# B Entanglement distance?

1. Jan 22, 2017

### megacal

Hello,
I had never heard of "entanglement" (in regards to particles) until today when I read
an article on it in Discover Magazine (July-August 2016, p.68).
I searched for and found several threads here, but were too advanced to
understand and/or did not seem to address my questions below.
BTW, have only had under-graduate physics & math for a BS in Microbiology (1972).
1. What distance is one particle theoretically able to be entangled with another?
2. How can they be said to be entangled if trillions of other particles separate them?

2. Jan 22, 2017

### phinds

There is no distance limit. A pair of entangled particles are a single system. It is irrelevant how far apart they are and it is irrelevant that other particles are between them.

3. Jan 22, 2017

### megacal

Ok, thanks. It's theoretically possible, and no offense, but so are pigs that fly.
Although I do think it's absolutely amazing that all that we see in the observable universe
is made up of incredibly (infinitely?) small particles tugging and repulsing each other, forming
structures, e.g. galaxies, galaxy clusters, super clusters, etc. that are connected by gravity
over such immense distances. So in that regard, I believe we are entangled.

4. Jan 22, 2017

### phinds

Yes, but pigs that fly have never been observed and entanglement has been experimentally shown to be true on numerous occasions so they're not really similar.

5. Jan 22, 2017

### megacal

How did they isolate the particles so they could be studied?
The Discover article described how polarized light, but I don't understand how you isolate a pair
of particles (photons or protons) to show they are entangled. You really are not working with
a specific pair of particles.

Also, you said the distance or number of particles in between the entangled pair is irrelevant......how can that be proven?
It may be mathematically possible, but there's no way to test it.

6. Jan 22, 2017

### Comeback City

It could be experimentally proven by simply increasing the distance that separates the entangled particles, and then performing the experiments to see if it yields the same results. I'm pretty sure the distance would be irrelevant, though, as Phinds stated.

7. Jan 22, 2017

### Mentz114

Entangled photon pairs can be created with PSDC, see the Wiki article

https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion

Experiments have confirmed this phenomenon many times, starting with Alain Aspect

https://en.wikipedia.org/wiki/Bell_test_experiments

Last edited: Jan 22, 2017
8. Jan 22, 2017

### DrChinese

The first thing is to understand some of the properties that make entangled particles different than un-entangled particles. There is something called "Bell Inequalities". Entangled particle pairs violate the inequality, others do not. So the answer to your question is to run a Bell test on particle pairs and look at the results. So there is a way to test this.

Second, as to distance: progressively longer and longer distances have been measured for entanglement. It went from being a few feet, to a few football fields, to even longer (this is 10 years old):

Abstract: Quantum Entanglement is the essence of quantum physics and inspires fundamental questions about the principles of nature. Moreover it is also the basis for emerging technologies of quantum information processing such as quantum cryptography, quantum teleportation and quantum computation. Bell's discovery, that correlations measured on entangled quantum systems are at variance with a local realistic picture led to a flurry of experiments confirming the quantum predictions. However, it is still experimentally undecided whether quantum entanglement can survive global distances, as predicted by quantum theory. Here we report the violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality measured by two observers separated by 144 km between the Canary Islands of La Palma and Tenerife via an optical free-space link using the Optical Ground Station (OGS) of the European Space Agency (ESA). Furthermore we used the entangled pairs to generate a quantum cryptographic key under experimental conditions and constraints characteristic for a Space-to-ground experiment. The distance in our experiment exceeds all previous free-space experiments by more than one order of magnitude and exploits the limit for ground-based free-space communication; significantly longer distances can only be reached using air- or space-based platforms. The range achieved thereby demonstrates the feasibility of quantum communication in space, involving satellites or the International Space Station (ISS).

https://arxiv.org/abs/quant-ph/0607182

9. Jan 22, 2017

### DrChinese

You asked about the theoretical limits, and phinds answered that there were none. The thing that ends entanglement is an observation. I gave you an example of entanglement over a large distance, and even larger are planned. There are even some attempts to perform observations of cosmic light particles - these are still in the very early stages of design. On the other hand, there has entanglement preserved for an hour - quite a technical feat.

As to "theoretical" flying pigs: please note that there is no theory of flying pigs (as in flying like a bird) because pigs do not possess a mechanism for flight. So as was pointed out: the word "theoretical" has a more specific meaning around here, and it is not a derogatory term.

Lastly: there are many scientists who believe entanglement is pervasive. In some sense, you could say that "we" are entangled. Note that there is nothing that limits entanglement to just 2 particles, and entanglement of billions of particles has been experimentally demonstrated. On the other hand, there seem to be some limits but they are not well understood at this time.

10. Jan 22, 2017

### megacal

Thank you for all the replies. I very much appreciate the time you take to help us who don't
have higher math ability. If I misused the term "theoretical", it was not meant to be derogatory.

