Quantum entanglement : need explanation

[Edgard Neuman]

Hi,

I try to understand quantum entanglement : I understand that you have a pair of particles that are somehow linked to each other, but in unknown states, and so knowing one make the other one different (so if you measure the second you have the complementary property from the first).
I'm willing to believe that it is a "quantum" special thing, but, I still don't see in what way this is different from randomly putting two cards in two envelopes. Knowing what one card is in one envelope (instantly) tells you the other one is in the other envelope.
So what's so different about quantum entanglement and how could it be used for teleportation ?
(please use understandable words)

[Sensei]

and how could it be used for teleportation ?

 

It cannot.

I try to understand quantum entanglement :

 

I suggest buying equipment and checking by yourself..

Lasers, polarization filters, beam splitters etc. are quite cheap.

http://en.wikipedia.org/wiki/Photon_entanglement

[Endy0816]

There is quantum teleportation.

 

It is all more about the properties than anything else.

[Strange]

I'm willing to believe that it is a "quantum" special thing, but, I still don't see in what way this is different from randomly putting two cards in two envelopes. Knowing what one card is in one envelope (instantly) tells you the other one is in the other envelope.

So what's so different about quantum entanglement and how could it be used for teleportation ?

There isn't really a simple answer to this (or I have never seen one). The thing is that quantum theory predicts probabilities for outcomes that are different from your envelope example.

 

This is one of the best simple explanations I have seen: http://www.drchinese.com/David/Bell_Theorem_Easy_Math.htm

[swansont]

Hi,

 

I try to understand quantum entanglement : I understand that you have a pair of particles that are somehow linked to each other, but in unknown states, and so knowing one make the other one different (so if you measure the second you have the complementary property from the first).

I'm willing to believe that it is a "quantum" special thing, but, I still don't see in what way this is different from randomly putting two cards in two envelopes. Knowing what one card is in one envelope (instantly) tells you the other one is in the other envelope.

So what's so different about quantum entanglement and how could it be used for teleportation ?

(please use understandable words)

 

 

The difference in QM entanglement is that you cannot say that the card you find in the envelope was there (i.e. say it was that card) the whole time.

[Enthalpy]

I suggest buying equipment and checking by yourself..

Lasers, polarization filters, beam splitters etc. are quite cheap.

http://en.wikipedia.org/wiki/Photon_entanglement

 

How do you produce the entangled photons with a simple and cheap setup?

I try to understand quantum entanglement [...]

I'm willing to believe that it is a "quantum" special thing, but, I still don't see in what way this is different from randomly putting two cards in two envelopes.

 

This was a long debate among the best scientists. Presently the near-consensus is that particles are random by nature, as opposed to cards in envelopes, which are fully determined but are not observable with some available methods, and could become observable once the adequate X-rays are invented - that was the "hidden variable" interpretation.

 

To my understanding, a single particle gives no answer between both interpretations. But entangled particles do tell "random", unless one seeks very bizarre alternative interpretations.

 

The deep reason with photon polarization is that we can observe the same source of pairs with linear or with circular detectors, as detailed there:

http://www.scienceforums.net/topic/83752-probabilities/#entry811449

and if the photon pair (or their source) decided upon emission which linear polarization the choose, there would be no correlation between the circular detectors - but correlation is observed. Same against a circular polarization chosen at emission: linear detectors show a correlation.

 

So the (very broad) mainstream interpretation is that the polarization is not determined upon emission but upon detection, and for entangled particles, that both "decide" the same way. More disturbing, and again decided by experiment: both particles correlate within a time shorter than light would take to propagate between the detectors.

 

Though, entangled particles are not so different from a single one in that aspect. A photon can be meters or light-years broad (or "its wave" is broad, for some people here) but if it's detected at one place, it gets immediately unavailable for the other detectors, including the remote ones. Entangled particles only differ in that they hamper the "hidden variable" interpretation, in this case the direction of the photon decided at emission.

[Sensei]

 

So the (very broad) mainstream interpretation is that the polarization is not determined upon emission but upon detection,

 

Do you mean that ANY photon has random polarization during emission (regardless of entanglement).. ?

