[RUTA]

It's not radical at all when you really understand the alternatives. The classical model has been falsified. It is useful at a macro level and may be the only way that we can visualize or comprehend the physical world, but it's wrong.

The wave-like properties of an object and the particle-like properties cannot logically persistently exist. The underlying substance, depending on the type of interaction it is undergoing, will manifest either wave or particle properties for the duration of that interaction. All we can measure, all we can see, is the effect substances have during interactions. We see what their properties are as they are interacting with other substances, but not what they are between interactions. Neither particle nor wave properties are basic persistent properties of matter. QM experiments prove that. We have no knowledge of any basic underlying structure, so we're forced to use these incorrect concepts in the meantime.

In the view I mentioned supra, there are "entities" fundamental to classical reality. However, these entities are not "things" like particles or waves in the sense of classical physics. Rather, the fundamental constituents of quantum mechanics are spacetime symmetries. Here is another quote on this idea from an earlier paper:

“It would appear, however, that the role of symmetry in relation to quantal physics has, so to speak, been turned upside down, and it is the purpose of the present article to show that quantal physics itself emerges when the coordinate transformations (the elements of spacetime symmetry) are recognized as the basic variables.”

A. Bohr & O. Ulfbeck, “Primary manifestation of symmetry. Origin of quantal indeterminacy,” Rev. Mod. Phys. 67, 1-35 (1995).

So, the basic idea is that the "things" of classical reality are co-constructed in a non-separable fashion from "relations" which are not themselves classical objects (in fact, relations shouldn't be thought of as "objects" at all).

 

[kote]

In the view I mentioned supra, there are "entities" fundamental to classical reality. However, these entities are not "things" like particles or waves in the sense of classical physics. Rather, the fundamental constituents of quantum mechanics are spacetime symmetries. Here is another quote on this idea from an earlier paper:

“It would appear, however, that the role of symmetry in relation to quantal physics has, so to speak, been turned upside down, and it is the purpose of the present article to show that quantal physics itself emerges when the coordinate transformations (the elements of spacetime symmetry) are recognized as the basic variables.”

A. Bohr & O. Ulfbeck, “Primary manifestation of symmetry. Origin of quantal indeterminacy,” Rev. Mod. Phys. 67, 1-35 (1995).

So, the basic idea is that the "things" of classical reality are co-constructed in a non-separable fashion from "relations" which are not themselves classical objects (in fact, relations shouldn't be thought of as "objects" at all).

Ruta, as I understand it, we're pretty much saying the same thing. Do you see a disconnect between what I said and the view you are describing? I am always interested in clarifying my thoughts / explanations. In general I'm in agreement with Bohr on the whole issue (although it took a while to actually understand his view - at least I assume I understand at this point).

 

[RUTA]

Ruta, as I understand it, we're pretty much saying the same thing. Do you see a disconnect between what I said and the view you are describing? I am always interested in clarifying my thoughts / explanations. In general I'm in agreement with Bohr on the whole issue (although it took a while to actually understand his view - at least I assume I understand at this point).

I wasn't sure if you were in total agreement with my point, since you talked about "underlying substances." The point I was trying to make is that there are no underlying entities or substances. But, frankly, I would've followed your post regardless because yours was a rare response to this idea -- pro or con -- so I'm never sure whether people understand the idea or not And it DOES address all the foundational problems of QM without preferred frames or superluminal causation, i.e., it's very reactionary, so I'm surprised no one bothers to argue it.

 

[kote]

I wasn't sure if you were in total agreement with my point, since you talked about "underlying substances." The point I was trying to make is that there are no underlying entities or substances. But, frankly, I would've followed your post regardless because yours was a rare response to this idea -- pro or con -- so I'm never sure whether people understand the idea or not And it DOES address all the foundational problems of QM without preferred frames or superluminal causation, i.e., it's very reactionary, so I'm surprised no one bothers to argue it.

I guess whether or not you believe there is an underlying substance or reality is a somewhat meaningless question. Is there something physical causing these phenomena and it's just literally impossible for us to know anything about it, or is it better to just say there's nothing there at all?

