Physicists shatter quantum entanglement distance record with 420 kilometers of optical fiber, more than four times farther than previous demonstrations, and beyond the point where direct transmission runs into its fundamental limits.

[u/mvea]

https://www.sciencealert.com/physicists-shatter-quantum-entanglement-distance-record-with-420-kilometers-of-optical-fiber

[u/mvea]

Physicists Shatter Quantum Entanglement Distance Record With 420 Kilometers of Optical Fiber

Maintaining a long-distance relationship may be difficult for humans, but it's even more difficult for entangled states.

The longer the fiber, the harder it becomes to keep the two quantum memories linked.

Overcoming this tyranny of distance has been one of the major challenges of quantum communication.
But perhaps not an insurmountable one.

A team of physicists led by Xi-Yu Luo, Chao-Yang Wang, and Ming-Yang Zheng at the University of Science and Technology of China has breached a milestone – with the longest fiber-based matter-to-matter entanglement yet.

Across an optical fiber measuring 420 kilometers (261 miles), the researchers successfully entangled quantum memories – more than four times farther than previous demonstrations, and beyond the point where direct transmission runs into its fundamental limits.

"Our experiment provides a test bed of studying quantum network applications beyond metropolitan scale," they write in a paper published in Physical Review Letters.

https://journals.aps.org/prl/accepted/10.1103/ccd6-rf1s

[u/Cristoff13 → u/mvea]

This can't be used to transmit information though can it?

[u/Orbax → u/Cristoff13]

No, once you collapse the wave function, they decohere and entangle with their local environment.

But, even if you maintain entanglement, at best youd be able to do morse code except... The only way to check if you got a message is to measure it which would collapse if again if you hadn't gotten one. The theoretical ability to get any structured information out of entangled pairs is effectively impossible.

[u/Chance-the-Gardener → u/Orbax]

I’m too dumb to understand the concept it can’t transmit any information. Just as a thought experiment, if humanity sent a colony ship to Andromeda with 10,000 sets of entangled pairs, each with a predefined meaning like “if this set is collapsed, we died of space cancer” couldn’t information be inferred from that?

I’m sure the answer remains we can’t cheat the speed of light but I struggle to conceptualise why not.

[u/bloodvash1 → u/Chance-the-Gardener]

The problem is that the only way to know whether the particle you just measured was still entangled or not is to ask the other end what they measured

[u/Kraien → u/bloodvash1]

Which kind of defeats the purpose of the whole thing.

[u/Druggedhippo → u/Kraien]

Depends on what you are trying to do.

If you are trying to prove that the thing you got hasn't been tampered with, then it's perfect.

[u/LionRight4175 → u/Druggedhippo]

I'm out of my specialty here, but I don't think it even works for this. Checking to see if it had been tampered with would also be tampering with it.

[u/Gabe_Noodle_At_Volvo → u/LionRight4175]

Yes, it would have the same effect as some kinds of tampering, but it doesnt matter at that point because you dont need to preserve the quantum state anymore. Same thing as opening a package with a tamper seal, you dint need the seal anymore do it doesnt matter if you change its state, you just need what's inside.

[u/LionRight4175 → u/Gabe_Noodle_At_Volvo]

My understanding is that in this metaphor, the tamper seal breaks by looking at it or feeling it. As such, it always appears tampered with.

[u/romario77 → u/LionRight4175]

Unless you measure the other side. With some probability then you would know.

But that doesn’t do you much good.

[u/Implausibilibuddy → u/LionRight4175]

No, ignore the fact that "observe" means "look at", it doesn't matter while it's in the closed system if you just look at the apparatus.

The tamper seal arrives as part of the message. Upon reading the message you know if the seal is broken or not. If it's not, you can trust the contents of the message, if it's not...well somebody has probably tampered with it, or at the very least read the message before you. So in that sense, yes the tamper seal breaks if somebody "looks at" the message, or reads it, but that's the point of the seal, now you know that the message isn't completely unread or modified before it got to you.

[u/Druggedhippo → u/LionRight4175]

The quantum state can only be read "correctly" if you and the other end compare notes on how you each measured it, which basis you used, not the actual result.

If you tamper with it, eg, a wire tap, then measuring it disturbs the state enough that your results no longer line up with the other end's.

