Talk:Quantum superposition/Archive 1

Archive 1

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User:65.104.118.82 added this text:

Is superposition native to any specific interpretation of quantum mechanics? How does the Kopenhagen and the Many-worlds interpretation deal with superposition respectively? -FredrikM kauss-at-aland.net

Alternative theory for the layman:

Although non-rigorous, another explanation may help the average person form a mental image of how a quantum object can have more than one value at once.

Since the speed of change between objects is limited by the speed of light, the smallest time interval that an object's characteristics can be measured ( or observed ) is also limited by the speed of light.

If we imagine a string that is vibrating at a frequency billions of times higher than light itself can travel in a nanometer, then the phase (position of midpoint) of that string would appear random every time we attempt to observe it. It would appear as if the string was holding all values until the observation event, at which time it would have only one value. This would be similar to a strobe light on a spinning fan.

When objects are connected with entanglement, this ultra-high vibration of one object is linked to its entangled partner such that there is a relationship between their quantum values. If there is a series of entanglements, then a quantum computation could occur.

Before the event of observation, the initial object would vibrate thus affecting its entangled partner at change rates far higher than the speed of light would seem to allow. Perhaps in the quantum foam, this is allowed ? In any case, qubits that are tied together are exercised through their entire range seemingly simultaneously, thus giving the impression that the output can have all values.

However, the way you get a single-valued answer out of a quantum computer is to observe the inputs over and over again until the right pattern is found. This "input search" will give the correct answer on the output when the inputs have been matched to the quantum computation machine. Each observation of the input set collapses all of the input objects' entangled partners (and their partners) thus collapsing the output to a single value.

A promising area of research is "functional entaglement" such that an object's state is a complex function of its partner's state, not simply a duplicate. This allows fewer qubits to achieve a specific formula.

This seems like unsourced original research to me, which is already enough to prevent inclusion in the article. It's also very confusing: what does vibrating at a frequency billions of times higher than light itself can travel in a nanometer mean? It should obviously be either the time it takes light to travel a nanometer or the distance light travels in a nanosecond, but neither one of those makes much sense either. Is the string actually moving faster than light or not? —Keenan Pepper 02:35, 30 June 2006 (UTC)

....................

I see your points. Well taken.

In the interests of keeping it short, I took too many liberties with the explanation. Maybe this can help:

The quantum object is not "vibrating" in the physical sense, but changing its internal characteristics. This change in characteristic is not movement, so it is not limited by the speed of light. It changes its internal state at orders of magnitude far higher than any possible physical observation can take place. (Observations are limited by the speed of light).

Because the rate of cycling through its entire range of possible characteristic states cannot be followed by fast sequential physical observations, it appears random. In the time that light moves in a nanometer (smallest distance we typically can observe), the state of the "uncollapsed" quantum object has already been through vast numbers of cycles.

The point being is that while there is no way to prove this theory via direct observation, the "simultaneous value" assumptions being used to describe some quantum phenomena may imply incorrect features in quantum computing. This alternate perspective may allow other avenues of thought in the goal of obtaining useful qubit results. —The preceding unsigned comment was added by 65.104.118.82 (talk • contribs) .

Okay, let's look at everyone's favorite example, the double-slit experiment. In this experiment, the electron has some probability of passing through either of two slits, so its position has two possible values. In your interpretation, the electron is moving rapidly back and forth between these two positions, which are separated by a macroscopic distance (say, a millimeter). The problem is that if the electron were moving back and forth, its kinetic energy would be much greater. Where would this extra energy come from? Also, your model doesn't explain the observed outcome of the experiment, which is that the electrons make a diffraction pattern on the screen, even if they are so infrequent that only one of them at a time is going through the slits. How can the electron interfere with itself as a wave if it's just moving rapidly back and forth? —Keenan Pepper 04:13, 30 June 2006 (UTC)

................

It is not moving back and forth in the physical sense. The internal characteristics are not the same as the external states such as electric charge, spin , etc. The internal characteristics arise from similar forces that allow quarks to interact. As an analogy, the speed at which you can change your mood is not related to how fast your body is moving.

But the electron has two different values of its position, an external, measurable property. How does your interpretation explain that?

.... All external properties are derived from the fluctuating internal properties at the exact moment of observation. Think of quantum particles as more like gas giant planets rather than rocky core planets. There is no surface boundary of a gas giant. An electron can be spread out in several places because its equivalent of an atmosphere is spread out. (pardon the wild analogies, but I hope to explain, not prove)

All right, this is definitely a nonstandard idea. What internal properties determine the external property of position, specifically?

Also, if the particle is spread out like a gas giant planet, that explains superposition by itself, and there's no need to bring light-speed fluctuation into the picture.

Also, electrons are not made of quarks.

.... I was not referring to quarks per se, but of the forces between them.

All right, what do the forces between quarks have to do with the internal characteristics of an electron?

Every quantum particle emits a field. Some particle fields have very short distance effects (i.e. nutrinos) and some have long distance effects (i.e. electron). After all, particles are nothing more than highly concentrated fields in which the concentration density gives the mass. Without fields there is no interaction possible.

In the double slit experiment the electron field outer fringe passes through the slit "before" the core. When the two parts of the leading portion of the field interact, it sets up a standing wave thus guiding the core one way or the other.

What do you mean by "core"?

.... There is a continuum of field concentration density from the outer fringe to the core. The core is the area of highest concentration and typically though of as the position of the particle. However, the core is not necessariy spherical, but could easily have the shape of a peanut shell.

But in the double-slit experiment, just after it passes the slits, the probability function for the electron's position has two maxima, one in front of each slit. Which one is the "core"? Or does it have two cores?

As the slits get closer to each other, the standing wave gets more definition and the interference pattern is sharper.

The fields I am referring to are the same ones that give rise to the quantum foam, where virtual particles spontaneously appear and disappear.

It would be interesting to see how much "faster" this field is compared to the speed of the particle. An experiment could be set up where a moving point charge is aimed at a stationary point charge of the same type, but at a slight angle as to cause a glancing blow. By varying the velocity of the moving charge and detecting where it begins to alter it's trajectory (taking momentum into account) it may be possible to time how long it takes for the particles to "recognize" each other and deflect.

This experiment would have the most interesting results if the moving particle was very close to the speed of light. —The preceding unsigned comment was added by 65.104.118.82 (talk • contribs) .

I have no idea what you mean by "the speed of the field" versus "the speed of the particle". How would the measurement in this experiment be performed? —Keenan Pepper 19:22, 30 June 2006 (UTC)

.... The field emanating from the particle is either at the speed of light or faster. If faster, it cannot be one of the familiar physical field such as electromagnetic, weak, or strong forces. The fields at play in the quantum foam that allow interactions between the virtual particles and between entangled photons certainly seem to be faster. The jury is still out on gravity.

This experiment could only take place in a particle accelerator. While the trajectory of one particle could not be tracked conclusively, a steady narrow stream might. First, measure the deflection of the stream at slow velocities like one-third the speed of light. On a flat target detector behind the stationary point charge, measure its deflection. Next, increase the velocity of the particles in the stream.

If the deflection goes to zero as you approach the speed of light, then there is no interaction faster than the particle itself. If there is a deflection (at the highest speed of the accelerator) and it is related to the strength of the stationary point charge, then there is some kind of interaction faster than the speed of light. Re-stated, does the deflection vary with the strength of the stationary point charge when the stream is at its highest speed possible ? ( at least 99.99% of speed of light in a vacuum).

It will be hard to separate out the effects of relavistic momentum at those speeds, but if we use heavier particles at slower speeds with the same momentum and same charge, it might act as a control reference (use a heavier isotope of the same ionic element). Keep in mind that we are trying to avoid actual collisions.