Edited July 24, 2014 by Sensei

[hoola]

as I recall, einstein's entanglement explanation was not with cards, but with left and right gloves. If one glove of a pair is right, the other must be left. From what I understand, the "spooky action at a distance" occurs whenever you disentangle the 2 particles. At that point, one seems to sense the disentanglement from the other regardless of the distance between them...if you "open the envelope" and expose particle A to the environment, it will cause a loss of entanglement to the B particle, instantaneously causing both particles to begin random angles of spin from a previously held stable position that was right angled to each other...

[swansont]

 

Do you mean that ANY photon has random polarization during emission (regardless of entanglement).. ?

 

No, you can have a definite polarization of a photon, but that photon would not be entangled.

 

Entanglement requires the correlation of the two states and that you can't distinguish between the particles.

[Sensei]

 

No, you can have a definite polarization of a photon, but that photon would not be entangled.

 

Yes, I know. That is why I was asking him to clarify his statement....

[Nicholas Kang]

Quantum Entanglement is used to deny the fact that black hole exist. Quantum entanglement only allows one particle to interact with one particle at the same time. So, if a particle falling into the event horizon is entangled with a particle leaving the black hole, then it cannot be entangled with another particle leaving the black hole. This mean untangling particles must be done. Thus, we suppose that a hot wall of particles exist around the event horizon because untangling particles require enormous energy, violating Einstein`s Theory of Relativity which states falling into black hole is as "relaxing" as free fall in space. Believe it or not, Hawking then propose an alternate version of grey hole theory.

[Mordred]

Quantum Entanglement is used to deny the fact that black hole exist. Quantum entanglement only allows one particle to interact with one particle at the same time. So, if a particle falling into the event horizon is entangled with a particle leaving the black hole, then it cannot be entangled with another particle leaving the black hole. This mean untangling particles must be done. Thus, we suppose that a hot wall of particles exist around the event horizon because untangling particles require enormous energy, violating Einstein`s Theory of Relativity which states falling into black hole is as "relaxing" as free fall in space. Believe it or not, Hawking then propose an alternate version of grey hole theory.

 

 

No that's not quite correct (though its not far off), your explanation is missing several key details. The grey hole theorem has to do with the firewall conjecture and the ads/Cft correspondance. In this particular model there is two quantum entanglement processes going on the Unruh radiation and the Hartle Hawking radiation. Also he removes the event horizon with an apparent horizon.

Information Preservation and Weather Forecasting for Black Holes

http://arxiv.org/pdf/1401.5761v1.pdf

 

I would love to read the technical paper to describe it properly the paper he published doesn't go into alot of detail

however entangled particles lose their entangled properties quite easily, its fairly unstable.

Edited July 25, 2014 by Mordred

[Nicholas Kang]

Oh My God. Wrong. Yes, the paper is too short, just 4 pages. I have never read such a short article before. Thanks for sharing. I have always wanted to find his article about grey hole theorem. This is the one. Thanks. Finally, what is the difference between a theory and a theorem?

Edited July 25, 2014 by Nicholas Kang

[Edgard Neuman]

Thanks to all !

I think I get it : correct me if I'm wrong : if somebody observe one of the particles, the other one "behave" differently (somehow, there are less possible measurements), which is different from an envelope, for there is no way to know if the other one has been opened.

Edited July 26, 2014 by Edgard Neuman

[Enthalpy]

"If someone observes" is already a risky step. "If the state of one particle is known" seems a more careful formulation, because action (like detection) at one particle won't affect the other.

 

Say, the detector is sensitive to vertical polarization only: it won't force the photon to be vertical. It may force the photon (hence pair) to decide whether it is vertical (then detected) or horizontal (then undetected), and the other photon then has the same polarization, so two detectors would give correlated results.

 

Well... At least one experiment by a known researcher claims to see the absorption of one photon influence the other, but over the two arguments he gives against a different effect at his non-linear crystal, one (amplitude) is weak to my eyes, and the other argument (linear amplitude instead of quadratic) may well result from the crystal's behaviour, which is less than simple. I haven't heard of a confirmation by an other group neither.