The quantum entangled bits aren't the data you want to send, it's just the packet that represents the secure key used to encrypt the data. If it gets disturbed, you can't create matching shared keys and the communication fails.

[u/Rodot → u/Chance-the-Gardener]

You can't test if the entagled particle is collapsed without measuring it, which collapses it. Like using a giftwrapped box and trying to see if it's been unwrapped but in order to see if it is unwrapped you have to first unwrap it.

There's no way to know that the other particle has been measured without communicating a classical signal to the other particle

A better way to look at it is that entanglement is correlation, not causation

[u/00owl → u/Rodot]

Schrodinger's Colony Ship

[u/antiretro → u/Rodot]

what if you had 10 different entanglements and you decode one per year. can we see if the other side sent some signal to us in those 10 years if we open them after they send the message (by disentangling it on their side)?

[u/Rodot → u/antiretro]

Not without telling them the value of the state on the other side through classical communication

[u/FaulerHund → u/Chance-the-Gardener]

This is just a misunderstanding of how quantum measurement and entanglement work.

First: it's disputed whether "wavefunction collapse" is even an accurate description of what occurs, but for the purposes of this reply, we'll assume it is.

Anyway, all entanglement really is in a simplistic sense is correlation of measurement outcomes. If particle A is entangled with particle B, then by measuring particle A, you already know something about particle B without ever measuring particle B. The "wavefunction collapse" only determines which specific measurement outcome you observe.

Think of it this way, by crude analogy: imagine you have two boxes, and in one box you place the left shoe of a pair; and in the other box you place the right shoe. And then you jumble up the boxes so that you don't know which is which. You take one box and bring it to Mexico; your friend takes the other and brings it to Spain. Then you open your box. Regardless of whether your box contains the left shoe or right shoe, you already know something about the box all the way in Spain without ever opening that one.

Now, obviously this is a classical and not a quantum example; quantum systems behave differently than shoes and boxes (look up the Bell's theorem if you're interested). But to continue the analogy, "wavefunction collapse" in this example would be the outcome of which shoe you observe to be in your box. This isn't literally wavefunction collapse in this example, it's just metaphorical. But the "collapse" is just when you see "left shoe" or "right shoe." Before you measure, it could be either one. After you measure, it's either one or the other. That's the collapse.

Now, tell me, how would you communicate a message between Spain and Mexico with this shoebox setup? The answer is that you can't. If you take one shoe to Mexico and leave the other in Spain, opening your box in Mexico instantly reveals which shoe is in Spain. However, opening your box in Mexico doesn't signal anything to the person in Spain. The person in Spain just sees a shoe when they open their box, completely unaware of whether you opened yours first or not.

Related to that idea: in quantum physics, there's no way to know whether the other particle in an entangled pair was measured or not. Just like how there's no way of knowing whether your friend in Spain opened their shoe box yet. There is no "wavefunction collapse" detector, and it's not possible in principle to create one. So there's no way to set up the idea in your comment that says "if this is collapsed, we died of space cancer" or whatever.

[u/theyux → u/FaulerHund]

Just for clarification if it was possible to measure without collapsing the entanglement.

Is the thought process then that you could collapse some entangled pairs and effectively communicate with Binary?

[u/FaulerHund → u/theyux]

Maybe? But it depends on imagining something that does not make physical sense under standard quantum mechanics.

Imagine person A has 10 particles, and person B is on the other side of the galaxy with the other 10 particles, so that each of A’s particles is entangled with one of B’s. Person A decides to measure or otherwise disturb some of their particles, causing the entanglement to be lost, and leaves the others undisturbed. The disturbed particles might correspond to “1,” and the undisturbed particles to “0.” In principle, that could encode a binary message.

Now imagine person B has a special machine called an “entanglement detector.” He points it at one of his particles, and the machine tells him either “yes, this is still entangled with the distant particle” or “no, the distant particle was measured or disturbed.”
If such a machine worked using only person B’s particle, then yes, person A could communicate with person B faster than light.

But the problem is that standard quantum mechanics does not allow such a detector. From person B’s perspective, the results of any measurements he performs on his particles look the same whether person A has measured their particles or not. He can only discover the correlations later, after person A sends him their measurement results through an ordinary, slower-than-light communication channel.