—65.104.118.82 22:34, 30 June 2006 (UTC)

I don't see how interaction at high beam energies (high particle speeds) implies interaction faster than the speed of light. Could you explain why you think that? Also, how in the world could you "avoid actual collisions"? I work at a nuclear accelerator, and I've seen the spot the beam makes on the target: it's a few millimeters across. There's no possible way to aim a beam at an individual atom, not to speak of one side of an individual atom. —Keenan Pepper 18:26, 1 July 2006 (UTC)

...... It would be similar to fighter jet flying through a canyon at the speed of sound. The pilot would never hear the echo of his jet. The results of the sonic wave interacting with the canyon wall would never catch up the the speeding plane.

If the distance between the moving particle's wavefront and the particle is less than the distance between its trajectory and the interaction point, then there can be no interaction felt by the particle (unless it is a collision). If the fields don't interact directly with each other, but only on the particles themselves, then the existence of the stationary field before the moving particle arrives should not matter.

Okay, now you're ignoring the principle of relativity. Sound waves move through a fixed medium: the air. They used to think electromagnetic waves also moved through a fixed medium (see luminiferous aether) but now we know that's false.

While I agree that getting this kind of accuracy would be very difficult, the beam could be narrowed with a hole in a thick layer of particle absorbing medium. Perhaps some kind of technique similar to integrated circuit lithography. (for electron beam type, not the UV type)

No, a narrow hole would make the beam spread out more. The particles that made it through the hole would come out the other side in all different directions. Confine the position and the momemtum becomes undefined: that's the uncertainty principle.

Besides, you don't really have to eliminate the occurence of collisions, just remove their effects from the analysis. A crescent or donut shape will form. The important measurement is the inside radius of the crescent/donut.

—65.104.118.82 22:34, 30 June 2006 (UTC)

Tell you what. Go take some courses on relativity and quantum mechanics, write these ideas up in a paper, and publish it in a peer-reviewed journal. Then I'll read it. —Keenan Pepper 19:26, 3 July 2006 (UTC)

If you don't like my comment, make an intelligent reply to it, don't vandalize this page by removing it. —Keenan Pepper 21:05, 3 July 2006 (UTC)

.... very well. The sound waves in air analogy was to illustrate a relationship at the boundary conditions - I am familiar with past ideas of aether and relativity. While its clear that a non-photon particle cannot hit the speed of light without infinite mass, there will be some time required for the interaction to catch up to the (almost) light speed particle. That would be evident in the deflection difference even it if is only a very small amount. This explains why a photon is only affected by gravity (and matter).

Ok, forget the hole. In whatever manner, the beam diameter and particle flux should be minimized at high energies. In any case, I am not an experimental physicist like yourself. Hopefully someone could create a valid experiment in the future. To get back to my original point of simultaneous values in a quantum computers, there may be alternative perspectives.

Someone just pointed out at Talk:Fermat's Last Theorem that an article's talk page is not the appropriate place for general discussion of its topic. Article talk pages should only be used for discussing possible improvements to the article. If you want to continue this discussion, let's do it at my user talk page. —Keenan Pepper 19:16, 5 July 2006 (UTC)

ok.

Lead

Any chance of including in the lead a more layman-friendly explanation, just a sentence or two before one gets to the details? zafiroblue05 | Talk 04:14, 19 May 2008 (UTC)

Questions

The following from the article is confusing:

For example, if a photon in a plus spin state has a .1 amplitude to be absorbed and take an atom to the second energy level, and if the photon in a minus spin state has a -.1 amplitude to do the same thing, a photon which has an equal amplitude to be plus or minus would have zero amplitude to take the atom to the second excited state and the atom will not be excited. If the photon's spin is measured before it reaches the atom, whatever the answer, plus or minus, it will have a .1 amplitude to excite the atom.

Should that be probability/chance/likelihood? --207.114.163.138 (talk) 18:29, 13 June 2008 (UTC)

It's correct as written. It's confusing because it is the essential (and confusing) point--- amplitudes behave like probabilities but are not probabilities. If the photon has equal amplitude to be plus or minus, then the atom will not be excited. If the photon has amplitude .707 to be plus and amplitude -.707 to be minus (note the sign change), the atom will be excited with twice the amplitude, or four times the probability. If the photon has amplitude .707 to be plus and .707i to be minus, it will be excited with sqrt(2) times the amplitude, or twice the probability.

Amplitude turns into probability only when all the states are perpendicular (in the example above when the two possibilities were i-multiples of each other), so that length-squared is additive. States are defined for the entire system, so macroscopically different states are always perpendicular, and when all states are perpendicular, the laws of quantum mechanics reproduce the laws of probability. This is called "decoherence".Likebox (talk) 21:44, 13 June 2008 (UTC)

Wiki Education Foundation-supported course assignment

This article was the subject of a Wiki Education Foundation-supported course assignment, between 20 August 2020 and 23 November 2020. Further details are available on the course page. Student editor(s): MAllison5.

Above undated message substituted from Template:Dashboard.wikiedu.org assignment by PrimeBOT (talk) 07:35, 17 January 2022 (UTC)

Vandalism on first paragraph

Someone has decided to damage the article out of frustration. "For any particular quantum system, the principle of quantum superposition states the existence of certain relations amongst states, respectively pure with respect to particular distinct quantum state analysers. This sentence was intended not to make any sense, in order that only people who already know what quantum superposition [1] is can learn what it is. There's an irony there, but you probably don't know how much. It is a fundamental principle of quantum mechanics." MDZhB (talk) 16:01, 19 July 2015 (UTC)

I have deleted the offending sentence. — Preceding unsigned comment added by JeremyDanielBenjamin (talk • contribs) 18:41, 19 July 2015 (UTC)

OhmylordJesusChristSomebodySaveme

I am using this article as an example of a case of scientific mental aberration so bad that it will take some time just to describe it. It is quintessentially ugly. And the question is, why is it here? The answer is because it is filling the void of a far simpler and better explanation that would not be allowed. It requires an explanation that is profoundly simple, that simply explains that superposition is a technical term for any system that has not yet been determined and is subject to entanglement. Then you can explain the terms and inputs and give examples from both science and magic. That's what I would do if I were allowed to write, by you Wikipedia dominators. --Xgenei (talk) 09:46, 1 May 2015 (UTC)

Incomplete, unbalanced and probably offtopic

Let us examine, of what topics the article currently consists.

• Probability theory and stochastic processes (~35%)
• Some examples with a repeating word “particle”, instead of “quantum system” (~15%)
• Path integral (~20%)
• Some stuff on eigenstates and observables, masked as a so named formal interpretation (~20%)

These mistakenly give an impression that the article is well developed and complete.

Is there an explanation about the difference between states and measurements? There is not. Does the article mention such directly relevant things (for n=2) as Bloch sphere and qubit? No. Something substantial about the light polarization? Almost no, even number of states was not given explicitly. For more than 2 states, is there something about difference between vectors and states, about a complex projective space which is an underlying mathematical structure (comparable to Euclidean space for macroscopic scale)? No such thing. About quantum entanglement which is nothing but a direct consequence of the quantum superposition? Also nothing. In other circumstances I tried myself to rewrite a text, but it would be hard for me to write such complicated things in English from scratch. Any volunteer, for whom English is a native language? Incnis Mrsi (talk) 10:19, 9 February 2010 (UTC)

Could be a bit more laymantastic (Needs to Include Writing that the Average Reader Can Understand)

Really needs sections for laymen on "how we know this is true", and "why it matters". At the moment the page basically seems to be saying "it's like probability, but for small stuff". The link to the double slit experiment, at nearly the bottom of the page, hidden amongst what (to a layman) is pure waffly white noise, just isn't enough.

