Physicists demonstrate new violations of local realism

June 10, 2015 by Lisa Zyga, Phys.org report
By extending Gisin’s theorem from pure states to mixed states that obey a certain property, the results of the new paper could have applications for quantum certificate authorization protocols, like the one shown here. Credit: Chen, et al. ©2015 Nature Scientific Reports

(Phys.org)—Erwin Schrödinger once famously stated that quantum entanglement is "the characteristic trait of quantum mechanics" that distinguishes it from classical theories. Now in a new paper, physicists have demonstrated a new family of entangled states that violates the principle of "local realism"—an intuitive concept that is a standard feature of classical theories, but disturbingly at odds with quantum theory.

When two objects are entangled, a measurement on one object instantly affects the state of the other, even more quickly than light could travel between them. This instantaneous action goes against our intuition that an object should be affected only by its immediate surroundings, a concept known as locality.

For years, physicists struggled to definitively answer the question of whether or not entangled states truly violate local realism—that is, do they violate either locality or realism, where realism is simply the assumption that objects exist even when they're not being observed?

Although it was long suspected that at least some entangled states violate local realism due to how they seem to instantly influence each other, it wasn't until 1991 that physicist Nicolas Gisin at the University of Geneva quantitatively demonstrated that all pure entangled states must violate local realism. This result is now known as Gisin's theorem.

In quantum mechanics, a "pure" entangled state is one that is clearly defined. However, the vast majority of entangled states are "mixed" to some degree, meaning they consist of a combination of multiple types of pure states. Although Gisin's theorem holds only for pure states, over the years physicists have extended the theorem by showing that some other types of states can also violate local realism.

In a new paper to be published in Nature Scientific Reports, Jing-Ling Chen, et al., from institutions in China and Singapore, have demonstrated that all mixed states that obey a certain steering property must violate local realism. This new family of entangled mixed states that violate local realism may lead to a better fundamental understanding of , as well as simplify the implementation of some quantum information protocols.

"Our enhanced Gisin's theorem is the first time that the theorem has been generalized from pure states to mixed ones, and includes the original Gisin's theorem as a special case," Chen, a physicist at Nankai University in China and the National University of Singapore, told Phys.org.

Two distinct concepts

Chen explained the problem in more detail:

"It has long been well-known, starting from Werner's seminal 1989 paper 'Quantum states with Einstein-Podolsky-Rosen correlations admitting a hidden-variable model,' that entanglement and violation of local realism are two distinct concepts. Some entangled quantum states admit a local hidden variable model and hence do not violate local realism. An important question arises. Can we pinpoint a condition that constrains quantum states to those for which entanglement is equivalent to a violation of local realism? A possible condition is purity. Any pure entangled violates Bell's inequalities. This is known as Gisin's theorem.

"For a more general case of mixed states, however, researchers have been concerned about a lack of such a condition. The more general condition is of great significance not only from the theoretical viewpoint of the need for a deeper understanding of quantum correlations. It is also important in experiments, and for quantum informational applications. Since a quantum system inevitably interacts with its environment, the quantum states practically always are to some degree 'mixed.' In this work, we address this problem and propose to use the concept of Einstein-Podolsky-Rosen steering as a condition to bridge entanglement and violations of local realism."

Three forms of correlations

As Chen explained, entanglement, steering, and violations of local realism can be thought of as three different forms of quantum correlations that form a hierarchical structure, with violations of local realism being the strongest form. Steering, the intermediate form, takes the correlations of entanglement a step further so that one system can control—or "steer"—the state of its entangled partner.

Here, the physicists demonstrated that, if two observers are able to steer each other's qubits into pure states by making a measurement on their own qubit that spontaneously collapses the state of the other's qubit, then even if the qubits were originally in mixed states, they must violate local realism.

"This proposed condition is more intrinsic, in the sense that Einstein-Podolsky-Rosen steering is by definition a form of quantum correlation that is intermediate between just entanglement and a much stronger one: violation of local realism," Chen explained. "Our result provides an important step forward to solving a long-standing problem of pinpointing a physical condition that automatically implies violation of local realism by an entangled state."

Overall, the findings help establish rigorous criteria for marking the borders between these three highly related yet different concepts.