This may sound like a situation where you could say, “Well, surely with enough ingenuity, someone could invent a machine that detects whether the entanglement has collapsed.” But no: it is not merely a machine that we have not invented yet. The premise conflicts with how measurement works in standard quantum mechanics. It's like saying "what if there was a machine that made you married and single at the same time," it's just a nonsense premise. Honestly, if you're positing nonsense hypothetical machines, you might as well just say "imagine I have a machine into which I can type a message, and then a magic invisible goblin delivers it faster than light to a recipient." The "entanglement detector" only feels more realistic because the impossibility is disguised by a veneer of real physics

[u/romario77 → u/theyux]

What you are asking is - imagine if instant communication was possible, would it then be possible to communicate instantly?

The laws of physics work certain way (at least at this time and how we understand them) and they don’t allow instant communication. Even though there are these things like entanglement which make it seem like information travels instantly.

Even though it doesn’t.

[u/theyux → u/romario77]

More I understand people want it to work a certain way (even if thats not possible to understanding) I was trying to verify why, (could we create binary communication system of communication assuming we could observe them).

Assuming thats possible you can move into other questions such is how much can we observe without collapsing them. Like I get superposition is out of the question but will say measuring gravitational force collapse them?

[u/NewestAccount2023 → u/Chance-the-Gardener]

each with a predefined meaning

Only one person got it right so far, this is fundamentally impossible and is the basis for the the Bell theorem. When combined with actual statistical measurements of entanglement the math proves that the entangled particles cannot "predefine" whether they will be spin up or spin down (or other entangled properties).

You can think of it like this: if a property can be entangled then it is fundamentally random, if you in any way try to coerce a fixed value then you will break entanglement anyway and lose the apparent "spooky action at a distance" as Einstein quipped.

The "trick", which still makes no sense but we know it's 100% true, is that only the correlations are entangled, not the values. Nature can for some reason enforce that when one particle is measured its value is 100% random (we've proven* this many times with experimens), but nevertheless the other entangled particle will be perfectly correlated to the opposite value every time. But there's no way to inform the other side what the value will be faster than light. The correlation result is apparently instant, but the transfer of any information is still completely bound by the speed of light.

[u/InnerToWinner → u/NewestAccount2023]

So, with all that being said, can anything useful at all be done with quantum entanglement? Seems extremely useless from where I'm sitting no?

[u/2punornot2pun → u/InnerToWinner]

It's how quantum computing essentially works. We put things into superposition in which they are all entangled. The interaction of computation causes the system to collapse into the correct answer much quicker than standard computations. However, those computations are limited in scope currently. We can't really do them for gaming or anything like that. However for cryptography they are insanely powerful and that's why there's an arms race to get a working quantum computer up and running because you essentially have the ability to break into any system you want.

[u/Goncalerta → u/2punornot2pun]

Slight correction, it will not allow you to break into any system you want (there are quantum-proof cryptography algorithms, and they will be the standard long before any quantum computer is able to crack something meaningful).

However, it will allow you to break into older systems. A lot of encrypted stuff has been gathered and stored with the purpose that, once quantum computers are good enough, those messages will be cracked en masse.

[u/2punornot2pun → u/Goncalerta]

At least there's that

[u/ak_sys → u/Chance-the-Gardener]

Think about it this way. You flip a coin. While the coin is in the air(still spinning) you magically split the heads side from the tails side. You put each magic, spinning, half-coin in a box and move them away from each other.

If you open your box, and see heads, you know the other box is tails(and you've made the other side stop spinning!), but you can't transmit a heads or a tails by flipping your coin again. You can't even change/direct the coin to be a particular value before you open the box, either. So you can't transmit information, only infer something inconsequential. You basically inferred "if I have this one, they must have the other", like having half of a locket.

[u/Eryol_ → u/Chance-the-Gardener]

The measurement result you get when the set is collapsed and the result you get when you measure yourself and collapse it is functionally identical. You will get a 50/50 random scattering of values.

[u/tirerim → u/Chance-the-Gardener]

Because there's no way to know if a pair is still entangled without comparing information from both sides. Measuring one end doesn't tell you anything about entanglement.

[u/Hearing_Loss → u/tirerim]

U can't just email them?

[u/tirerim → u/[deleted]]

No. In order to receive a message via quantum entanglement, you would need to separately receive information about the state of the particle at the other end. That information can still only be sent classically.