I generally go with the idea that there is an underlying structure and natural laws and the problem is epistemological. It can steer the conversation away from pure instrumentalism vs realism. Really though, that point is impossible to argue either way.

 

[RUTA]

I guess whether or not you believe there is an underlying substance or reality is a somewhat meaningless question. Is there something physical causing these phenomena and it's just literally impossible for us to know anything about it, or is it better to just say there's nothing there at all?

I generally go with the idea that there is an underlying structure and natural laws and the problem is epistemological. It can steer the conversation away from pure instrumentalism vs realism. Really though, that point is impossible to argue either way.

Absolutely, an interpretation is really only valuable to physics if it suggests new experiments and/or theory. This interpretation suggests both, but that's not an issue for this thread.

 

[DrChinese]

Anyone doubted the fact that one can also describe, say, wavelike behavior using standard photon approach needs to look at the Marcella paper that I've cited here numerous times. This clearly shows that, without having to switch gears from "particle" to "wave", we can clearly derive all those so-called wave behavior (diffraction, interferences, etc.) using the same, SINGLE, formalism.

As I've mentioned earlier, where's the "duality" here?

Zz.

Marcella paper:

http://arxiv.org/abs/quant-ph/0703126

 

[kote]

Anyone doubted the fact that one can also describe, say, wavelike behavior using standard photon approach needs to look at the Marcella paper that I've cited here numerous times. This clearly shows that, without having to switch gears from "particle" to "wave", we can clearly derive all those so-called wave behavior (diffraction, interferences, etc.) using the same, SINGLE, formalism.

As I've mentioned earlier, where's the "duality" here?

Zz.

If you consider that single probabilistic formalism to be basic, you are necessarily rejecting the basic existence of classical properties. There is nothing wrong with doing this. The duality, however, is in the visualization of this single formalism.

How can you visualize a photon that does not have a persistent / defined location, mass, velocity, polarization, etc? All of those properties are classical concepts which, by definition, have certain deterministic causal implications.

A photon with a certain persistent momentum can't bend its path around different slits. Momentum is not a persistent property of a photon. It must therefore be an emergent property of your underlying single formalism.

The problem is that we can only ever measure the classical emergent properties. At some point in the chain we have to measure and consider the classical properties of a photon - either its particle properties or its wave properties. Although neither type of property is basic, they are necessary for our measurement of the phenomena, and they are the only properties that we have access to. Because we only have evidence of underlying basic properties through emergent and fleeting classical properties, we can have no conception of what the underlying persistent properties might actually be. We are forced back to varying our conception / visualization using the dual and falsified classical concepts of waves and particles.

 

[Naty1]

Thanks, Dr Chinese for the reference...
The Marcella paper IS interesting as Zapper indicated:

...We determine the statistical distribution of scattered particles for four different slit systems. The results are in agreement with the well-known interference patterns obtained in classical wave optics.

and

Quantum interference can occur only when a large number of identically prepared particles are observed. These particles are detected at different locations, one at a time... A single particle is always detected as a localized entity and no wave properties can be discerned from it. It is interesting that for particles scattered from a double slit, the probability amplitude that gives rise to the interference is due to a superposition of delta functions.

Kote brings up an interesting point:

If you consider that single probabilistic formalism to be basic, you are necessarily rejecting the basic existence of classical properties.

I'm not sure I agree that accepting one means rejecting the other; rather, it appears to me that the "quantum" interference approach in marcella's people complements the classical wave theory approach...both give the same experimentally observed distributions. Sounds similar to the five different string theories converging via m theory...that is, different views of the same phenomena.

anyway, good discussions...

 

[Naty1]

Kote posts:

The problem is that we can only ever measure the classical emergent properties.

Is this really true? why?

 

[kote]

Kote posts: Is this really true? why?

Take the double slit experiment for example. The actual measurements are measurements of classical particles colliding with a detector. We infer that some craziness must have happened in the meantime to position them in certain patterns and place them in strange locations, but we only directly measure a collision. We know that there are strange underlying forces from inference, but we can only see the result of those forces on the classical macroscopic measurements we take. We can't measure the quantum forces directly.