"In this hierarchical structure of entanglement, steering, and violations of local realism, the former contain the latter as a subset," Chen explained. "[Marking the borders between them] is a nontrivial problem since, in general, it is not easy to reduce a superset [] to a subset [violations of local realism] by imposing extra constraints, which is just EPR steering in our work."

As the scientists explain, the new family of states that violate local realism could provide a new resource for quantum information tasks by reducing the number of entangled particles needed to perform a task. One example is the Third Man cryptography protocol, also called "secret sharing," in which a third party can control whether two people are allowed to secretly communicate with each other. Previous versions of this protocol required three entangled qubits, but because the fidelity of three-particle entangled states is currently still below about 90%, it is very error-prone. Using the new states, the protocol can be implemented with just two entangled qubits, which has a fidelity of more than 99% and therefore a much lower error rate.

Another potential application is quantum certificate authorization, in which a person sending a confidential message through the internet to another person can ask a third party to verify that person's identity. One way that the third party might do this is by ensuring that both the sender and the receiver can steer each other's qubits into pure states. If they can, the must violate , which ensures a secure protocol. The physicists plan to use the new family of EPR-steerable mixed states to experimentally realize these protocols in the near future.

Explore further: Quantum test strengthens support for EPR steering

More information: Jing-Ling Chen, et al. "Beyond Gisin's Theorem and its Applications: Violation of Local Realism by Two-Party Einstein-Podolsky-Rosen Steering." Sci. Rep. 5, 11624; DOI: 10.1038/srep11624. To be published. Also at arXiv:1404.2675 [quant-ph]

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Sonhouse
2.4 / 5 (7) Jun 10, 2015
So does this work imply that communications can now take place faster than c? Actual coding of the quantum states and observing the behavior at the other end, a stream of such particles either entangled or not or some variation of that?
antialias_physorg
4.6 / 5 (10) Jun 10, 2015
So does this work imply that communications can now take place faster than c?

No. We're talking quantum information, here - not classical information (those are two different critters)
In classical information you can encode a state. In quantum information you do not know which state you encode (as that would constitute a measurement and break entanglement). In quantum information you can only be sure that both entangled parties will get the same measuerement later on. That's why it's useful for encryption (because encryption does not constitute information transmission)
TulsaMikel
4 / 5 (1) Jun 10, 2015
Is every particle entangled with another particle?
robandlezlye
1.5 / 5 (6) Jun 10, 2015
So does this work imply that communications can now take place faster than c? Actual coding of the quantum states and observing the behavior at the other end, a stream of such particles either entangled or not or some variation of that?


So does this work imply that communications can now take place faster than c? Actual coding of the quantum states and observing the behavior at the other end, a stream of such particles either entangled or not or some variation of that?


...no, but possibly particles arise from then return to the aether at c, and as such when entangled their intrinsic connection via that medium causes instantaneous change, seemingly faster than c, when perturbed ... a kind of Einstein-Rosen Bridge between two particles via the vacuum. There is far more empty space than matter, and what little matter there is arises from the 'vacuum'. Maybe dark energy and matter is unrealized reality contained within the vacuum before its observed or measured.
PhysicsMatter
1 / 5 (5) Jun 10, 2015
These are more metaphysical consideration than anything else. The issue is what is material and what is not? and whether transfer of information is material or not?. So called non material entities could travel at any speeds such as those in deHoffmann-Teller frame of reference or well known phase velocities of the waves.

An interesting take on all these exaggerated metaphysical clams and paradoxes of quantum mechanics at:

https://questforn...-quanta/
MandoZink
2 / 5 (1) Jun 10, 2015
...no, but possibly particles arise from then return to the aether at c, and as such when entangled their intrinsic connection via that medium causes instantaneous change, seemingly faster than c, when perturbed ... a kind of Einstein-Rosen Bridge between two particles via the vacuum.