If entanglement has been verified by multiple experiments (over 30 yrs?) and accepted as reality by the Physics
Community, then I must accept it as well, even if it seems impossible.

I'm surprised I hadn't heard of it until now, though it appears
to be common knowledge here. I'm way behind the curve!

I can't spend more time on it at the moment, but will in the coming days, and
may have a few more questions later.

11. Jan 22, 2017

### phinds

This will hardly be the last thing you encounter that you will find impossible if your pursue knowledge of quantum mechanics and/or cosmology.

12. Jan 22, 2017

### sanman

Is it possible for an entangled particle to undergo tunneling, and yet still remain entangled?

13. Jan 22, 2017

### Staff: Mentor

You should give Louisa Gilder's (fascinating, not technically demanding) book "The age of entanglement" a they.

14. Jan 22, 2017

### Staff: Mentor

You are stringing words together in a way that doesn't make sense. There's no such thing as "undergoing tunneling", because tunneling isn't something that happens to or is done to a particle. There's a probability of finding the particle in various locations. These probabilities are different from what classical theory predicts and we use the word "tunneling" to describe the differences.

15. Jan 23, 2017

### rockart

I'd guess that since a photon is two dimensional, not existing in its direction of travel, that once it has been emitted, it will remain entangled with whatever emitted it until it interacts with another three dimensional object.

16. Jan 23, 2017

### phinds

As far as I am aware, this makes no sense at all. It seems that you misunderstand photons, EM radiation, and entanglement.

Light does not travel as photons, it travels as waves. Photons are the result of an EM wave interacting with a particle. Entanglement is not something that just automatically happens by virtue of a photon having been emitted.

The language on this gets a bit fuzzy. For example, we say that a photon is emitted, but the resulting EM radiation is a wave, not a photon. Actually, to be more precise about it, a photon is a quantum object. It is not a wave and it is not a particle. If you measure it for wave characteristics, it exhibits wave characteristics and if you measure it for particle characteristics, it exhibits particle characteristics, but it is not one or the other. It is a quantum object. Period.

Also, you can, with some rationale, talk about the size (amplitude) of an EM wave, but to say that a photon has 2 dimensions is not meaningful.

17. Jan 23, 2017

### Comeback City

Photons can be thought of as quantum packets of electromagnetic energy. Can you even measure energy in dimensions?

18. Jan 23, 2017

### Staff: Mentor

No, and this is only one of several reasons why the idea that "a photon is two dimensional, not existing in its direction of motion" is nonsense.

However, all of this is a digression in this thread. Any further discussion should happen in a new thread - but please do look at some of the many threads we already have about what a photon is.

19. Jan 23, 2017

### megacal

This Wiki article, Quantum Entanglement, seems the best overview I've found so far (thanks to earlier links provided by Mentz114),
and just ordered a copy of The Age of Entanglement by Louisa Gilder recommended by Nugatory.

Also appreciate the info by Dr.Chinese, and am studying the pdf of the 144km experiment using
the Optical Ground Station (OGS) of the European Space Agency he linked to.

This is going to take me a while to digest....fascinating stuff!

Last edited: Jan 23, 2017
20. Jan 23, 2017

### mikeyork

First of all, it is a god idea to think of entanglement as a fundamental QM phenomenon that operates at a level in which space-time is irrelevant. So, even though we necessarily see it as happening in our space-time context, it is independent of any space-time co-ordinates or measurements such as distance.
Second, the presence of trillions of other particles is also irrelevant unless the entangled particles have interacted with any of them. If they dont "see" or experience them in any way then they can have no effect. On the other hand, if they did interact then yes, they would interfere with the prior entanglement.

21. Jan 23, 2017

### sanman

Pardon me for articulating myself poorly. So I just wanted to know if an entangled particle which is then observed to have tunneled across some potential barrier, would still be able to maintain its original entanglement.
Is that more clearly stated now?

There are all kinds of interactions which will collapse the wavefunction indeterminacy into some particular state, thus losing the entanglement with other particles. I just want to know if that entanglement would be lost if tunneling subsequently occurs.

22. Jan 23, 2017

### DrChinese

If the entanglement was on a basis not relevant for the tunneling, it would not be affected per se. If it were entangled in energy, then it would serve as a kind of measurement to have tunneled.

23. Jan 23, 2017

### megacal

- Quantum Entanglement, Wiki
I accept that it as reality, but how does one particle "know" what the other particle is doing or what is being done to it???
It's as though they are connected by a thread (or string?) that has no elasticity if they react instantly to the measurement of
the other(s) in the system.

24. Jan 23, 2017

### mikeyork

The phrase "It thus appears" is important here. It refers to how things appear from a classical viewpoint -- which we know to be wrong. Confusion continues around this in the popular literature (and in Wikipedia). Understanding QM requires a willingness to abandon classical ideas that no longer serve us.

25. Jan 24, 2017

### megacal

Are you saying the Wiki article is inaccurate?