I mean, if I read a news article like "someone has shown that macroscopic objects can be put into quantum superposition", I expect to come to this page and be able to find out at least something on what that means in real terms, and how it'd ever be provable, since it appears that if taking a measurement of an object is enough to make the states collapse, so you can never directly see it being superpositioned. If you can't measure it (I naively thought) you'd never be able to prove it. I still have no idea what the equivalent of the double-slit experiment for a macroscopic object would be, though. —Preceding unsigned comment added by DewiMorgan (talk • contribs) 19:28, 21 March 2010 (UTC) ~~

Wikipedia is a public use encyclopedia. Articles should be written to be intelligible to the average reader where possible. Scientific language may be used, but should be in parallel to language that the average reader can understand.

98.245.148.9 (talk) 07:21, 5 December 2010 (UTC)

-- Some topics can't be explained or discussed in language intelligible to the average reader, nor should they be, if in attempting to water down some concept the meaning is changed or even partially lost. This isn't commenting specifically on this article but on scientific/technical articles in general. If it was possible to explain everything in layman's terms, there would be no need for higher education. — Preceding unsigned comment added by 137.78.30.234 (talk) 19:54, 7 July 2014 (UTC)

Cries for improvement

I think that the message of the previous posts, well mostly, is that the lead in to this article sucks. I have rewritten the introduction. It is still not so good but I hope that it help get this article rolling. Also the section on superposition is pretty bad. I need to sleep on this article in order to figure out what needs changed and how. If anyone has some ideas lay them on me please. Phancy Physicist (talk) 07:58, 9 June 2010 (UTC)

Agreed with the above-comment. This article, as all physics-related articles, should have a cogently written summation in terms a layperson can fathom before going headlong into the esoteric equations. —Preceding unsigned comment added by 174.16.133.235 (talk) 00:08, 28 February 2011 (UTC)

Not quite, I think: while a readable lede is a good start (cough), my main problem with the page is that there's nothing here to explain why I shouldn't vote "yes" to a deletion vote. Nothing to say why it's notable, what it means, why it gets billions of dollars in funding, where the research is being done, what kind of experiments are being done, what kind of equipment is needed, what the historical advances were, who they were made by, what awards were given...

What are the potential real-world implications of this? Are there any? If there are, why are they not described on this page? Does quantum superposition impose a limit on data storage densities, and if so, does this affect us yet, and if not, when will it, if Moore's law holds? Does quantum superposition allow us to imply something about the history of the universe, and if so, what observations have been made, and what did we learn? Does it have an effect on materials science? Weather forecasting?

What are the potential real-world uses of this? Are there any? If there are, why are they not described on this page? Does it allow faster than light communications? Does it provide a way to perform massively parallel calculations on every possible value at the same time? Does it allow secure, uninterceptable communications over a distance? Is this being used by financial institutions today? Are there sensors made that use this effect, and if so, what do they detect, and why is this effect useful?

This is really exciting stuff, and yet, somehow, this page makes it seem like a tedious homework assignment, while at the same time managing not to answer even the most basic questions like "now we're superpositioning stuff big enough to see... why is that cool (other than that it's cooled to 0K), and what can we do now that we couldn't before (other than see the stuff we're working on)?"

With all the questions above in mind, it's hard not to look at the "Experiments and applications" section and just facepalm. How did these people get funding for particle accelerators if they can't even explain in English why this stuff is so exciting? There's a hell of a lot more to this subject than just a lardy dollop of math. DewiMorgan (talk) 17:59, 1 September 2011 (UTC)

Why aren't your writing the version you want to read? 1Z (talk) 18:03, 1 September 2011 (UTC)

Thank you, I'm honestly complimented that you think I could even begin to write with any authority on this. I'll take that to mean that I'm at least asking questions that look relevant. Unfortunately, you wouldn't WANT me to touch this page: I don't know the answers to any of the stuff I asked. I know only what I've read in places like New Scientist and the Discovery channel: dumbed down stuff at about the same level as the lede. I can't even answer "how do we know quantum superposition exists?" - Young's slit experiments could have many explanations, so how did we pick this one? I've no idea if particle accelerators give us any info on this stuff, either, though I mentioned them above. For all I know, all research on this is done with a couple of razor blades and a black-painted slide.

What's pretty obvious, though, from the people posting asking for a more accessible article, is that there's interest amongst the public: we know this effect exists, we know it's exciting and interesting, and we'd really love to understand more about it. But unfortunately, where the wave equation is represented as a complex vector, the probability will be extracted from the absolute value of an inner-product of the coefficient matrix and its complex conjugate realign the sensor dish through Geordie's visor. DewiMorgan (talk) 00:28, 2 September 2011 (UTC)

Amen. An exciting topic turned into mush for the average reader thanks to "advanced" mathematical notation. David Spector (talk) 21:50, 10 February 2012 (UTC)

Use good examples and stuff

Really, this is boring and there's too much math. Can't you use some better examples, like Schroedinger's cat or some cool alternate universe hypothesis, or wormholes or something? You wonder why nobody finds this interesting, it's because it is so dry. —Preceding unsigned comment added by 142.150.48.174 (talk) 22:17, 9 February 2011 (UTC)

Too Technical to be Layman and too Small to be important

I don't think quantum superposition stands well enough on its own to warrant an article. It is essentially the same as superposition with the added feature that each contributing wave is part of a singular entity and contributes with a weighting factor equal to the chance/reality of that wave. This is relatively simple for those versed in quantum mechanics but not so for those who retain a deterministic and macroscopic worldview. The importance of quantum superposition likely isn't enough to warrant a full article leading the reader through a shocking epiphany and worldview change in order to understand that a simple particle can be in more than one place at the same time (for lack of a better description) and that the amount of that particle (or the probability of it) at a certain location is a factor in determining superposition contribution. Essentially quantum superposition should be expanded on the superposition page with the relevant links to quantum mechanics. Since an understanding of QM is required to get QM-superposition it is simply impossible to put it in layman's terms. 64.114.134.52 (talk) 14:42, 4 September 2011 (UTC)

I do think quantum superposition stands well enough on its own to warrant an article. It is essentially different from a superposition of a field because, even applied to the field concept, does not imply linearity of interactions (cf. superposition principle). Moreover, there are important consequences of the q.s. for composite quantum systems, as there are tensor products of quantum states, but there are no tensor products of fields like electromagnetic or acoustic waves. Also I could say that quantum states are an illusion at all and the quantum superposition is nothing but a certain property of observables, but this would bring me to philosophical discusses. Essentially quantum superposition should not be expanded on the superposition page either with the relevant links to quantum mechanics, or without ones. Q.s. is a superposition of quantum states, which are the thing very different from physical waves but frequently confused due to historical reasons. Waves itself, as such non-layman terms as "singular entity", actually are not relevant to the phenomenon of quantum superposition. Incnis Mrsi (talk) 18:41, 4 September 2011 (UTC)

Some of these points show the difference between classical wave superposition and quantum wave superposition and should be made in this and neighbor articles. All these articles could be made a lot more intelligible to the average reader. David Spector (talk) 21:57, 10 February 2012 (UTC)

"too technical"?

"This article may be too technical for most readers to understand".