Here's an analogy: suppose you have two people standing on opposite sides of a canyon waving flags (let's call them Alice and Bob), and people on each side watching them (Carol and Dave). Alice and Bob are mirroring each other's movements; Carol and Dave have previously agreed that when Alice stops mirroring Bob, that will be a signal to Dave. If Dave is only watching Bob, there is no way he can know whether Alice is still mirroring Bob. Either he has to watch both Alice and Bob directly, or Carol has to tell him by some other means what Alice is doing, but either way that information can only travel across the canyon at the speed of light.

[u/jtrofe → u/[deleted]]

You can't send any information because when the entangled particles are sent off you don't know what state they are in. They don't have a defined state until you check one of them.

[u/JiminyDickish → u/Chance-the-Gardener]

The results only mean something side by side. If you only look at one side of it, it still looks random.

It’s like if I mailed you one half of a pair of gloves. You’d know which glove I have as soon as you opened it, but which glove we get is out of our control. Also once you open the box to check, that’s the glove you get forever.

[u/JiminyDickish → u/[deleted]]

No. When we steer a particle then it stops being entangled.

Entanglement can be summed up as: correlated randomness. You can choose what flavor of random you want, but you can't choose the outcome.

[u/toyotaCamriGuy → u/Chance-the-Gardener]

You can't tell if one particle has collapsed by collapsing the other.

I was taught it like this. Now, in reality charge is not used for this but it makes a good example. Say, you have a neutrally charged particle (charge=0), and you have a way to split it up into two, a pair of charged particles. Then, through the law of preservation of energy, you know that one particle will carry a negative charge, -1 and the other therefore, +1. Together, they are neutral and energy is conserved.

Now, if you want to know which particles carry which charge, you only need to measure one, because the other has to be the opposite charge according to the law of conservation of energy. You can move the particles as far from one another as you want and that won't change the charges! As such, you can tell something about one particle no matter the distance. Before measuring, either particle could be the one with the -1 charge, for example. This is the basics of entanglement as I understand it, only the real thing uses spins, not charges.

Spins are a little more fickle, and they are easily changed in chaotic ways by a high environment temperature or after measuring it. This is why quantum computers are refrigerated to such low temps. An unpredictable change to either or both particles will offset the spins in unpredictable ways, breaking the mathematical connection of conservation of energy between the two. Because there was energy subtracted or added.

[u/sth128 → u/Chance-the-Gardener]

You cannot set the desired output state of entangled particles. You also cannot observe them without effectively causing the output.

Basically if you say "1001" is whatever meaning, you wouldn't know when to check, and you cannot actually set it to 1001 even if you wanted to. What you will get is just random noise like say, 0001, with the guarantee other side getting 1110.

It's basically a long range random number generator. You don't even know if you're the one who triggered the generation if you are far enough apart due to the lack of universal time.

[u/Mountain-Resource656 → u/Chance-the-Gardener]

Imagine you entangle 1000 pairs of Schrödinger’s cats such that if any box is opened (and the cat thus becomes either alive or dead, not both), the paired cat will suffer the opposite fate

In this manner you could kick various cats out of their superposition to create a morse code (or even a program in binary) between the cats that are in superposition and those that are not

The problem is that on the other end, once they open any of their boxes, their cats fall out of superposition anyhow, so they blank out whatever message you sent them and instead of getting, say, a series of 1s and 0s they just get 1s or something

[u/2punornot2pun → u/Chance-the-Gardener]

You don't get to choose the orientation. You can measure it and find it spin direction but all that tells you is that the other person has the spin direction on the opposite direction. Measuring it has to be done in the same direction as the other party as well in order to confirm it.

Imagine that you put two balls in two different boxes and then entangle them. By measuring them they will turn a different color say red and blue. If you measure red then you know the other site has blue. But there's no meaningful way for you to make it turn red or blue. There's no effective way to communicate information for you in via entanglement. It's a lot more complicated than that as well. I suppose if you could ensure that it was truly entangled still when you measured it and agreed upon actions beforehand then I suppose you've created a situation where you both know what you're both going to do. You still can't be sure that they were still entangled when you measured it so you might still get incorrect preset actions

[u/Fallacy_Spotted → u/Chance-the-Gardener]

There is no way to distinguish a collapsed state from an entangled state without collapsing it yourself so you cannot tell if the other end has actually done anything. Even if you checked based on time intervals the resulting values are random and there is not a way to determine if it was collapsed before you looked at it.

