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In my poor understanding of quantum physics, quantum entanglement means that certain properties of one of two 'entangled' quantum particles can lead to change over infinitely large distances when the other particles' properties are changed.

Disregarding this already mind-boggling event taking place over say 10 meters distance; how have physicists been able to demonstrate, beyond reasonable doubt, that this can take place over infinitely large distances?

For instance: have they done some of these tests between ISS and Earth perhaps?

How can they be so sure?

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    $\begingroup$ Just as soon as we put two detector infinitely far apart. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Apr 28, 2011 at 21:30
  • $\begingroup$ Since the ISS is only around 350 km above Earth, entanglement has been shown at a reasonable fraction of this distance. Of course, this is nowhere near infinity. $\endgroup$
    – Peter Shor
    Commented Apr 28, 2011 at 21:31
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    $\begingroup$ Infinitely large distances should be replaced by arbitrarily large finite distances. $\endgroup$
    – Qmechanic ♦
    Commented Apr 28, 2011 at 21:37
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    $\begingroup$ The answers below are good, but noise in the apparatus ensures that it gets increasingly difficult to construct an experiment that verifies the violation of Bell inequalities as the distance increases. The ESA experiment is not desktop technology. Beating the effects of noise makes longer range experiments progressively more expensive and requires progressively more ingenuity. We have to introduce not only more effective shielding from noise but also more sophisticated ways to allow for the effects of the noise that can't be shielded. $\endgroup$
    – Peter Morgan
    Commented Apr 28, 2011 at 22:17
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    $\begingroup$ @knzhou this question is asking about the distances involved in experiments being extrapolated to much larger ones, rather than about whether we have proof about entanglement as in the proposed dupe. Can you clarify what is duplicated between these two? $\endgroup$
    – Kyle Kanos
    Commented Dec 16, 2018 at 12:55

2 Answers 2

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First of all, it is very important to note that quantum entanglement is not a spooky action-at-a-distance as Einstein once called it! It is a strong correlation of measurements that is stronger than any classical correlation could ever be. This has been experimentally verified by the so-called violation of Bell's inequalities.

Second, quantum entanglement has been successfully demonstrated for distance of some 100 km, as the Wikipedia article on quantum teleportation states (references in Journales such as Nature are given there)

Zeilinger’s experiments on the distribution of entanglement over large distances began with both free-space and fiber-based quantum communication and teleportation between laboratories located on the different sides of the river Danube. This was then extended to larger distances across the city of Vienna and most recently over 144 km between two Canary Islands, resulting in a successful demonstration that quantum communication with satellites is feasible. His dream was to bounce entangled light off of satellites in orbit.[1] This was achieved during an experiment at the Italian Matera Laser Ranging Observatory.[4]

Admittedly, that is not a demonstration at infinite distance, but then, nothing has ever been demonstrated at that length scale and we would never be able to do and to demand such a demonstration is unreasonable. But please not the different scales involved here! Usually, we think of quantum mechanics to govern the microscopic world, involving length scales of under a micrometer, i.e. $10^{-6} m$. Achieving quantum effects on a length scale of $100 km = 10^5 m$ means you span 11 orders of magnitude. You can jokingly call this "infinite for all practical purposes".

The misunderstanding is that nothing changes in the moment of your measurement. There is a classical analogue that is wrong on the details but emphasizes the main idea: Suppose you take two playing cards, one is a king and the other a queen. You shuffle them and let your friend pick one at random while you keep the other one. Now, you have the king with probability $1/2$ and the queen also with probability 1/2, but you don't know which one you have without looking. You also don't know which one your friend has. But as soon as you look at your card, you immediately know which one your friend has: If you got the king, he must have got the queen. There is no spooky action at a distance that somehow changes what the card of your friend looks like!

In quantum mechanics, it is admittedly a bit more complex, because not only would you not know what card you have without looking, it isn't even determined until you look. It is, however, possible to go through the math and the mechanisms and come up with a satisfactory explanation that does not involve faster-than-light communication and processes.

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    $\begingroup$ Good answer! One additional point. As you say, you can never test at infinite distances, so the best you can do is test at very large distances. When someone says "large", you should always ask "large relative to what"? In the case of these experiments, all of the natural length scales associated with the particles (wavelengths, etc.) are microscopic, so doing an experiment over 100 km is a truly enormous distance -- probably billions of times bigger than any other relevant length scale. So while this isn't infinite distance, it's very nearly as good! $\endgroup$
    – Ted Bunn
    Commented Apr 28, 2011 at 21:17
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    $\begingroup$ My intro mechanics professor used to say "for our purposes, infinity is 5, maybe 10" ;-) $\endgroup$
    – David Z
    Commented Apr 29, 2011 at 2:49
  • $\begingroup$ The fundamental point is that QM is not based on probability, but on probability waves (or fields) which, differently from normal statistics, exhibit interference. $\endgroup$
    – Sklivvz
    Commented Apr 30, 2011 at 16:44
  • $\begingroup$ @Sklivvz There's a lecture and article by Scott Aaronson where he argues that all the axioms of QM are exactly what you expect when you define a notion of probability that uses the 2-norm instead of the 1-norm $\endgroup$
    – Lagerbaer
    Commented Sep 20, 2011 at 16:46
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Since my Alma Mater was involved I can point to this:

A team of European scientists has proved within an ESA study that the weird quantum effect called 'entanglement' remains intact over a distance of 144 kilometres.

enter image description here (Source)

In September 2005, the European team aimed ESA's one-metre telescope on the Canary Island of Tenerife toward the Roque de los Muchachos Observatory on the neighbouring island of La Palma, 144 kilometres away. On La Palma, a specially built quantum optical terminal generated entangled photon pairs, using the SPDC process, and then sent one photon towards Tenerife, whilst keeping the other for comparison.

There is an effort for Quantum Entanglement in Space Experiments.

Experimentally is has not been proven to work over an infinite distance, since that would be impossible.

To better understand Quantum Entanglement (and why distance is not a problem) I suggest starting with the EPR Paradox.

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