Um, how could it possibly not be? Wouldn't any simplification give false impressions? — Preceding unsigned comment added by 71.22.244.212 (talk) 20:53, 8 November 2011 (UTC)

I think that most of the mathematics could be made intelligible, just by losing some rigor. Why must we say that space is Euclidean, or Minkowski? Why must we speak of a manifold, instead of just Space? Everyone understands 3-dimensional space. It is true that non-cartesian coordinates and non-Euclidean spaces can be more natural in some situations, but why bring them up when they are not needed for understanding the topic of a QM article? Why not compare and contrast ordinary EM and gravity waves with the QM equivalents, without much math? It would be a good start. Why not motivate with the single or double slit, and maybe some polarization and some superfluidity? David Spector (talk) 22:10, 10 February 2012 (UTC)

I agree, this is a very succinct, 'general' explanation that presupposes the reader has at least a rudimentary understanding of physics, as otherwise they should be watching a YouTube explanation, not a written one. I honestly found the general explanation to be more than satisfactory, and certainly helpful to someone who was able to understand what was written, and was illuminated in the process. Many thanks to the author, Steph. — Preceding unsigned comment added by 27.33.118.194 (talk) 02:40, 11 November 2013 (UTC)

Paint analogy

The following paragraph was recently added to the "Concept" section:

Imagine making green paint by mixing blue and yellow paint; the tertiary color will be greenish blue or greenish yellow, depending on the amounts of each primary color in the mix. Yet, no matter the ratio of blue to yellow, the paint will never be pink. Suppose by closing one eye you could filter the blue from the green and see only yellow, and by the closing the other eye, filter the yellow and see only blue. Opening both eyes, green emerges. Green is a synthesis; it is neither blue nor yellow. Green is a paradox; it is both blue and yellow. If our eyes could see differently, we could perceive green, yellow and blue all together and separately, simultaneously.

Besides appearing to be original research, it doesn't provide an accurate analogy for understanding quantum superposition, so I've deleted it. Instead, a better analogy would be: Imagine mixing 20% quantum blue paint and 80% quantum yellow paint inside a can, with the contents hidden from view. Then imagine opening the can and, instead of seeing a yellowish green mixture, seeing entirely quantum blue paint 20% of the time, and entirely quantum yellow paint 80% of the time, when the process is repeated many times.

The strangeness of quantum superposition is revealed in the collapse of the wavefunction.J-Wiki (talk) 02:33, 30 January 2012 (UTC)

You probably confuse the quantum measurement with one of its consequences (Von Neumann's state vector reduction). But you are right in the point that quantum superposition should not be explained without an example with measurements in different bases. Almost incredibly, current version of the article even does not mention Stern–Gerlach experiment, which was the first experiment in the history to demonstrate the superposition in the context of different bases for a system now called q-bit. Incnis Mrsi (talk) 09:27, 30 January 2012 (UTC)

My understanding is that to say "a measurement has been made" is equivalent to saying "the wave function has collapsed":

"[T]he 'reduction of wave packets' always appears in the Copenhagen interpretation when the transition is completed from the possible to the actual. The probability function, which covered a wide range of possibilities, is suddenly reduced to a much narrower range by the fact that the experiment has led to a definite result, that actually a certain event has happened. In the formalism this reduction requires that the so-called interference of probabilities, which is the most characteristic phenomenon of quantum theory, is destroyed by the partly undefinable and irreversible interactions of the system with the measuring apparatus and the rest of the world." Heisenberg (1958)

When is this not true?J-Wiki (talk) 02:46, 1 February 2012 (UTC)

Energy level splitting

I recently improved that article. Although English is not probably good, examples with a non-diagonal Hamiltonian may be useful for improving this ugly "quantum superposition" article, as a mechanism which give some non-classical physical effects. Incnis Mrsi (talk) 16:43, 19 August 2012 (UTC)

Needs Complete Rewrite

The article is totally confused as to what a "state" means. A state is only a description of a PROBABILITY of being in a conventional classical state ON MEASUREMENT, such as a real position X, or real momentum P. The quantum state says absolutely nothing about any real position X, or real momentum of the system. Values simply cannot be assigned to a system at all until it has been measured, let alone assigning it two values at once!

|Q> = a|X1> + b|X2>

Is a PROBABILITY equation. The + sign is a probabilistic OR operator. It says that on measurement, |Q> will be EITHER X1 OR X2.

The main confusion is not understanding the distinction between |X>, the probability of having the value X, from X itself. QM only makes statements on probabilities. A system in a mixed state means that the system is in such a condition that it may result in one of several potential values when measured, in contrast to a classical system where there would be only one possibility. Summary |X> -> probability of the value X not, |X> -> X . Nothing in QM relies on any assumptions that prior to measurement a system resides in mulitple real physical states. Indeed, theorems essentially prevent such identifications. Kevin Aylward 16:37, 21 April 2014 (UTC) — Preceding unsigned comment added by Kevin aylward (talk • contribs)

References to the probability theory

This question is discussed for 3 years, and AFAIK no one favors the current situation with Quantum superposition #Analogy with probability and Quantum superposition #Quantum mechanics in imaginary time. I can preserve few examples and reasoning actually relevant to Quantum Mechanics, but demolish all unsourced researches towards the probability. Objections? Incnis Mrsi (talk) 10:51, 10 February 2013 (UTC)

Define "measured" and "observed"

To really give people a much better understanding of what superposition is, we need to define what "measured" and "observed" mean in the phrase "when measured or observed". I'm actually not sure of this myself, but the wording there is incredibly vague to the average reader. Most people think thoughts related to a ruler when they hear "measured", and think about cameras when they hear "observed". Neither of these come anywhere close to describing what collapses a superposition. Fresheneesz (talk) 05:45, 30 May 2014 (UTC)

I added a wikilink to Observation#Observational_paradoxes, to address this.J-Wiki (talk) 20:35, 7 July 2014 (UTC)

neutrality

The current lead of the article starts:

"Quantum superposition is a fundamental principle of quantum mechanics that holds that a physical system—such as an electron—exists partly in all its particular theoretically possible states (or, configuration of its properties) simultaneously; but when measured or observed, it gives a result corresponding to only one of the possible configurations (as described in interpretation of quantum mechanics)."

I think more logically, the doctrine about what exists is interpretation, not essential to the definition of quantum superposition. Partial existence is not a logical option, in ordinary language. And Bohr insists that ordinary language is the only ultimately valid one. The orthodox and perhaps the Copenhagen interpretation, as I understand them, take the ontological position that the superposition is an ontological fact of the physical world. That is to say, the cat is exactly one of dead or alive, really and factually and ontologically both at the same time, until it is observed, when it becomes one or the other, not both. The ontological physical fact is not agreed by all reasonable physicists to go that far. Unorthodoxly, some physicists think that perhaps the superposition is a way of describing a situation of which the observer does not have complete knowledge, that is to say that it is an epistemic fact. I think it is definite that the fundamental quantum physical principle is that states exist, unobserved, in which one must regard it as unpredictable which, dead or alive, will be observed. But to say what is actually the case for the unobserved state is not knowable as directly ontologically factual. It is I think the orthodox or Copenhagen interpretation that asserts that it is directly and ontologically factual that the unobserved case is in a sense in both quasi-states at the same time. This is part of the Bohr–Einstein debate. Bohr won, but he did not convince Einstein. I think Wikipedia should not dogmatize, that more or less by definition, Bohr was in ontological fact right and Einstein was in ontological fact wrong. I think the Wikipedia position should be that for many physicists, perhaps most reasonable physicists, Bohr won the debate. But I don't think it is Wikipedia's neutral point of view that because Bohr won the debate, therefore by definition Einstein was ontologically factually wrong. Superposition is real and factual, but not knowable to be so at an ontological level. The ontology is a matter of interpretation, for Wikipedia neutrality, I think. The current first sentence of the lead I think takes an unequivocal ontological position, not admitting the possibility of interpretation. I have read the Schrödinger equation, and even looked at it with my strong spectacles, and even with my rose-coloured spectacles, but I don't see the word 'ontological' in it. Still I think the Schrödinger equation is a key element of quantum mechanics, and I think it is entirely correct and reliable and real and factual. But what philosophical kind of reality or factuality? No answer. I am not making an immediate edit to that leading sentence because I think it might well be the subject of differences of opinion.Chjoaygame (talk) 07:22, 1 December 2014 (UTC)

You said you weren't planning to make any changes to the lead -- then replaced it! For easy reference, your new version is:

Quantum superposition is a physically observable relation between quantum states. It is exhibited by the phenomenon of interference. A single beam of quantal entities (for example photons) is input to a beam-splitter (such as a crystal) and emerges from it having been split into several beams. Each distinct emergent beam is pure with respect to that beam-splitter. Then a copy of that beam-splitter is placed in the path of the split beams so as to restore the initial input unsplit beam. The restored quantum state is the superposition of the several split quantum states. That the split beams are of distinct quantum states is to be demonstrated by putting in their paths objects that differentially affect their paths of flight; then an interference pattern can be observed.