Any quantum experiment will measure classical events like momentum transfers and will infer probabilistic relationships about what results the underlying quantum forces had. If we could measure and describe the underlying quantum forces directly then we could causally explain why one result happened instead of another. We would also be able to predict with certainty the results of future quantum experiments. Unfortunately, we can't do either.

Hidden variables theories arise from the knowledge that there are parts of QM that are hidden from our direct measurement. Whether or not you believe in hidden variables is just a (very interesting) question of semantics regarding the appropriateness of ascribing a real existence to something that we can inherently never test for or falsify (hidden variables, by virtue of being hidden, are unfalsifiable).

 

[DrChinese]

Hidden variables theories arise from the knowledge that there are parts of QM that are hidden from our direct measurement. Whether or not you believe in hidden variables is just a (very interesting) question of semantics regarding the appropriateness of ascribing a real existence to something that we can inherently never test for or falsify (hidden variables, by virtue of being hidden, are unfalsifiable).

Again, we are talking about non-local hidden variables. Local hidden variables can and have been probed, and found not to exist.

 

[kote]

Again, we are talking about non-local hidden variables. Local hidden variables can and have been probed, and found not to exist.

Absolutely! Thanks for the clarification. Hidden variables as a general concept are unfalsifiable. Any more specific interpretation may potentially be found to be self-inconsistent or just absurd.

 

[RUTA]

Take the double slit experiment for example. The actual measurements are measurements of classical particles colliding with a detector. We infer that some craziness must have happened in the meantime to position them in certain patterns and place them in strange locations, but we only directly measure a collision.

I don't think you can say that much with certainty. All you have is a detector click, i.e., a spacetime location, or collection thereof. That's all you KNOW at the end of the day.

 

[Naty1]

RUTA posts:

I don't think you can say that much with certainty

I agree...I don't see how post #20 answers my question...is this true:

The problem is that we can only ever measure the classical emergent properties.

Is a measured probability distribution classical??

I thought classical measures are real value functions only, quantum measures a superposition of real and complex...How about quantum computing: How do we make use of quantum bits, which can take on contradictory values at the same time, if we can make only classical measurements?

 

[kote]

Is a measured probability distribution classical??

I thought classical measures are real value functions only, quantum measures a superposition of real and complex...How about quantum computing: How do we make use of quantum bits, which can take on contradictory values at the same time, if we can make only classical measurements?

Probabilities are inferred mathematically from collections of measurements. We don't measure probabilities, we calculate them. There are plenty of examples of classical probability distributions as well... take temperature.

Also, when you take the location of a blip on a detector, you'll get a real value for a reading. You'll get the x/y coordinates of the blip. That's it. This is true of all measurements. No measuring device will ever return an imaginary number unless it is doing some complex math between taking the measurement and reporting it to you.

I agree with the comment that we can't even say a collision is taking place, all we can know is the reading on the instrument. But then again, you can't KNOW anything has happened in the physical world. You can only know the contents of your own perception :). It was probably a poor choice of words on my part to offer an explanation in terms of classical particles when earlier in the thread I said the idea of classical particles has been falsified... That language comes from the idea that they can exist temporarily, just not in their true, original, persistent sense.

 

[Naty1]

I thought classical measures are real value functions only, quantum measures a superposition of real and complex...How about quantum computing: How do we make use of quantum bits, which can take on contradictory values at the same time, if we can make only classical measurements?

Charles Seife in DECODING THE UNIVERSE (2006), an information based look at physics, discusses quantum bits (qubits) and quantum computing pages 180 -216. He says the "law of information" has not been completely established, but makes pretty clear that while we can theoretically utilize the benefits of qubits in more rapid computing, as soon as we make a measurement the superpositioned state collapses and we read a classical result.

As an example of an analogous effect, he makes note of the quantum Zeno effect: Keep measuring a radioactive nucleus over and over and you can prevent it from decaying. Each rapid remeasurement "resets" the quantum superposition preventing the nucleus from decaying. "Somehow, quantum information is tied to the laws that govern how matter behaves."