How do you determine that a particle is/was entangled?

[DrChinese]

Surely an entangled photon cannot affect or 'act on' its partner, it is only correlated and even that is only a synchronisation of state observables wrt time. If bob is observed as an x then alice instantly becomes a y?

I would say that is the question, and the answer is not known. But you cannot assume this to be true.

 

[LaserMind]

I would say that is the question, and the answer is not known. But you cannot assume this to be true.

Dr C -
hmmm, can you give me a reference(s) to experiments or theorems that indicate otherwise? Sounds like a UFO hunt to me, but I am eager to dig into it and change my opinion.

 

[JesseM]

Dr C -
hmmm, can you give me a reference(s) to experiments or theorems that indicate otherwise? Sounds like a UFO hunt to me, but I am eager to dig into it and change my opinion.

You could take a look at Bohmian mechanics, an interpretation of QM which makes all the same physical predictions as any other version of QM, but which features hidden variables that can influence one another instantaneously at a distance.

 

[DrChinese]

Dr C -
hmmm, can you give me a reference(s) to experiments or theorems that indicate otherwise? Sounds like a UFO hunt to me, but I am eager to dig into it and change my opinion.

JesseM correctly points out Bohmian Mechanics (BM, also referred to as de Broglie Bohm or dBB). In this view, the state of particles that are non-local to the entangled particles themselves provide critical influences that are intended to describe and explain the entanglement mechanism. Some groups of scientists have studied this concept in depth. While the ideas are still imperfect, the basic Bohmian programs are able to reproduce the testable essentials of QM. However, there is no relativistic version yet and there is no "one" version of BM as there are competing versions.

You can also imagine that there might be force carriers, previously unknown and with no other known footprint, which have the ability to travel FTL. Perhaps these are exchanged between entangled particles. This, of course, would be pure speculation and there is no evidence whatsoever to support the view... but it is possible.

Either way, it is *possible* that non-locality is a part of nature.

 

[Dbar_x]

May I offer a criticism of the Walborn, Padua experiment? Note carefully that the two terms superposed in equation 2 describe the probable results of a measurement of the compound object, composed of a photon (described by psi) and the which-path marker (described by M). The authors show that results of a measurement of the compound (for emphasis, COMPOUND) system, specified by QM as the absolute square of Psi (that’s capital Psi, the left side of equation 2), include no cross terms, indicative of no interference. Again, carefully now, this means that if one measured the compound system with some as yet unspecified apparatus, that compound system would not exhibit interference. It would have no interference because the eigenfunctions of M are orthogonal.

But it’s not an interference pattern of the compound system that is supposed to disappear and then be restored via quantum erasure. Instead, the interference pattern of a single photon, described by psi, is what we are interested in. It is absolutely not the case that because a compound object exhibits no interference, then its constituent systems will also exhibit no interference. (For simplicity, this is like an exploding artillery shell, composed of fragments. We cannot tell where each fragment went by looking at the center of momentum of the entire shell. The compound object doesn’t tell us everything about each constituent. The single photon wavefunction, psi, determines everything about the photon, not the compound wavefunction, Psi.)

The implied inference the authors give us, that the photon interference is gone, is incorrect, and misleading. It’s no surprise, then, that if the interference never disappeared, it can be made to reappear.

It seems to me that we ought always to keep in mind, when discussing quantum erasure, the original theoretical analysis from 1978 that has generated all these QE experiments over the years. (Sculley, et al., Phys. Rep. 43, p. 485) (Am I actually the only person who has read that article carefully?) Scully and his colleagues meticulously described the quantum mechanics for a heavy molecule of spin one-half traveling through a modifoed Stern-Gerlach magnet. They placed a measuring apparatus, in this case a bi-level atom, in one arm of the magnet. They say that if the molecule goes that way, it will always kick the atom into its excited state, thus measuring which path was taken.

They use Schrodinger’s equation, of course, to show that after passing through the magnet, the density matrix for the molecule-atom system will be almost diagonalized. Meaning that the off-diagonal terms are small compared to terms on the diagonal. (That’s not really a diagonalized matrix, by the way.) Their analysis depends on the crucial assumption that because the molecule is arbitrarily heavier than the atom, there will be an arbitrarily small momentum transfer to the spinning molecule. This, they assert, implies no “significant” change to the molecule’s wavefunction at measurement, so continuous Schrodinger evolution continues, they claim.