This research was not intended to extend into a hypothetical multiverse. Your comment on an alternate universe in which an "aether" might actually exist is indiscriminate speculation, yet it does stimulate the imagination. I would think that in a universe where "The Force" exists, violations of of local realism may be endless - possibly requiring the additional presence of just such an aether. Just saying,

I guess now we're both way off topic here, eh?
AmritSorli
1 / 5 (3) Jun 11, 2015
quantum entanglement between ˇ"local" particles is function of "non-local" quantum vacuum where time has only a mathematical dimension.
http://www.degruy...14-5.xml
theon
2 / 5 (4) Jun 11, 2015
Bell had the audacity not to allow hidden variables for the detectors, and when he did, he did it wrong. The "contextuality loophole" can not be closed. Bell's "theorem" is a mathematical triviality which does not have any say about physics. One can not make any sound argument out of it. This post is one more without meaning.
DarkLordKelvin
5 / 5 (1) Jun 11, 2015
Bell had the audacity not to allow hidden variables for the detectors, and when he did, he did it wrong. The "contextuality loophole" can not be closed. Bell's "theorem" is a mathematical triviality which does not have any say about physics. One can not make any sound argument out of it. This post is one more without meaning.


Hmmm .. I am pretty sure that the contextuality loophole can be closed by simply having spacelike separations between all possible source-detector, and detector-detector pairs, can't it? In fact, I thought I saw a PO post about an experiment that accomplished this for single photons, a few months back. What am I missing?
swordsman
1 / 5 (1) Jun 11, 2015
It is well known in electrical and mechanical systems that a single measurement is NEVER completely accurate. Modern physicists are re-defining this issue and giving it glitzy names. Everything in the universe is "entangled" to various degrees, most of which are relatively insignificant. In fact. atoms are "entangled" by virtue of the Coulomb and magnetic forces that act between them.
anywallsocket
not rated yet Jun 11, 2015
hmm, so we entangle particles traditionally, which are "mixed" because their wave-functions are interacting with those of the system, then we use the system to "steer" them into a more isolated state or "pure" state, where they can basically merge with the environment or system, ceasing to be, and thus violate "local realism"?

that's my two cents.
docile
Jun 11, 2015
This comment has been removed by a moderator.
machapungo
1 / 5 (1) Jun 11, 2015
All aspects of the universe are entangled whether we currently realize it or not. As I may have asserted before, the universe is a whole single entity that is composed at it's most fundamental level as multiple FIELDS of energy that are all constantly interacting to manifest reality as we know it. Until this is realized and investigated by the physics community they will not achieve a big TOE where the disagreements between QM and STR / GTR are resolved. These fields are universal in scope and range. Universal, in effect, means infinite since the universe is all that is, by definition. Fields Rule! They are the foundation of things that we perceive, such as, motion. Forget about multiple universes and time as a physical reality. Regards "The Crackpot".

theon
not rated yet Jun 15, 2015
Bell had the audacity not to allow hidden variables for the detectors, and when he did, he did it wrong. The "contextuality loophole" can not be closed. Bell's "theorem" is a mathematical triviality which does not have any say about physics. One can not make any sound argument out of it. This post is one more without meaning.


Hmmm .. I am pretty sure that the contextuality loophole can be closed by simply having spacelike separations between all possible source-detector, and detector-detector pairs, can't it? In fact, I thought I saw a PO post about an experiment that accomplished this for single photons, a few months back. What am I missing?


You're missing a whole research line. More and more scientists get convinced of it. For them it's trivial: the information travels with the particles (surprise, surprise.). The popular press fails to pick up the point, it's not so sexy.
DarkLordKelvin
not rated yet Jun 15, 2015
Bell had the audacity not to allow hidden variables for the detectors, and when he did, he did it wrong. The "contextuality loophole" can not be closed. Bell's "theorem" is a mathematical triviality which does not have any say about physics. One can not make any sound argument out of it. This post is one more without meaning.


Hmmm .. I am pretty sure that the contextuality loophole can be closed by simply having spacelike separations between all possible source-detector, and detector-detector pairs, can't it? In fact, I thought I saw a PO post about an experiment that accomplished this for single photons, a few months back. What am I missing?


You're missing a whole research line. More and more scientists get convinced of it. For them it's trivial: the information travels with the particles (surprise, surprise.). The popular press fails to pick up the point, it's not so sexy.

That's directly contradictory to established physics. Got a reference?

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