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There is a small debate over the comment session in NASA Lands InSight on Mars video uploaded by JPL. The 1 minute and a half video is about the essential part of InSight's EDL event cropped out from the full live broadcast back on Nov 26.


The commenter posted a comment: "The actual celebration is 7 Minutes in delay." Under the reply session, a Indian user named SCIENCE SIDE claims that he's currently studying PhD in Theoretical Physics, IISc Bangalore. He replied that we actually received the signals in just 1-2 seconds with the help of MarCO CubeSats. Later on followed up with confusion from other users with his statement and asked for clarification, he then mentioned that it is relating to quantum entanglement for communications.

Whether or not that is accurate, I'd still like to ask the following: Could using entangled photons be used instead for interplanetary communications (between Earth & Mars) to shorten the real-time latency as much as possible in future?

A statement about Quantum Entanglement from Wikipedia

Entanglement is considered fundamental to quantum mechanics, even though it wasn't recognized in the beginning. Quantum entanglement has been demonstrated experimentally with photons, neutrinos, electrons, molecules as large as buckyballs, and even small diamonds. The utilization of entanglement in communication and computation is a very active area of research.

**Do note me if this is considered as cross-site question with any other SE site. Enclose a relatable useful reference regarding to communication in space.

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    $\begingroup$ I made small edits for two reasons 1) entangled photons work in principle for radio or light or any other wavelength, 2) the comments may not be true, but you don't want that to invalidate your question, which is a good one! $\endgroup$ – uhoh Nov 28 '18 at 1:48
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    $\begingroup$ Relevant xkcd: xkcd.com/1591 . $\endgroup$ – David Hammen Nov 28 '18 at 7:16
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    $\begingroup$ I think the best you can obtain is to halve the time required for cryptographic communication (which would otherwise take twice as much as unencrypted). You can use quantum entanglement for FTL communication if your communication consists entirely of white noise. And while the white noise itself doesn't carry any useful information, it can be used to create cryptographic keys. The cryptogram still needs to be sent by traditional means, but you don't need to wait to receive the key - you may generate it from a stockpile of entangled particles the other half of which the other party has. $\endgroup$ – SF. Nov 28 '18 at 8:30
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    $\begingroup$ Quoting: "I'm dammm sure that it's used for communication...I'm damm sure dude....!! Even I know it's mathematics." Moral of the story: ignore armchair "scientists" in YouTube comments. $\endgroup$ – Lightness Races in Orbit Nov 28 '18 at 10:30
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    $\begingroup$ @SF I'm sure there's some very important physical distinction you're making, but in terms of signalling and cryptography I don't think it's relevant: some material has been transmitted in advance which is used to create the key. This is similar to shipping an algorithm with secret parameters from which the real key is derived. A set of non-entangled particles in a known state could be transmitted in exactly the same way, and used as a one-time key in exactly the same way. The advantage of using entangled particles is the inability to clone them, not the ability to get them anywhere faster. $\endgroup$ – IMSoP Nov 28 '18 at 13:22
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Apparently not:

I like this Quora answer. Here's part of it, the rest is worth reading as well:

No experiment conducted using entangled photons has ever demonstrated faster than light communication!

There have been many such experiments. They were not looking for faster than light communication. They were testing quantum mechanics against Einstein's local hidden variable hypothesis. They essentially have shown that quantum mechanics works as advertised.

Furthermore, there are no proposals for faster than light communication using entangled particles!

If ever there was the slightest hint that FTL communication were possible, expect a media explosion… The world would definitely notice and there would be no need to ask such questions on Quora.

As it stands, quantum mechanics is still consistent with causality and is compatible with special relativity. That means no information transfer faster than the speed of light.


Additional background:

The Forbes article/blogpost The Real Reasons Quantum Entanglement Doesn't Allow Faster-Than-Light Communication Goes into great detail on this topic. The illustration helps remind us what communications using entangled photons really means. You generate a pair somewhere else, and each of the two parties in question receives one member of the entangled pair.

Naively one might think that if the two paths were equidistant and if one person "does something" to one photon upon receiving it, this would "simultaneously" "result" in the other photon "knowing about it" and to therefore "do the other thing". Per helpful comments below, varying interpretations of the word "instantly" may be where the common misapprehension lies. This is the extent of my understanding of why some people including myself get the idea that entangled photons could allow FTL communication.

enter image description here

Also enjoy Veritasium's video Quantum Entanglement & Spooky Action at a Distance which does a great job of explaining some of the basics of entangled photon experiments and at the end addresses why FTL communication is not thought to be possible using them.

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    $\begingroup$ While "instant" or "simultaneously" are indeed not valid terms to describe space-time relations, their precise possible meanings are not the issue here. (One scenario sufficient to be hopeful of "very fast" communication is to determine the quantum state of one particle entangled with another one far away, then traveling there to tell, and the observer at the other place says "I have known that for a long time".) The issue is that you cannot communicate anything that way. $\endgroup$ – Peter A. Schneider Nov 28 '18 at 15:46
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    $\begingroup$ For a more elaborate and longer answer on why this question even has been asked so often and what is the proper answer to it, please watch this video youtube.com/watch?v=dEaecUuEqfc (The Quantum Conspiracy: What Popularizers of QM Don't Want You to Know / Ron Garret - the title is a joke, there is no actual conspiracy or even the hint of it presented.) $\endgroup$ – Ashnur Nov 30 '18 at 15:16
  • $\begingroup$ @Ashnur "... there is no actual conspiracy..." that's what they want you to think! Just kidding! I've seen one or two Google Tech Talks before and they were great. Thank you very much for this link! I will give it a watch this weekend. $\endgroup$ – uhoh Nov 30 '18 at 15:23
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He replied that we actually received the signals in just 1-2 seconds with the help of MarCO CubeSats. Later on followed up with confusion from other users with his statement and asked for clarification, he then mentioned that it is relating to quantum entanglement for communications.