But, consider this: no matter how heavy the molecule is, it always kicks the atom from ground to its excited state. That’s a definite quantized energy. Energy is conserved, so the molecule always loses that same quantum of energy. Each distinct, total energy state of the molecule is specified by a unique, linearly-independent eigenfunction. Each such eigenfunction specifies a distinct, independent, measured state of the molecular system. So, this claim we often hear, that the measurement did not disturb the object measured, is not justified by this analysis.

Scully et al. then imply that since the molecule’s wavefunction evolved continuously through measurement, we ought to be able to reverse that evolution by reversing (erasing) the state of the atomic detector. That’s what was meant, initially, by quantum erasure. Thus, disappearance of interference, evidence for a measurement, is supposed to be restored by returning the apparatus to its ground state. But it’s not a credible physical theory.

I realize that those who’ve advocated for quantum erasure have changed its meaning over the years, as successive experiments have proved unpersuasive. We now hear of object-apparatus entanglement, and sub-ensemble sorting. But, if the theory is not consistent and comprehensible, its not scientifically sound.

DocMike

 

[JesseM]

I realize that those who’ve advocated for quantum erasure have changed its meaning over the years, as successive experiments have proved unpersuasive. We now hear of object-apparatus entanglement, and sub-ensemble sorting.

Object-apparatus entanglement? Where do we "now hear" of that? If you're referring to my posts on the "Interference seen in a member of an entangled pair" thread, you didn't read very carefully: I made it quite clear that object-apparatus entanglement would only be necessary to analyze a wholly impractical thought-experiment where a macroscopic apparatus can remain completely isolated from the environment for a long period of time (long enough that if you measure it at an earlier time you will gain which-path information, but if you measure it at a later time it will give you no which-path information), akin to the Schroedinger's cat thought-experiment. In practical quantum eraser experiments, the "marker" that has the potential to give you which-path information (unless it is measured in such a way that this information is 'erased', or if you prefer, never existed, which as I said in post 56 here is just a semantic issue) is just another entangled particle, like the "idler" that can give you which-path information for the signal photon in the delayed choice quantum eraser. There is no need here to imagine that the measuring apparatus becomes entangled with what it measures, the assumption that measurements collapse the 2-particle wavefunction should work just fine. If you know a practical quantum eraser experiment where the authors felt the need to assume the measuring system becomes entangled with the particle being measured, please point it out.

 

[Dbar_x]

Object-apparatus entanglement? Where do we "now hear" of that?

Mohrhoff's article in the American Journal of Physics in 1996 was the most pointed criticism of quantum eraser theory that I know of. The response of Scully and his colleagues was, it seems, to find a different meaning for quantum erasure. They did some calculations, not entirely persuasive, which led to an explanation involving entanglement and the sorting of coincidence measurements into sub-ensembles. One sub-ensemble correlated with interference, the other sub-ensemble with no interference.

DocMike

 

[JesseM]

Mohrhoff's article in the American Journal of Physics in 1996 was the most pointed criticism of quantum eraser theory that I know of. The response of Scully and his colleagues was, it seems, to find a different meaning for quantum erasure. They did some calculations, not entirely persuasive, which led to an explanation involving entanglement and the sorting of coincidence measurements into sub-ensembles. One sub-ensemble correlated with interference, the other sub-ensemble with no interference.

What do you mean by "sub-ensembles" in this context? And what does this have to do with entanglement between the particles and the measuring apparatus? If these papers are not available online, perhaps you could quote a relevant paragraph or two?

edit: The ensemble interpretation of QM interprets the wavefunction as just giving statistical predictions about experimental results on an ensemble of trials where the system is prepared in the same initial state and measured the same way. So are sub-ensembles just the theoretical analogue of a coincidence count, like the probability the signal photon will be detected at different points on the screen given the assumption that we're looking at the subset of trials where the idler was detected at, say, detector D2?

 

[p764rds]

Wheelers delayed path does not require backwards in time theories provided we use the CI approach. If its presented as a typical wave particle conundrum then all types of horrors emerge (read Wheeler's analysis) - I am expecting the same of the Quantum Erasure claims by Scully and am requesting a re-evaluation.