Entanglement without Classical Correlations

August 27, 2008 By Lisa Zyga, Phys.org feature

Quantum mechanics is full of counterintuitive concepts. The idea of entanglement – when two or more particles instantaneously exhibit dependent characteristics when measured, no matter how far apart they are – is one of them. Now, physicists have discovered another counterintuitive result that deals with the line between the quantum and classical worlds.

Normally, when two or more particles are entangled (and seem to communicate with each other instantaneously), they not only share quantum correlations, but also classical correlations. Although physicists don’t have an exact definition for classical correlations, the term generally refers to local correlations, where information does not have to travel faster than the speed of light.

So if entangled particles demonstrate correlations across large distances, you might assume that they will also have correlations across shorter distances. After all, if entangled particles can communicate at faster-than-light speeds, they should be able to communicate at slower-than-light speeds.

But a team of physicists from the National University of Singapore, Mediterranean Technology Park in Barcelona, the University of Leeds, and the University of Bristol has demonstrated something different. They’ve theoretically shown that any odd number (greater than one) of entangled particles can exist without classical correlations. They explain this paradox in a recent issue of Physical Review Letters.

“One way of seeing this is as follows,” Vlatko Vedral, Professor of Quantum Information Science at the University of Leeds, told PhysOrg.com. “Entanglement means being correlated as far as many different measurements are concerned. Classical correlations mean being correlated as far as one particular measurement is concerned. That is why researchers usually think that when there is entanglement, there are also classical correlations. However, our paper shows that you have to be careful about making this inference.”

As Vedral explained, generally when physicists measure entanglement, their measurements destroy the quantum correlations first, and then the classical correlations.

“Entanglement represents excess of correlations, over and above classical ones. In other words, whatever cannot be accounted for locally is due to quantum entanglement. When you make local measurements on entangled particles, then you will invariably be destroying their correlations (both classical and quantum). Since quantum is in excess of classical, it is possible that you can first get rid of entanglement, but are still left with some classical correlations.”

But to do the opposite of this – to get rid of the classical correlations and have only quantum correlations – is more difficult to comprehend.

“Imagine that I tell you that I am a billionaire,” Vedral said as an example. “You would then infer that I certainly have 100 million somewhere in my assets. You would be very surprised, indeed, if I told you that this was not true and that I am actually not also a millionaire. You can't have more, without have less as well (by definition).”

This is not the first time that physicists have demonstrated entanglement without classical correlations. In 2006, Toth and Acin found an example of a three-qubit system that also shows this phenomenon. This three-qubit example has already been observed in the laboratory, and the physicists hope that their new example with any odd number of qubits can also be observed. They also expect that even numbers of qubits should exhibit the same effect, but do not yet have an example.

“The key is that we are using one particular definition of classical correlations, which is in fact the main one used in the solid state physics (and is used to mark phase transitions among other things),” Vedral said. “This is based on average values of a set of observables and the key is that this set is not complete. However, when it comes to two particles (and two point correlations is what all solid state experiments are about) then you cannot have the situation that we found with three and more particles. Namely, if classical correlations vanish for two qubits, then so do the quantum ones.”

The paradox that quantum correlations can exist without accompanying classical correlations could have some thought-provoking consequences. For instance, physicists often use a test of Bell inequalities to determine if local realism has been violated and that quantum correlations have occurred. But since Bell inequalities are based on classical correlations, the test doesn’t work for this example. This leads to the need for a new way to detect quantum correlations, based on different concepts.

The study may also affect how physicists view the boundary between the classical and quantum worlds – a question at the foundation of physics. With this demonstration of the existence of a state that has quantum correlations without classical correlations, the physicists suggest that local realism might be used as the criteria to characterize the classical world.

The result could also have practical applications – for instance, as a possible method for detecting phase transitions. Using quantum correlations only (instead of both quantum and classical) for detecting phase transitions could provide a more universal measurement than conventional methods.

More information: Kaszlikowski, Dagomir; Sen(De), Aditi; Sen, Ujjwal; Vedral, Vlatko; and Winter, Andreas. “Quantum Correlations without Classical Correlations. Physical Review Letters 101, 070502 (2008).

Copyright 2008 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

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not rated yet Aug 27, 2008
I would assume quantum correlations would come first, then classic ones... It only seems counter-intuitive because we live in Classicalville?
1.3 / 5 (3) Aug 27, 2008
I wonder if this has any implications for possible instantaneous 2 way communication. If you can change either classical or quantum correlations and be left with the other, does that mean that useable information might be "read" from the remaining correlation?
not rated yet Aug 27, 2008
No, Modernmystic, it is impossible, unfortunately. In opposite case the special relativity will be dead.
not rated yet Aug 27, 2008
I like the example of Vedral it describes the physics behind it perfectly.
not rated yet Aug 27, 2008
I think what is meant by quantum and classical correlations is being misunderstood here. Classical correlations, conservation of momentum or energy, only have two states. Its almost like only a heads and tails on a coin, thus 3 entangled particles in a two state system are not really entangled. But there are various properties that particles have that could correspond to a more than two state system. Such quantum correlations could allow for entanglement in more than a two state system. I think the thing they were trying to achieve was a pure quantum measurement without affecting the classical correlations. This is pretty cool...quantum "transistors" anyone? :)
not rated yet Aug 27, 2008
Entangled Communication


not rated yet Aug 28, 2008
Well, the definition of classical correlations they used seems to be questionable. Have a look at:


Anyway, interesting!
2 / 5 (3) Aug 28, 2008
billion = 1000 million, shame on you Physorg!!!!
1 / 5 (1) Aug 28, 2008
Quantum correlations imply instantaneous contact, whether over a distance of parsecs or femtometers. I have (already 7 yeras ago) created a thin rod of entangled electrons (billions of them) in my laboratory. When injecting an electron at one end another jumps out immediately on the other. In other words this phase is not just entangled but also consituites the most perfect superconductor possible: By the way, once formed it is stable up to temperatures at which the surrounding materials fail. Thus, this is superconduction way above room temperature. Obviously the "experts" on superconduction reject this result because there are no phonons and "Cooper-pairs" involved. It is really sad that they have not yet caught on that "Cooper-pairs" cannot explain the most fundamental property of superconduction: namely, how a conservative electric-field is cancelled between two contacts when a supercurrent flows.
not rated yet Aug 28, 2008
Could this possibly become a means of power transmission from earth to a space vehicle?
1 / 5 (1) Aug 28, 2008
Ok I'm sorry, but I'm having trouble not seeing how this can be used for communication. If you can shoot electrons through a tube and have them come out on the other side that AT THE VERY LEAST can be used as Morse code unless for some reason you can't get enough control over the electrons going into the tube to make it intelligible.
1 / 5 (1) Aug 28, 2008
For that matter if you can measure the change in spin no matter the distance then isn't that a form of information..."hey the spin on the bugger has changed there's a 1, oh now we have changes in spin on two of the buggers there's a 0" bingo digital communication. Granted you'd have to take a lot of entangled electrons with you on your trip and they'd run out eventually but I don't understand why it wouldn't work in principle.
3 / 5 (2) Aug 28, 2008
In order to use entangled electrons as a conduit for power transmission one will require such an entangled wave to stretch from earth to the space vehicle. There is, however, the possibility that dark matter might be neutral entangled macro-waves. If this is correct, they already exist over parsecs. If one could entangle a neutrally-charged object with one them, one might have teleportation over parsec distances. My electron-wave forms a rod with a diameter of about 2 microns and a maximum length of about 25 microns. Since it has no smaller parts which can act as charge-carriers an electron is "teleported" from one contact to the other. By the way, we have sent a current of 20 mA "through" such a rod. It stays black even when the surrounding materials heat up. It is easy to calculate that there is no known material (not even carbon nanotubes) that can carry such a current density without exploding.
5 / 5 (2) Aug 28, 2008
Modernmystic, I am a foreigner and it isn't easy for me to discuss in English, such special matter, but I will try, anyway. Let consider an electron-positron anihilation and the result with two gama-photons. Every of these photons has spin 1 or -1 (in h/2pi units). But we can't say what of them has 1 and what -1. They are in mixed condition. To say this, we must make a measurement. And if after the measurement we find photon one has spin 1, then automatically photon two will get spin -1, independently on the distance between the photons. I.e. we have something as "momental communication". But the problem is that we don't know the spin of photon one before the measurement. So that we aren't able to manipulate the spin of photon two. I.e. this process is useless for one or two way communication. In my opinion all similar quantum phenomenons suffer from such problem.
1 / 5 (1) Aug 28, 2008
English problems aside you've just cleared up all my misunderstanding. I forgot that you can't know the state of the entangled photon on the sending end without measuring and hence changing it's state and spoiling the whole concept. I knew the implications on the receiving end, but was failing to apply them across the board.

Actually a pretty stupid mistake on my part :)

Thanks for setting me straight.
1 / 5 (1) Aug 28, 2008
Obviously, when entangled, you cannot talk about 'photon 1" and "photon 2". They do not exist as independent entities: Only when you measure you create independent entities and they HAVE TO CORRELATE. It is really quite simple!
not rated yet Aug 31, 2008
can the measurement of a multipartite particle cluster be achieved without the direct measurement of one of the single particles within the cluster? either by measuring the particles or a field that surrounds(or permeates) the multipartite cluster?

if so, can this be done without destroying any, or at least "some" of the multipartite system's entanglement?
not rated yet Sep 01, 2008
phystic, I'd assume it's along the lines of action/reaction. If you can read the state of the cluster from one point because the rest of the particles impart information to that one particle, then any changes to that one particle would be reflected in the rest.

I'd be curious to know if you can read the particle with X amount of change and then "unread" it with -X amount of change. I do believe there was an article about such behavior, at least that's how I interpretted it.
1 / 5 (2) Sep 01, 2008
Is it the proverbial goose chasing its own tail or the golden eggs?
1 / 5 (1) May 10, 2009
phystic, I'd assume it's along the lines of action/reaction. If you can read the state of the cluster from one point because the rest of the particles impart information to that one particle, then any changes to that one particle would be reflected in the rest.

I'd be curious to know if you can read the particle with X amount of change and then "unread" it with -X amount of change. I do believe there was an article about such behavior, at least that's how I interpreted it.

What article are you talking about? Does it really exist? The library says you never returned it.
not rated yet May 11, 2009
What article are you talking about? Does it really exist? The library says you never returned it.
How would you know Neil. The transcript from your court case shows that you were banned from the library.

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