Problems with this new text:

1. Quantum superposition is not physically observable. Fringe patterns are observable. It is conceivable that there could be principles, different from quantum superposition, that these patterns are consistent with. But such principles would not belong to quantum mechanics. They would be part of a different theory.

2. An example is given, but not described as such. As an example, it is necessarily limited, so it is important to describe it as an example.

3. You don't provide any references for the text.

Superposition was described by Paul Dirac as a principle. What do you consider wrong with that description?

I agree that the lead that was there should be adjusted to address the issue of ontology that you rightly point out, but the new lead you wrote is not a suitable replacement.J-Wiki (talk) 02:07, 2 December 2014 (UTC)

Thank you for this. I am sorry for the delay of this response. The cable internet connection has been out of action for upgrade in the past day.

I am glad to read your careful comments. Happily we seem to agree that the word "exists" (or the phrase "exists partly") is a ground for concern.Chjoaygame (talk) 05:43, 3 December 2014 (UTC)

• "Quantum superposition is not physically observable." Fair comment, but not necessarily the ultimate judgement.

The term 'observation' in my sentence refers to the relation between states. Indeed as you say, fringe patterns are directly observable. But observation of a relation is often a more abstract thing. For example, one can observe that the cat is on the mat. That is their relation. The word "on" is a preposition that tells of a relation. So I would say that in the context of the whole sentence (the right context to consider), it is ok to say that the relation is observable. Ordinary language is not compositional, but is to be constructed and construed sentence by sentence, not word by word, with one and only one meaning and construction for every word. For mathematics, one has a near approach to compositionality. But ordinary language is not mathematics. And fringe patterns are often in fact composed of many dots in a particular kind of relation to one another. So if one wanted to say that a relation is not observable, one would have to accept that fringe patterns are unobservable. In the very narrowest sense, I think, subject to review, that for quantum physics, the only thing that is strictly observable is a detection of a single quantum by a detector, or perhaps more broadly a count of successive detections by a detector. Just for example, Dirac says "When we make an observation we measure some dynamical variable." It is not quite obvious that observing a fringe pattern is measuring some dynamical variable. Perhaps it is, somehow? I am not sure whether or not Dirac alone is an ultimate authority.

Here I stand by the sentence.Chjoaygame (talk) 05:43, 3 December 2014 (UTC)

You wrote: "quantum superposition is a physically observable relation..." It is not a relation. It is a principle regarding relations. Principles are not physically observable. Hence, quantum superposition is not physically observable.J-Wiki (talk) 06:27, 4 December 2014 (UTC)

You go too far in saying it is not a relation. Obviously it is a relation, at least in many usages. You choose to read it as a principle instead; a defensible choice, and one that of course also occurred to me; but I think not the best choice. You take a particularly hard line in your claim that principles are not physically observable, but I don't need to refute it, since I am not saying that the word superposition primarily means 'principle'.Chjoaygame (talk) 22:10, 4 December 2014 (UTC)

• "Superposition was described by Paul Dirac as a principle."I think more precisely he talked about "the principle of superposition of states".

I didn't find him anywhere writing the exact words "superposition is a principle". He felt it necessary to use the phrase "principle of superposition". The principle is a principle of something. On the construction that superposition is a principle, one would then read 'principle of principle'. Not a good layering of levels of abstraction. Indeed there is a principle of superposition, but a thing is what it is, and something else. If there is to be a principle of superposition one needs a notion of superposition on which to define the principle. Of course, I agree that the principle of superposition is a fundamental principle of quantum mechanics, but this article is not entitled Principle of superposition. This article is entitled Quantum superposition. I accept that these remarks are trivial, but we are talking fine points here, and I think it fair to make trivial points in this context.

My concern is not that I am worried about the idea that the principle of superposition is a fundamental principle of quantum mechanics. I am worried about the rest of the sentence that stood in the version that I edited. As I write this it has been restored and is still there and currently reads "Quantum superposition is a fundamental principle of quantum mechanics that holds that a physical system—such as an electron—exists partly in ..." My concern is that the principle is stated in a non-Wikipedia way, that is to say a non-neutral way. The principle is stated with a very definite interpretation, the hard-line "orthodox interpretation". I am not sure how that may or may not relate to the Copenhagen interpretation. But interpretation it surely is. That was my reason for editing. More precisely, my concern is with the word "exists". I didn't find Dirac saying that the principle is that such and such exists. The key to this principle is relation, not existence. So I think the article should emphasize relation, and admit that what actually exists is a matter of interpretation of the formalism. That was the burden of my pre-edit talk-page comment.Chjoaygame (talk) 05:43, 3 December 2014 (UTC)

Dirac's use of the phrase "the principle of superposition" implies that superposition is a principle. See Merriam-Webster definition 8a of the word "of": "used as a function word to indicate a particular example belonging to a class denoted by the preceding noun <the city of Rome>".

Although Dirac didn't use the word "exist" (regarding the states themselves, rather than relationships between them), he did imply it: "whenever the system is definitely in one state, we can consider it as being partly in each of two or more other states." His use of the words "is" and "being" are equivalent to using the word "exist". Merriam-Webster definition 2a of the word "be": "to have an objective existence".

That said, insofar as the principle of quantum superposition is used in other interpretations of quantum mechanics, I agree that the word "exist" as presently used in this article may be problematic and require correction.J-Wiki (talk) 06:27, 4 December 2014 (UTC)

It is the word 'exist' that makes it "Copenhagenist". It is the "Copenhagenism" that makes it non-neutral. I will wait to see you remedy the fault.Chjoaygame (talk) 07:55, 4 December 2014 (UTC)

You choose to read Dirac's use of "of" as appositional, but I think it not a good choice. Dirac actually wrote "[The principle] requires us to assume that between these states there exist peculiar relationships ..." On many occasions he uses the word superposition to refer to these relationships. I think it is better to read this as its primary reading. One thinks of the the principle of conservation of energy. Again, conservation is better read as a relation. The principle asserts its occurrence or actual existence in certain circumstances. I think it a bad idea to telescope the meanings in the way that you propose. I was mistaken to write above that "I didn't find Dirac saying that the the principle is that such and such exists." I didn't find him saying that the states exist; but on looking again I did find him saying that the relation exists. Superposition is primarily the name of the relation, and derivatively the name of the principle. So my reading is well sourced in Dirac.Chjoaygame (talk) 22:10, 4 December 2014 (UTC)

• "It is conceivable that there could be principles, different from quantum superposition, that these patterns are consistent with. But such principles would not belong to quantum mechanics. They would be part of a different theory."