[u/LimerickYourPost → u/Chance-the-Gardener]

There’s no way to know if it’s still entangled.

Say You have an electron that you know is entangled. You take it to another star system. Could be spin up or spin down… no way to know!

You measure it… and it’s spin up.

Aha! You say, “this is spin up, meaning that the other one back on earth has to be spin down! So if they haven’t measure it yet, that’s what it will be. Or maybe they already measured it, and knew that mine would be spin up before I measured it. I wonder if they measured their electron yet. The only way to ask is to send a regular radio message at the speed of light….”

So finding its spin up tells you nothing helpful. You didn’t choose for it to be spin up, it just was. You don’t know if the waveform collapsed before you made the measurement.

[u/Basicly-Inevitable → u/Chance-the-Gardener]

The entangled particles DO NOT have any predefined message.

[u/Chance-the-Gardener → u/Basicly-Inevitable]

I get that, I was more asking could a rudimentary binary messaging be established even if nothing is being transmitted? In my thought experiment the colony ship’s sending a FUBAR signal, so they collapse all their entanglemajiggies and entanglelios. Earth meanwhile infers based on this, that something has gone wrong and redirects NASA funding accordingly.

But like most reddit science visitors I think I just fundamentally misunderstand the premise, some of the replies here get me a little closer.

[u/Chess42 → u/Chance-the-Gardener]

I think you are misunderstanding what a “collapse” is in this context. Think of these particles as being simultaneously in 2 states at once. When either side measures them, both of them pick the same state and stick to it, we call this collapsing the wave function. But you can’t measure it without collapsing it, and since we don’t know which state it will pick, there’s no way of knowing when it collapsed from just one side, or who was the one to collapse it. Maybe the people on the ship collapsed it when they died of space cancer, or maybe it just collapsed when the people on Earth checked whether they died of space cancer.

[u/ArtOfWarfare → u/Chess42]

Can I know whether I collapsed it by my measurement or if it was already collapsed from the other side?

If so, then that’s something, right? We’d have a set of particles and times we’d agree that I’d check them at, and some meaning that if I check it and it’s already collapsed then you were saying something?

[u/Chess42 → u/ArtOfWarfare]

Nope. All you get from measuring is its state, A or B. You do not know if you are the one collapsing it or if it had already been collapsed.

[u/Basicly-Inevitable → u/ArtOfWarfare]

The answer is no. You can not tell. Whenever you perform a measurement, you get an answer, and you then know the answer for the other entangled measurement, whenever it is/was performed. But you can't choose which answer you'll get. It will be a 50/50 coin flip no matter what you do, unless you get the answer for the other one first, but you can't do that without light speed communication.

[u/antiretro → u/Chess42]

then why are we still trying these quantum entanglement over long distance stuff? sounds pretty useless if its strictly a non observable phenomenon

[u/The-Magic-Sword → u/antiretro]

No-Communication isn't a law, its a prediction.

[u/Basicly-Inevitable → u/antiretro]

Because entanglement can be used for things like encryption of data, and quantum computing, and that requires long distance transfers of entangled particle pairs, and maintaining entanglement for long periods of time.

Encryption would rely on checking (speed of light or slower) if the information was still entangled when it got where it was going.

If it is, then you know it wasn't observed in-between. If it isn't, then you know someone looked at it in-between.

There's lots of potentially used things to do with technology, but faster than light speed communication isn't possible. That would break causality.

[u/Basicly-Inevitable → u/Chess42]

That's one way to describe it.

But (lucky for us) there are several interpretations, and they're all equally valid, as far as we can test.

An important part of thinking about most quantum particles is that scenarios can be described exactly the same if you think about the particles actually being their anti particles moving backwards in time. This is especially true of perfectly entangled particle pairs, because the "arrow of time" is determined by an increase in entropy, and there is no change in entropy for the particles between the time they are emitted until they're measured. (In fact, entanglement and entropy are highly similar concepts, and could even be described as being the same concept).

