Physicists settle controversy over identical particle entanglement

identical particle entanglement
Illustrations of (a) two spatially separated identical qubits, (b) two identical qubits in the same space, and (c) two identical qutrits in the same space. Credit: Sciara et al. Nature Scientific Reports

(Phys.org)—In a new study, physicists have shown a way to establish real entanglement between two identical particles—a topic that has been disputed until now. The results provide a better understanding of the fundamental nature of entanglement between identical particles and have potential applications in quantum information processing.

One of the many strange features of quantum is that, if two particles are identical, then they automatically appear to be entangled. In this case, "identical" means that the particles are of the same type—for example, any two photons are considered identical because there is no way to tell any particular photon apart from another.

This type of entanglement, which call "entanglement due to indistinguishability," arises because of the standard way that identical particles are labeled. Although the particles are identical, physicists assign them different labels in order to tell them apart. Because of the way that entanglement is determined, with respect to their labels, identical particles inevitably appear to be entangled.

Although this type of entanglement is different than entanglement between non-identical particles, there is disagreement among physicists over what exactly it is and whether or not it is useful for applications. In one viewpoint, the entanglement holds even for distant particles but cannot be exploited for practical use. In another viewpoint, the entanglement is simply an artifact of labeling and should not really be considered entanglement at all.

Ideally, an appropriate way to settle the debate would be to investigate the entanglement between identical particles using a process called Schmidt decomposition, which is commonly used to study entanglement between non-identical particles. Unfortunately, Schmidt decomposition doesn't apply to identical particles in a straightforward way, and its use in this area has been controversial and inaccurate, giving obviously incorrect results for identical particles.

The main result of the new study is that the physicists have generalized Schmidt decomposition so that it can be applied to identical particles equally as well as non-identical particles.

"The results show that entanglement between identical particles is not merely a mathematical artifact, but that, under certain conditions, the entanglement due to indistinguishability is true physical entanglement," coauthor Rosario Lo Franco at the University of Palermo, Italy, told Phys.org.

When applied to two identical qubits, the method shows that, in certain situations, particles can be physically entangled only when in close proximity, and not when spatially separated as it previously appeared.

"The key to the achievement was to do away with the standard practice of assigning labels to identical particles, and instead simply describe the particles in terms of their states," said coauthor Giuseppe Compagno at the University of Palermo.

The researchers also discovered a somewhat surprising result for qutrits—three-level which are relevant for storing quantum information—that contrasts with a previous result using a different method. The researchers said that this difference requires further investigation.

Overall, the physicists expect the method to provide a new type of entanglement resource for applications.

"In the case where the particle wave functions overlap, entanglement due solely to indistinguishability and unveiled directly by the Schmidt decomposition can be utilized operationally in protocols of ," Lo Franco said. "This achievement may first require a separation of the as obtained by the so-called extraction procedures."


Explore further

Entanglement for identical particles doesn't follow textbook rules

More information: Stefania Sciara, Rosario Lo Franco, and Giuseppe Compagno. "Universality of Schmidt decomposition and particle identity." Scientific Reports 7, 44675 (2017). DOI: 10.1038/srep44675
Journal information: Scientific Reports

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Mar 27, 2017
See, this is why I hate QM. Probability distributions used to create massive confusion, not clarity!

Mar 27, 2017
Quite right Hyperfuzzy.
However it is not QM you hate, I suspect, but the logical brainfart of a mindfuck that is being written on the subject. This goes on and on, because QM is a statistical description with simple principles, but 'physicists' have to publish, don't they? Or perish.
Read this paragraph from the article: "When applied to two identical qubits, the method shows that, in certain situations, particles can be physically entangled only when in close proximity, and not when spatially separated as it previously appeared."
This is mostly theological argument not unlike: how many devils can fit in a stomach of an angel?
Particularly, for a multitude of identical entangled particles simply by the virtue of being identical, the concept itself is meaningless. They are identical, all right, stop trying to make more out of it than there is.

Mar 27, 2017
What I am looking forward to is practical implementations of many interesting properties. Explaining the nature of bogus concepts like entanglement of all identical particles, like photons, etc. is pointless waste of effort. Time to focus on finding deeper theories, or shut up and teach students. And more physical experiments - like last sentence points toward, just do it then. Countless theological discussions based on the same 5 principles create only more confusion.

Mar 27, 2017
See, this is why I hate QM. Probability distributions used to create massive confusion, not clarity!


Translation: Physics is hard! Let's go bake cookies for the boys.

Mar 28, 2017
Notice the diagonal alignment of of "entangled" bits in part "B" of the image?
It's because there is something else (another vector's force) affecting it...
That means they are not truly entangled.

Mar 28, 2017
form the link;

"...if two particles are identical, then they automatically appear to be entangled. ..."

shouldn't that be;

"...if two particles are entangled, then they automatically appear to be identical..."

?

If 'two' particles are 'identical' then they are the SAME particle thus not 'two' at all, right?

Mar 28, 2017
This comment has been removed by a moderator.

Mar 29, 2017
Is there a manner to frame these apparent but not actual identity differences for fundamental modes without dispersing the operator? Super Yangmils thinks so.

Zero fundamental modes, degrees of freedom and naturalists who do not believe in frame dependence don't.

Mar 30, 2017
Is there a manner to frame these apparent but not actual identity differences for fundamental modes without dispersing the operator? Super Yangmils thinks so.

Zero fundamental modes, degrees of freedom and naturalists who do not believe in frame dependence don't.

Well, then... Maybe it's time to rotate the reference frame...

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