The article is entitled Quantum superposition not Superposition in quantum mechanics. There are other kinds of quantum theory than quantum mechanics, if you count de Broglie's theory as different, or if you count quantum field theory as different. You are here offering some commentary about other principles that the patterns fit. My sentence did not stray into that territory, so at present I see it as a distraction and not needing a response. Perhaps you will clarify if necessary?

I was not offering any commentary, merely explaining why it is a principle.J-Wiki (talk) 06:27, 4 December 2014 (UTC)

• "An example is given, but not described as such."

I wrote "A single beam of quantal entities (for example photons) is ..." I am not clear why you say "not described as such".Chjoaygame (talk) 05:43, 3 December 2014 (UTC)

You provide photons as an example of quantal entities! Where do you say that the entire scenario you describe is an example of quantum superposition?J-Wiki (talk) 06:27, 4 December 2014 (UTC)

• "You don't provide any references for the text."

This is true. But it isn't necessarily decisive. The version that I replaced did not cite a reference. In the lead about abstract matters, it is common enough that the lead is accepted as a summary that is not immediately supported by citations.Chjoaygame (talk) 05:43, 3 December 2014 (UTC)

The version you had replaced is a summary of information that appears in the main body of the article, in particular, the quote from Dirac, which is referenced. This is not true of the sentences you added.J-Wiki (talk) 06:27, 4 December 2014 (UTC)

See above for my defence of sourcing in Dirac.Chjoaygame (talk) 22:10, 4 December 2014 (UTC)

• You conclude "I agree that the lead that was there should be adjusted to address the issue of ontology that you rightly point out, but the new lead you wrote is not a suitable replacement."

Maybe. The problems you find in my edit are in my view not fatal objections, and perhaps do not add up to a justification for undoing it. You haven't offered ways that you might find acceptable to improve my edit. I will think about it.Chjoaygame (talk) 05:43, 3 December 2014 (UTC)

I did offer ways to improve it: State that the example you provide is an example. Don't say that quantum superposition is observable. But keep the previous text that explains that it is a principle of QM and summarizes what it holds, but let's correct the problem with the use of the word "exist". For now, I'll simply mention that the summary is according to the Copenhagen interpretation.J-Wiki (talk) 06:27, 4 December 2014 (UTC)

Perhaps I should clarify by actually giving a statement of the principle of superposition, as different from saying what superposition is. The latter is the subject of the present article. The principle of superposition states that "superposition occurs in quantum physical systems, and is represented in the quantum mechanical formalism". The principle, I think, is more along the lines of an assertion of occurrence than of a definition or description.Chjoaygame (talk) 08:57, 3 December 2014 (UTC)Chjoaygame (talk) 22:21, 4 December 2014 (UTC)

Usage of the word is very often as meaning relation. For example, <David, F. (2015). The Formalisms of Quantum Mechanics: an Introduction, Springer, Cham, ISBN 978-3-319-10538-3.> at p. 61 "Linear superpositions of pure states ...", at p. 68 "the superposition of two states", at p. 68 "a linear superposition of two states"; at p. 137 "in a superposition state". At p. 26 we read "This is the so-called superposition principle." The writer is directly indicating that the term superposition is used here as an adjective, obviously derived from its basic meaning as relation. He writes "Pure quantum states are rays of the Hilbert space ... Of course a consequence of this principle is that ... This is the so-called superposition principle." This indicates that he sees the "superposition principle" as derived from a more general principle, and in sense therefore he is not accepting it as truly a first principle.Chjoaygame (talk) 00:18, 5 December 2014 (UTC)

Big defect of article is failure to clarify how superposition is different from mixture

A big defect of this article is that fails to clarify how mixture and superposition differ.Chjoaygame (talk) 09:54, 4 December 2014 (UTC)

new animation

I would like to undo this edit which added an animation. It may be a fine thing in some suitable context, but seems to be a gee-whizz promotion here. Superposition by resonance and decoherence measurement by Rabi oscillation are not dealt with in this article. Consequently the animation is out of place here.Chjoaygame (talk) 17:50, 16 January 2015 (UTC)

virus could perhaps be prepared in a pure state

I am here responding to this edit, which is covered by the edit summary "removed speculation from unqualified/poorly informed author".

The removed comment was unsourced and so vulnerable to arbitrary removal.

Never mind whether that "author" is or is not "unqualified/poorly informed".

The physics is straightforward. A living organism in general continuously metabolises, and so cannot be prepared in a pure quantum state. Therefore it is not covered by the simple principles of superposition. But a virus does not metabolise. It is practically a crystal, perhaps a liquid crystal. So it could in principle, and quite likely in practice, be prepared in a pure state. Then it would be covered by the simple principles of superposition, and it would make sense to try to demonstrate this experimentally, according to suggestion that is cited in the article. But if, contrary to fact, a 'virus' were to metabolise, then, again contrary to fact, it would not make sense to use it to try to exhibit interference experimentally. So even if the "author" should be "unqualified/poorly informed", the deleted editorial comment is valid as physics.

The deleted comment would be useful, because for the reader it would clarify the physical scope of superposition.Chjoaygame (talk) 04:47, 23 April 2015 (UTC)

The deleted comment was not valid. Complex systems, living systems, crystals, viruses, bacteria can all be put into quantum superposition as per leading theory so long as they are properly isolated. That they could not be is a misconception stemming from the belief in a line of demarcation between living and nonliving systems arising between the level of complexity separating viruses from bacteria.

This is very fundamental QM. That it "only makes sense because ____" to you is wrong at best, speculative and unsubstantiated at worst. — Preceding unsigned comment added by MintyFreshBreath (talk)

It is good that you care about accuracy in Wikipedia, and that you deal with it on the talk page. You can most easily sign your posts here by using the four tilde format. I have added the colons to maintain the usual format for talk pages.

You propose that the comment was based on a "misconception stemming from the belief in a line of demarcation between living and nonliving systems arising between the level of complexity separating viruses from bacteria". No, that was not the basis of the comment. It was perhaps a fair guess at what someone might have been thinking, but it wasn't the basis of the comment.

The comment already gave its reason, that viruses don't metabolise. That is not a matter of "level of complexity" pure and simple. It is a fact about viruses. It means that a species of virus can be prepared in a state that doesn't change its molecular form, and that every single virus of the species is of that one and the same molecular form. This means that one could perhaps prepare a beam of virus particles in a quantum mechanically pure state. Such a state is needed to demonstrate superposition. Bacteria are living in the sense that they metabolise, meaning that they continually take in molecules and give out other molecules. They are thus continually changing their molecular constitutions, even while each bacterium maintains its individuality. In general, each bacterium has a slightly distinct molecular individuality. In general, this rules out the experimental possibility of preparing a beam of bacteria in a quantum mechanically pure state.

The concept of "level of complexity" is not involved here. It is too vague to be decisive, and moreover not relevant. The concept of metabolism is here decisive. Viruses replicate by way of the metabolism of living cells, but viruses as such don't metabolise. That is why they might be experimentally prepared in a quantum mechanically pure beam.

The phenomenon of superposition refers to pure states in the quantum mechanical sense. It is not just a matter of "isolation". It is a matter of identity of molecular form. That is needed for quantum mechanical state purity, and for demonstration of superposition. This is the relevant quantum mechanical fact.Chjoaygame (talk) 03:00, 3 May 2015 (UTC)Chjoaygame (talk) 03:06, 3 May 2015 (UTC)Chjoaygame (talk) 03:13, 3 May 2015 (UTC)

I do not know what your motivation here is, but you were the person who added the statement "This makes sense only because a virus is not in the ordinary sense of the word a living thing. It is a static arrangement of molecules that does not metabolize, whereas metabolism is one of the essential features of living things." to the page on March 3, prompting me to create an account to correct this misinformation. The statement did not add itself.