 

So presently people live with ideas like: both particles decide their state late and in a correlated way, faster than light, but action at one particle doesn't influence the other, so no instantly data transfer is possible.

 

Also good to know: the entanglement quality is limited by the uncertainty relations. Say, if photons come from a small generator, their directions may be linked, but within an accuracy limited by the source's dimensions - the diffraction limit - as per the position versus momentum uncertainty.

[Edgard Neuman]

We have two observers receiving one particle of the pair located at two different spot, unable to communicate.
One (the actor) do something. The other (the observer) observe the other particle.

The quantum thing may be weird and probabilities, but once observed, it's a collection of "measurement".
They repeat it several times.

What ever the explanation of the phenomena, there's only two possibilities in facts :

- Something travels from the actor to the observer. Somehow, the observer see different things whether the actor did something or not (something at least is slightly different after many experiment).

- nothing travel. (Even-though results may be statistically correlated one to the other like in the any classical situation).

Edited July 27, 2014 by Edgard Neuman

[Enthalpy]

- Something travels from the actor to the observer. Somehow, the observer see different things whether the actor did something or not (something at least is slightly different after many experiment).

 

"Actor" is already misleading. There are only observers. If one observer sees "vertical", he knows the other will also, but he can't force the polarization. This has no influence at the other particle.

 

It doesn't need many experiments. The production of entangled particles can be inefficient, but the common behaviour of entangled particles is strong.

[Strange]

- nothing travel. (Even-though results may be statistically correlated one to the other like in the any classical situation).

 

That's it. When they get together again and compare results, they will find a correlation. But the correlation cannot be explained in the classical way.

[DParlevliet]

What ever the explanation of the phenomena, there's only two possibilities in facts :

 

 

Perhaps you are still confused. Suppose the cards have one blue and one red side. One observer open randomly the first envelope, so will see a red or blue side. If the second observer opens the second envelope, then in the classical world he will see a red or blue side, not correlated to what the first observer sees.

 

But if the cards are entangled photons, then if the first observer sees a red side, the second observer will 100% sure see a blue side (and the other way around), at exactly the same time. Nobody knows yet how this works, that is the quantum mystery. There is no travelling because that would go faster then light, which is not possible (a guess would be that both photons are connected through another dimension).

[Enthalpy]

The blue and red cards in envelopes would still be reasonable outside quantum theories, as their state would be pre-defined. The way two entangled particles isn't especially disturbing: it can be the polarization of an intermediate state of the radiating molecule that produces two photons. QM becomes weird because experiments show that the common state of both particles is not defined before observation.

 

Presently people often say that the observed state is just a choice at detection, a state common to both particles, so it doesn't need the particles to exchange a signal. Though, I could live with some sort of underlying instantaneous communication channel between the particles, because this channel isn't available to the particles' observers.

 

Maybe I can reformulate that two entangled partices are not more weird than a single particle in that aspect. When a wide particle gets detected at one location, it becomes instantly unavailable elsewhere - though the particle's spread may take years to span at light speed. In that sense, the collapse of the wave packet is the tricky observation; whether the wave describes one or two particles changes little.

[Enthalpy]

In post #20 I outlined that the instant collapse of the wavefunction isn't special to entangled particles and happens for a single particle as well. The experiment with one photon is easy (already done probably).

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Take a light source that produces short pulses, say 50ps long, and emits one photon during some pulses, most often none, so that pulses of several photons are scarce. Split in two beams sent apart, say over 1m each at 90°, so they are 5ns away from an other where detected.



The photon is expected to be detected at either one end or the other, and after one propagation time within the pulse duration, so that no data can be transferred so quickly between the detector locations. Here as well, the wave function collapses instantly.

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The photon does take both paths: this is checked easily by modifying the experiment's downstream part into the usual Michelson interferometer - typically by removing the two fast detectors.
http://en.wikipedia.org/wiki/Michelson_interferometer



Fringes, reaction to the arms' length... reveal interferences resulting from two paths taken by the single photon.

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The source can use for instance a pulse-driven VCSEL:
http://en.wikipedia.org/wiki/Vertical-cavity_surface-emitting_laser
VIS offers the V40-850C1 (more references at other companies) for 100Gb/s Ethernet at 850nm in multimode fibres, with 10ps rise and fall time, and 30µm emitting diameter. 3mW light mean 600,000 photons per pulse.