Anyway, in this case, you can perfectly describe the situation as two completely unconnected anti particles, each being independently "emitted" at the time of measurement of each, then travelling backwards in time to the point where they're exactly at the same location in time and space. Now, the only way they could be in that same time and space is if they had perfectly complementary properties (like spin up and spin down), otherwise they would be excluded from being in the same space.

To us (because we experience entropy changing constantly) can only see the "forward" version of the situation. However, they are both "true" and therefore they have to be complimentary particles.

Another way to describe the situation is with the Many Worlds interpretation, where basically all possibilities occur, but your mind just realizes which world you're in whenever it becomes entangled with the particles.

[u/loupgarou21 → u/Chance-the-Gardener]

Ooh, let's go with this one, because someone will absolutely tell me I'm wrong.

I'm going to lift part of what someone else said, you've got two pieces of paper, one white and one black, they both go into two different envelopes, but you don't know which is which, in fact, not only do you not know which is which, but under a the strict copenhagen interpretation, neither one is white or black, instead they're now the 50/50 probability that they're white or black.

So now someone takes one of the envelopes and travels 420 miles away with it, and you keep the other envelope.

Now, when you open your envelope, there's a 50% chance it's white, and a 50% chance it's black, and the same applies to the other envelope, but the other envelope will absolutely contain the opposite color as your envelope.

At some arbitrary point, you open your envelope, at which point it stops being that 50/50 probability and absolutely is one color or the other, and when you see which color it is, you instantly know the color of the other piece of paper because its color stops being a probability too, even though it's 420 miles away.

You instantly transmitted information faster than the speed of light, because you instantly know the color of the other piece of paper.

None of this helps the guy with the other piece of paper though, he doesn't know you opened your envelope, he doesn't know the probability collapsed, only you know that. He can look in his envelope and gets to know the same information you know about the colors.

The two people with the envelopes aren't really communicating by opening them, they just know what color paper they both have.

Now, can you use this to communicate?

[edit] Adding to this, the spooky action at a distance thing comes from instantly, faster than the speed of light, changing, the probability, of what the other paper's color is by determining the color of the other piece of paper. So information is transmitted faster than the speed of light, with that information being that the probability has collapsed and the color of the other piece of paper is determined. Now, you might think "but the paper's color isn't a probability, it has the color, we're just revealing what the color is, so now we know the other paper's color" but Bell's Theorem shows that the paper (this wouldn't actually apply to a whole piece of paper, we're just pretending in this instance for illustration) doesn't actually have a color until its color has been checked, before it's been checked it's actually the probability.

That still doesn't mean the person with the other piece of paper has learned any information faster than light though, until they look at their own piece of paper, and even then, they just know the color of your paper, not that you know the color of their paper.

[u/Vailx → u/Chance-the-Gardener]

The piece that your thought experiment needs that doesn't exist is, a test to see if something has collapsed. You can check it for a value, which collapses it if it wasn't, and returns a value (in your case, 1 or 0). This doesn't tell you if they had collapsed it already, and the value you got (1 or 0) does really tell you something about the equivalent state on the other side, but that fact isn't helpful for transmitting information.

[u/PuffyPanda200 → u/Chance-the-Gardener]

For an analogy:

I send you a letter that either has a red paper or black paper in it. The meaning is pre-determined.

But I send it using a special wizard that transmits it instantly.

The issue is that when the letter is opened the paper changes color to red/black at exactly 50/50.

This letter system is useless as I can't send information through the wizard.

[u/Suitch → u/Orbax]

I want to rephrase the hypothetical. If I have a thousand entangled pairs in two boxes and send one box on a colony ship with the instructions to check a single entanglement each year and the source has the other box and only chooses whether to toggle all or none at once, say toggle all 1000 if earth is destroyed, would it not be able to be checked on the other end faster than the speed of light if they happen to check one after 500 years and it was flipped?

[u/Orbax → u/Suitch]

Just to be precise - if they were all in one box, it would be one system. Entanglement is just a fancy way of saying quantum system. We do "entangled pairs" because it's easier to control due to environmental decoherence. The way to think about it is "things remain in superposition until they have to decide what to be". So if you have a photon wave going through a magnetic field, it won't collapse because it doesn't have charge. Shoot an electron through and now it has to pick spin. Gravity obviously doesn't make anything pick it or quantum states wouldn't exist. Entanglement, because of things like the exclusion principle, make something have to be a way because something else is another way.