If you believe that metabolism (a form of complexity) prohibits a quantum system from being put into superposition, then you are wrong. No limits to the "scope" of quantum superposition have been discovered, regarding mass or changes in the chemical arrangement of the particles involved.

If you are getting this implication from a textbook or a popular science book of some kind, please post it here.

For the time being, the experiment regarding viruses in superposition should be included in this list (without speculation). It has been published in formal journals and that TechnologyReview article linked the proper Arxiv paper associated to the proper author after a layman's explanation.

"Matter-wave interference with particles selected from a molecular library with masses exceeding 10000 amu" (S Eibenberger, S Gerlich, M Arndt, M Mayor, J Tüxen) should also be listed.

As well, "3,000 atoms entangled with a single photon" (V Vuletic, R McConnell, H Zhang, J Hu, S Cuk) belongs in this list for the interested reader.

MintyFreshBreath (talk) 06:16, 3 May 2015 (UTC)

I am sorry to need to argue with you here. You are asking me to do your work. The editor who wants to post an item has the duty of justifying it. That is why you could delete my post: that I had not spent the necessary time supporting it with reliable sources. Now you are asking me to explain superposition to you, and you seem to intend to try to post something on the basis of perhaps slender or inadequate sourcing. ArXiv is not a reliable source in general. Perhaps you are also changing the subject of conversation, to judge from your just above references? My comment was not about molecular weight, as you are now perhaps referring to. My comment was about metabolism.

It is not quite clear what you think is misinformative in my comment. Are you saying that viruses metabolize? Are you saying that living things do not metabolize? Are you saying that a virus is not nearly enough a static arrangement of molecules?

My concern is that newsletter article on viruses suggested that not only viruses, but indeed living things, could be made experimentally to exhibit superposition. I accept that it might perhaps be possible with viruses, because they do not metabolize, but not with living things that are metabolizing.

It is evident in what you write that you are confusing in your mind between change and complexity. Metabolism is complex, but the crucial point here is not whether it is or is not complex, as you suppose. What matters here is whether it is or is not a form of stochastic molecular change that would prevent a preparation of a beam in a pure quantum state. At best for your case, a metabolizing living thing is in a quantum mechanically mixed state, not a pure quantum state. One cannot arrange things so that a molecule of a metabolite shall enter or leave living cell in a quantum mechanically pure way.

There is a difference between living cells such as living bacteria, and viruses. It is not here relevant here how relatively complex they are, as you are proposing. It is whether they exhibit unpredictable or stochastic molecular change. It is the speculated possibility that a beam of viruses could be prepared not showing unpredictable molecular change. I am saying, without prejudice, that such a speculation might be right. At the same time I am saying that one could not prepare a beam of living bacteria not showing unpredictable or stochastic molecular change. That is why the proponents of the speculated experiment are proposing to do it on viruses, not on living bacteria. To negate what I am saying here, you would need to propose a means of preparation of a beam of bacteria the would be pure to the last molecule and the the last spatial arrangement of its constituent molecules. I think you won't be able to do that.

As for superposition, may I recommend to you the textbook of P.A.M. Dirac, The Principles of Quantum Mechanics. In some ways, the first edition (1930) has its advantages, though mostly one relies on the fourth edition (1958). The first chapter is largely about superposition. I will here quote only a couple of leading ideas.

In the first edition, Dirac is a little more accommodating of the reader's need for a simply physical account. He writes of "any" state, but he means "any of a relevant class of states", as he makes clear by writing: "This, of course, is true only provided the two states that are superposed refer to the same beam of light, i.e. all that is known about the position and momentum of a photon in either of these states must be the same for each."

Dirac tells us that: "Each photon then interferes only with itself. Interference between different photons never occurs."

It would be too much for me here to quote further from Dirac. The fourth edition is the easiest to access, if you don't already have a copy.

• Dirac, P.A.M. (1930). The Principles of Quantum Mechanics, 1st edition, Oxford University Press, Oxford UK.
• Dirac, P.A.M. (1935). The Principles of Quantum Mechanics, 2nd edition, Oxford University Press, Oxford UK.
• Dirac, P.A.M. (1947). The Principles of Quantum Mechanics, 3rd edition, Oxford University Press, Oxford UK.
• Dirac, P.A.M. (1958). The Principles of Quantum Mechanics, 4th edition, Oxford University Press, Oxford UK.Chjoaygame (talk) 11:17, 3 May 2015 (UTC)

I'm not going to play games with this, Chjoaygame. If your first language is not English, then I understand your confusion. However, the potential of any system to be put into quantum superposition is absolutely fundamental to students learning QM; they are the ones who browse Wikipedia before choosing a course of study. Asserting that living things (virus, bacteria, your neighbor's cat) could not be put into QM because it disagrees with your personal belief based on a hard-to-falsify hypothesis is absolutely irresponsible on your behalf.

If you are able to find a credible author to support your opinions on this matter, I recommend that you create a new article to discuss this hypothesis.

Also, the Arxiv paper which was referenced by TechnologyReview has been published by IOPScience.

MintyFreshBreath (talk) 22:41, 3 May 2015 (UTC)

"[T]here is no reason why it should not be possible to observe quantum superpositions of living systems someday. For example, there is no fundamental reason why one should not be able to observe a quantum double-slit experiment for an amoeba or a very small bacterium. Surely, many experimental challenges would have to be surpassed first. For example, we would have to invent ways to shield such a small living creature from the hostile environment of the experiment. All of these experiments would have to happen in a vacuum. More generally, a system showing quantum interference should not interact with the general environment, as this would destroy its quantum state. Now a living system does not like the idea of living in a vacuum or of being completely isolated. It needs nutrition, it needs oxygen from the air, it needs to live at a certain temperature, and so on. But if we just play the game of science fiction for a moment, we can easily imagine that using nanotechnology, we might build a small housing around the small bacterium or amoeba that protects it from the environment. Such a living being could then survive the flight through a vacuum apparatus and similar challenges. Again, this is science fiction, but there is no reason why we should not succeed someday." (Emphasis added.) - Anton Zeilinger, Dance of the Photons (2010), p. 249.

J-Wiki (talk) 18:58, 4 May 2015 (UTC)

There are users just above who seem to think that a living thing could have a wave function, and that viruses are alive even though they don't metabolize. The above quote from Zeilinger, though it equivocates by saying "this is science fiction", includes the following relevant and valid remark: "More generally, a system showing quantum interference should not interact with the general environment, as this would destroy its quantum state." It is relevant here because a living thing always interacts with the general environment, that is to say, metabolises. A virus can be prepared in a temporary but reasonably prolonged non-interacting state without impairing its future ability to infect. So it could perhaps be made to exhibit quantum interference. By the same token, it is not alive. Nobel Prize winner for quantum mechanics, Willis Lamb, writes: "Only a very dead and thoroughly isolated cat could have a meaningful wave function."

The key here is neither in the size of the object, nor in its complexity or non-complexity; it is whether it is a living or non-living thing. I think this is simple and obvious (and that Lamb thinks so too), and that it does not call for a Wikipedia article of its own.Chjoaygame (talk) 15:01, 12 September 2015 (UTC)

deleted speculative item that was probably misleading

I have deleted the item about a speculated experiment with viruses. There are several reasons for my deletion. First, the list it appears in announces that it is of actually performed experiments, while the removed item is speculative. Secondly, the item says it is about "living things", while viruses, though biological, are not strictly living things, or at least they differ in a very important respect from nearly all "living" things, namely, that ordinary living things metabolized while viruses do not. The difference is profound and closely relevant, because metabolism rules out pure states, which are necessary for superposition. Thirdly, the reference is to a newsletter, not a refereed journal article.Chjoaygame (talk) 01:26, 30 April 2015 (UTC)

living thing?