A 1µm pinhole improves the optical quality and leaves <700 photons per pulse. There, a first offset detector can serve as a time trigger. A lens makes a nice beam.

An optical attenuator leaves <<1 photon per pulse, say mean 0.001 photon with <5 optical densities, so that only one pulse over 1000 contains several photons.

A pulse repetition period of 20ns still emits 50,000 photons per second, of which 10,000 may be detected, or one million in 100s.

The optical attenuator, or a set of, eases the adjustment of the setup with more light.

Marc Schaefer, aka Enthalpy

[Reaper79]

It looks like this has been asked and answered, but I've heard of a different way understanding this, without a Phd

 

The basics of it are, if you have 2 particles which are quantum entangled, one will always have a (positive charge) and the other cannot, no matter the distance involved if one is altered so too is its pair. But this leaves the question "how is that even possible?"

 

Imagine you own a fish tank, and in it you keep a single tropical fish, the fish tank is traditional in dimensions, rectangular 2 foot x 4 foot. Now you rig up two cameras, one on the wide side, and one on the narrow side, each camera feeds to a separate monitor. Place each of the monitors side by side and watch the fish swimming about. No matter it does on monitor 1, it does the exact opposite on monitor 2.

 

Right away it doesn't sound very reasonable to compare fish on monitor screen to accomplished science, but it did help me get a fingernail on what was actually happening.

[imatfaal]

It looks like this has been asked and answered, but I've heard of a different way understanding this, without a Phd

 

The basics of it are, if you have 2 particles which are quantum entangled, one will always have a (positive charge) and the other cannot, no matter the distance involved if one is altered so too is its pair. But this leaves the question "how is that even possible?"

 

Imagine you own a fish tank, and in it you keep a single tropical fish, the fish tank is traditional in dimensions, rectangular 2 foot x 4 foot. Now you rig up two cameras, one on the wide side, and one on the narrow side, each camera feeds to a separate monitor. Place each of the monitors side by side and watch the fish swimming about. No matter it does on monitor 1, it does the exact opposite on monitor 2.

 

Right away it doesn't sound very reasonable to compare fish on monitor screen to accomplished science, but it did help me get a fingernail on what was actually happening.

 

Hmm Reaper - I am not so sure about your analogies.

 

Firstly, you seem to be positing a classical situation ie that the states are already set; it isn't that one is heads and thus the other must be tails or one spin up etc. It is that both are in a quantum superposition of states until one is measured; and once measured both entangled particles will resolve to the two states that were part of the superposition. Entanglement is the state in which the characteristics of two quantum particles cannot be given individually - all that can be given is the description of the shared state; but once you actually interact with one of the particles you know what that particles individual state is and thus the shared nature collapses and the other particle will be found to be in the opposite state.

 

Secondly, "no matter the distance involved if one is altered so too is its pair." this bit seems to promote the old chestnut that if you change one side you can read the change on the other. This is not the case. Again it can only really be explained and thought of in non-classical terms - but suffice to say that the "old chestnut" would allow super-luminal communication of information and this is most certainly not the case. Basically if you know the state of one characteristic of a particle then it cannot be entangled through that characteristic.

 

This is not trying to say entanglement isn't jaw-droppingly weird - it is! But it is strange in very complicated ways - and almost entirely not in the ways that pop-sci and sci-fi would have you believe.

[swansont]

I will add that any classical analogy will fail because this isn't a classical phenomenon. And it's not a matter of the analogy failing because it was taken too far, it fails immediately because a crucial aspect of entanglement — that the states are indeterminate until measured — is not part of any of these analogies.

[Reaper79]

Yes, your right of course, entanglement is much more complicated that watching fish swim about on monitors.

 

Sorry about the poor analogy... I have no scientific background, I just watch 100's of hours of documentaries, talks from the world science fairs and anything else I can get my hands on. If anything I'm an enthusiastic observer. Sometimes too enthusiastic (smiley face).

Edited August 26, 2014 by Reaper79