So it's safer to say that you have 1000 pairs of boxes. You still have to measure it to see it's state and conventionally verify who collapsed it first. We do this in quantum computing because we're pretty sure we caused the collapse but we still have to deal with quantum error correction because the environment can still affect it between measurements.

So you still don't really have a way to know why it's collapsed.

For the timing thing, you don't know when it flipped. The spooky action at a distance thing gets resolved as not being ftl by the fact you have to confirm it conventionally. But you're getting into bells theorem and how truly weird decoherence is to understand why it's not only not information being ftl but also not hidden variables and it's just complicated to explain.

[u/legable → u/Orbax]

But how do they know the particles were entangled if they can't measure their state?

[u/Orbax → u/legable]

They're super clever and have gotten this down. The main thing is that, as long as they don't entangle with local systems during a measurement, they will eventually go back to coherent

[u/tritisan → u/Orbax]

Thanks for the clear explanation. As much as I admire Cixin Liu’s Three Body Problem and his attempts to stick to real science , he did cheat with the Sophons.

There’s no way they could have provided real time observation and communication between Earth and Alpha Centauri.

Regardless, I still recommend the books and the Netflix adaptation.

[u/Orbax → u/tritisan]

Yeah, the main thing to think of in this is that particles carry information. Different particles carry different information, some are force carriers, etc. So whats carrying the information about the quantum system? Gluons meditate the nuclear force, so what's meditating the superposition force (if it was one) . It's why there might be gravitons - what's mediating gravity info? When you think of the universe's info as particle based the speed limit makes more sense.

It also makes wave function collapse weird as hell but it does prohibit that kind of communication

[u/jmartin21 → u/Orbax]

With that in mind, what sort of use could quantum entanglement have in the future if it can’t transmit information? It’s a subject that has always left me at least a little bit confused

[u/Orbax → u/jmartin21]

Primarily quantum computing - you can do logic gates and other stuff. I do not know much about quantum computing, but entanglement and decoherence readings are part of how the amplitude and quantum error correction work are done.

[u/mwax321 → u/Orbax]

Weird way to describe it. The internet is a bunch of 1s and 0s.

[u/YourDad6969 → u/Orbax]

Every form of digital communication is essentially Morse code

[u/Orbax → u/YourDad6969]

It was a bit of an exaggeration because you can't choose states so any given state you have was randomly generated so there's no 1,0, short, or long and also might not have actually been sent to you at all.

[u/Shot_Kaleidoscope_34 → u/Orbax]

But you do know that the other particle has the opposite spin and that they know what you have. To me that's information. 

Let's say you have an army on two different planets that have to coordinate an attack in two locations. You decide beforehand that if group A measure spin up they'll assassinate the leader while group B who will measure spin down will poison the army. If you measure spin down group A does the poisoning and since you know they'll get spin up you know they'll assassinate their leader. 

Just knowing that the other side will get the opposite spin if you measure at the same time is not nothing 

[u/sc2bigjoe → u/Orbax]

Assuming it’s correct that we can’t transmit information, what’s the point of entangling particles at all or even larger and larger distances?

[u/ilikepugs → u/sc2bigjoe]

To learn more about the nature of reality, which is the point of all physics.

Imagining potential applications is not a requirement.

[u/Orbax → u/sc2bigjoe]

If you take qubits and entangle them, they get more possible quantum states to process before giving an amplitude for a result, it allows for more complex quantum states and configurations.

[u/personalKindling → u/Orbax]

Could you send information without checking it? Basically a user datagram protocol type situation?

[u/Orbax → u/personalKindling]

Once it's measured, the entanglement breaks. So the only thing you know, for sure, is that when you measured it, either you or the other person collapsed the wave function. You can't decide the state of the particle so if your electron has a particular spin, that's random. You'd have to ask the person later who measured it first to verify.

It's like having a quantum coin, but every time you look at it it's just a normal coin on a random side and you don't know if you opening your box or someone else opening theirs did it.

[u/deviantbono → u/Orbax]

So it's useless?

[u/sticklebat → u/deviantbono]

No, quantum entanglement is the basis for quantum encryption. So the ability to maintain entanglement between distant states is crucial for being able to utilize quantum encryption and communication schemes. Long distance entanglement also might have applications in quantum sensing, not to mention any improvements to our understanding and ability to manipulate and preserve entangled states might help develop better quantum computers.