This is nonsense; the MarCOs received the signals in 1-2 seconds, but relaying the signals to Earth took the usual several-minute speed-of-light delay.

Quantum entanglement doesn't presently offer any way of achieving faster-than-light communication. The reasons are complex, but you can read an overview here. MarCO were not equipped with any quantum-entanglement communication devices.

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  • $\begingroup$ I edited the question at the same time you posted but I don't think it changes the suitability of your answer. $\endgroup$ – uhoh Nov 28 '18 at 1:49
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    $\begingroup$ In fact I didn't believe his statement in the first place as well; in which that doesn't make any sense at all. That is why I tried to focus my question on Quantum Entanglement in interplanetary communications rather than debates over his statement that would twists my question and its meaning :) $\endgroup$ – Boosted Nub Nov 28 '18 at 1:57
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    $\begingroup$ "doesn't presently offer any practical way" - It will never offer any way, practical or not. $\endgroup$ – JollyJoker Nov 28 '18 at 8:56
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    $\begingroup$ @JollyJoker unless we're wrong about something, which may be the case. $\endgroup$ – OrangeDog Nov 28 '18 at 15:14
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    $\begingroup$ Removed "practical", but saying "never" about technological development is usually a poor bet. $\endgroup$ – Russell Borogove Nov 28 '18 at 18:12
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As someone deeply involved in quantum information/entanglement research: You cannot, under any circumstance, use entanglement to communicate faster than lightspeed. Ever.

The "no-signalling principle" says that no information can be transferred faster than lightspeed, even using all the tricks in the quantum book. If it were to be violated, a bunch of weirdness can occur, mostly violating causality. No one has seriously proposed that we can violate the no-signalling principle; computer scientists have looked at the consequences and they're.. exciting.

You can use quantum entanglement to distribute random information between separated parties, but even that relies upon correlating your measurements either ahead of time or after the fact. This cannot be used to send particular bits of information.

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    $\begingroup$ "You cannot, under any circumstance, use entanglement to communicate faster than lightspeed. Ever." -- In the framework of quantum mechanics, in other words provided quantum mechanics is a sufficiently correct theory. Of course, everybody expects reality to be causal (meaning no faster than light communication), but "Ever." is too strongly worded ;) $\endgroup$ – s.harp Nov 28 '18 at 17:14
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    $\begingroup$ @s.harp: It's not too strongly worded. If such a thing were possible, it would be completely outside of what QM models, and thus not "using quantum entanglement" but using some new theory. $\endgroup$ – R.. Nov 28 '18 at 20:23
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    $\begingroup$ Can you distribute random information faster than the speed of light? $\endgroup$ – Graph Theory Nov 29 '18 at 1:36
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    $\begingroup$ @GraphTheory: That view is a consequence of the awfulness of the Copenhagen interpretation, where the word "collapse" suggests some physical change. If there were any such physical change, it's outside the scope of anything QM or any other physical theory is capable of describing or making predictions based upon, so from a standpoint of the ability to apply QM to make predictions/"do something", it's not there and not useful. $\endgroup$ – R.. Nov 29 '18 at 19:25
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    $\begingroup$ A better way of thinking about "collapse" is like a conditional probability, where upon discovering new information, your ability to make predictions about other correlated things improves. For a classical example, as a random person you may have an X% chance of dying from a certain condition, but after having DNA tests done, your personal chance becomes Y% (maybe less, maybe more). Nothing (well, except state of neurons, etc.) physically changed from the new knowledge; rather you just got new information that lets you make better predictions. $\endgroup$ – R.. Nov 29 '18 at 19:36
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I tried to understand this topic for years but never really got it. But the first answer provides a great article which made it finally click for me.

The part that is missing in a lot of explanations and the most important part is:

It’s a brilliant plan, but there’s a problem: entanglement only works if you ask a particle, “what state are you in?” If you force an entangled particle into a particular state, you break the entanglement, and the measurement you make on Earth is completely independent of the measurement at the distant star.

I'm not sure if what the article is saying is technically not correct or why other sources do not mention it, but it helped me greatly understand the problem.

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    $\begingroup$ A non-quantum analogy: You order two pizzas, one with garlic and one without. You give two addresses, your own and a friend's. When you get a pizza without garlic, you instantly know your friend got the one with garlic. Now this is the tricky part: If you put garlic on the pizza you have, it will not remove the garlic from your friend's pizza. $\endgroup$ – JollyJoker Nov 29 '18 at 11:12
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    $\begingroup$ That would be Einstein's hidden variables. In QT, both outcomes are possible. $\endgroup$ – choeger Nov 29 '18 at 16:26
  • $\begingroup$ monk order's a hot dog: "make me one with everything". Einstein orders a pizza, "hold the hidden variables". Great answer, thank you very much! $\endgroup$ – uhoh Dec 3 '18 at 0:50

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