There is a temptation to put in 'gee-whizz' items. In particular, I refer to the claim implicit in one example here that viruses are living things. And now we have as another example a proposal to put a bacterium into a state that has a wave function. At best, such a state would be one of suspended animation. I have not bothered to struggle with the editor who passionately claims that viruses are alive, apparently because they are complex, while avoiding the fact that they don't metabolize. The new proposal about bacteria, in its cited text source, says they would be alive. The key physical fact is that for superposition, one needs a wave function, as stated clearly by Willis Lamb (see just above). This is so obvious that not many writers would bother to say it explicitly, and yet it does seem to escape the attention of too many. The present use of two such examples, without note that they rely on at least suspended animation, puts us in near 'gee-whizz' territory. I would prefer to see both removed because of their dodgy form, but I don't intend to struggle over it.Chjoaygame (talk) 00:49, 22 September 2015 (UTC)

"Concept"

Most of the material in the "Concept" section needs either a complete rewrite, or should simply be removed. For example the term "pure state" has a very specific meaning in quantum mechanics, and has little to do with anything described there. For a specific example, "pure with respect to the filter" is gibberish - a quantum state is either pure or mixed, there's nothing relative about it. The reference given (Messiah) says nothing that supports what is written. Another example: "For perfect superposition it is essential that the intermediate beams are mutually coherent." Again, this is gibberish - superposition is an exact principle, it's never "imperfect", nor is it necessary for beams to be "coherent" for their corresponding quantum states to be superpose-able. Given the major problems with these sections and the lack of sources that support what is written, the content there now detracts significantly from the article and from the understanding of anyone seeking to learn from the article. Therefore I am deleting the entire section. Hopefully it can be replaced later with a correct version. Waleswatcher (talk) 23:08, 5 November 2015 (UTC)

Something to add - there is really no reason to discuss "beams" at all, especially not in a "concept" section that is supposed to gently introduce the idea of quantum superposition. Superposition is a totally general concept in QM that applies to everything from individual isolated particles to macroscopic systems (assuming QM applies to them). It's far more general than "beams". Also, a minor note on English usage. "Quantal" is really not a common word, either among professionals or lay-people. Best to avoid it. Waleswatcher (talk) 23:13, 5 November 2015 (UTC)

It is always a pleasure to be corrected by you.Chjoaygame (talk) 01:00, 6 November 2015 (UTC)

I didn't check who wrote those sections - but if it was you, you're welcome. Please refrain from edits to wiki articles about quantum mechanics in the future, because it is apparent that you do not understand the basics of the topic (that's not meant to be insulting, very few people do). Thanks in advance. Waleswatcher (talk) 01:20, 6 November 2015 (UTC)

Silly old Feynman, he also could do with some correction by you.<Lectures, volume III, page 5–5>: "The answer is this: If the atoms are in a definite state with respect to S, they are not in the same state with respect to T—a +S state is not also a +T state. There is, however, a certain amplitude to find the atom in a +T state—or a 0T state or a −T state."[Feynman's format of —]Chjoaygame (talk) 01:14, 6 November 2015 (UTC)

What Feynman says is correct (assuming S, T are non-commuting operators). It has nothing to do with the material that I deleted, or with whether the state is pure or mixed. Waleswatcher (talk) 01:20, 6 November 2015 (UTC)

Quantal response equilibrium, Quantal theory of speech, Quantal neurotransmitter release.Chjoaygame (talk) 01:25, 6 November 2015 (UTC)

None of those have anything to do with quantum mechanics, which makes my point for me. To repeat myself, I'm not saying it's not a word, I'm saying it's not the right word in this context. And in any case that's the least important of the points I made above. Waleswatcher (talk) 03:43, 6 November 2015 (UTC)

It's always a pleasure to be corrected by you. Hopefully, the section "can be replaced later with a correct version" by you.Chjoaygame (talk) 03:58, 6 November 2015 (UTC)

"much like waves in classical physics"?

Dirac 4th edition (1958), p. 14: "It is important to remember, however, that the superposition that occurs in quantum mechanics is of an essentially different nature from any occurring in the classical theory, as is shown by the fact that the quantum superposition principle demands indeterminacy in the results of observations in order to be capable of a sensible physical interpretation."

Dirac 4th edition (1958), p. 17: "The assumption just made shows up very clearly the fundamental difference between the superposition of the quantum theory and any kind of classical superposition."

The second sentence of the lead of the present article currently reads "... much like waves in classical physics, any two (or more) quantum states can be added together ..." How much like?Chjoaygame (talk) 15:31, 21 November 2015 (UTC)

Very much like, in precisely the sense stated. The biggest difference is that ANY two states can be superposed, not just linear waves. Waleswatcher (talk) 12:18, 30 November 2015 (UTC)

Silly old Dirac, how wrong he was!Chjoaygame (talk) 12:46, 30 November 2015 (UTC)

He's not wrong, nor does what is written in the article now contradict what he says in either of those places. On p.17 he is pointing out that QM states must be normalized, so adding a state to itself doesn't change the state, where as superposing a classical wave with itself changes the amplitude. That doesn't conflict with the statement that waves can be added together, and so can QM states. Furthermore, the mathematics (solutions to linear diff. eqs. can be added together, and the result is still a solution) is identical in the two cases. Waleswatcher (talk) 15:45, 30 November 2015 (UTC)

It remains incongruous that Dirac repeatedly points out the profound differences between classical and quantum superposition, whereas the present lead of the present article seems to minimize or hide it. Classical waves do not exhibit wave–particle duality, a prime fact exhibited by quantum mechanics.Chjoaygame (talk) 18:13, 30 November 2015 (UTC)

This article is about quantum superposition. There is a very close connection between quantum superposition and the superposition of classical linear waves (mathematically, they are exactly the same thing). Yes, there are differences between quantum wave functions and classical waves, and I certainly don't object to pointing them out. However, bear in mind that the differences aren't really connected to superposition itself so much as they are to the interpretation of the wave. Waleswatcher (talk) 20:11, 30 November 2015 (UTC)

The Dirac passage quoted above is specifically about the addition of a state to itself with two different coefficients. In classical waves the result is a wave with different magnitude; Dirac gives the example of a drum head. In quantum waves, the magnitude is a probability amplitude; the scalar multipliers in QM are the eigenvalues. Johnjbarton (talk) 21:51, 5 November 2023 (UTC)

Experiments

Maybe we could eliminate the whole section Experiments and applications. Of course these are some examples, but isn't any QM experiment a demonstration of quantum superposition? All QM experiments are under the assumption they follow somehow Schrodinger equation or some version of Dirac-von Neumann axioms. I agree we could leave some examples but not as experimental evidence and only if it is argued why is a good example of Q superposition, like the slit and the qubit in the lead. --MaoGo (talk) 16:38, 14 May 2018 (UTC)

I partially agree. Some of the experiments on that list are interesting because they involve relatively large systems (like buckyballs or flu viruses), and it's interesting to check that such large systems can be put in coherent superpositions (of position eigenstates, for instance) and can demonstrate interference. I think it's a good idea to keep some discussion of that somewhere in the article. But much of what's there in that section is fluff. Waleswatcher (talk) 17:19, 14 May 2018 (UTC)

We might just move the list to pages like mesoscopic physics or quantum interference, they are maybe not suited here. --MaoGo (talk) 14:36, 15 May 2018 (UTC)