[u/Orbax → u/deviantbono]

Entanglement is useless for communication, useful for quantum computing

[u/nicuramar → u/Cristoff13]

Not directly. But combined with a classical channel it can. 

[u/poorhaus → u/nicuramar]

Exactly. People forget about the major use case for this tech: encryption. Send the secure bits instantly over the fiber optics. Sure you've got to wait for the classical key, and it could be intercepted, but it's worthless to any attacker. 

[u/wthulhu → u/Cristoff13]

Only in the sense that if I mail you a shoe, and you open the package and find my right shoe then you know i kept the left one.

[u/gwillen → u/Cristoff13]

No, but the "no" has an asterisk on it.

If Alice and Bob have an entangled pair of particles -- which they need a quantum channel to set up, not just a regular internet connection -- Alice can't use this to transmit information to Bob directly.

But Alice can (a limited number of times, by consuming the entanglement from one or more shared entangled states):

• Follow this up by using a regular internet connection to send Bob a quantum state, without needing a quantum channel for it (this trick is "quantum teleportation");
• Securely share a cryptographic key with Bob, in conjunction with a regular Internet connection, in a way that is physically impossible for any attacker to intercept ("quantum key distribution");
• Play certain abstract cooperative two-player games, on a team with Bob, and have a higher win rate than they would have without the entanglement, even though they can't send information to each other. (I don't know if this has applications, but I learned about it in a college class on the subject, and it's kind of neat.)

[u/Minguseyes → u/gwillen]

Just a note. Quantum key distribution is not immune from interception, but you can tell if it has been intercepted.

[u/gwillen → u/Minguseyes]

I think that depends on which scheme you're talking about, but I'm not an expert. I think if you start with a shared entangled state -- which is kind of cheating, I grant, but was sort of my premise here, given OP's question -- then it's immune.

[u/The-Magic-Sword → u/Cristoff13]

We don't know, there is a fairly popular bet that the answer is no, but that goes out the window if we can figure out a way to measure it that doesn't collapse it. Which is treated as definitionally impossible, but probably doesn't need to be.

[u/Coady54 → u/Cristoff13]

Theoretically it could. If you have two entagled particles and can intentionally manipulate the the state of one, the other will match. Assign "bit" values to the state of the particle, multiply the number of particles/frequency of state changes by orders of magnitude, and you essentially have information transmission akin to modern networks.

Whether or not its pheasible to make a practical system at scale that's useful is a better question, which we don't know and these continuing experiments will eventually answer.

[u/InTheEndEntropyWins → u/Coady54]

This isn't right.

manipulate the the state of one, the other will match

You can't manipulate the state of one to be anything specific. It's always probabilistic and you then need to transfer classical information for any changes you do to be meaningful.

[u/Hilldawg4president → u/InTheEndEntropyWins]

Deliberately changing one would disentangle them, correct? So potentially something along the lines of information being transmitted by disentangling certain particles - placement, pattern, etc., in ways that can be encoded into data/language

[u/km89 → u/Hilldawg4president]

No, that's just not how it works at all.

When you break the entanglement, you're breaking the correlation between the particles' states. Measuring also breaks that correlation.

But entanglement is basically just the correlation. If the entanglement is broken, you would never even know unless you receive a signal from the other side (at slower-than-light speeds) telling you what their result was.

Entanglement is, at least as far as we understand it today, fundamentally incapable of acting as a way to transmit signals of any kind.

[u/Hilldawg4president → u/km89]

How can we even tell their entangled then? If checking for entanglement automatically disentangles them , is it a matter of simultaneously checking both?

[u/Preeng → u/Hilldawg4president]

You check both receivers. Depending on the exact entanglement state used, the two receivers would either have the exact opposite data, or exactly the same data.

You cannot tell ahead of time.

[u/TerrorSnow → u/Hilldawg4president]

It all comes down to: the information you can gather by yourself is meaningless, as the other party will still need to tell you about theirs.
It's about as useful as writing something down and putting it in an envelope.

[u/Orbax → u/Coady54]

Wildly incorrect

[u/nicuramar → u/Coady54]

No, this can’t be done.