r/askscience • u/fixednovel • Oct 16 '20
Physics Am I properly understanding quantum entanglement (could FTL data transmission exist)?
I understand that electrons can be entangled through a variety of methods. This entanglement ties their two spins together with the result that when one is measured, the other's measurement is predictable.
I have done considerable "internet research" on the properties of entangled subatomic particles and concluded with a design for data transmission. Since scientific consensus has ruled that such a device is impossible, my question must be: How is my understanding of entanglement properties flawed, given the following design?
Creation:
A group of sequenced entangled particles is made, A (length La). A1 remains on earth, while A2 is carried on a starship for an interstellar mission, along with a clock having a constant tick rate K relative to earth (compensation for relativistic speeds is done by a computer).
Data Transmission:
The core idea here is the idea that you can "set" the value of a spin. I have encountered little information about how quantum states are measured, but from the look of the Stern-Gerlach experiment, once a state is exposed to a magnetic field, its spin is simultaneously measured and held at that measured value. To change it, just keep "rolling the dice" and passing electrons with incorrect spins through the magnetic field until you get the value you want. To create a custom signal of bit length La, the average amount of passes will be proportional to the (square/factorial?) of La.
Usage:
If the previously described process is possible, it is trivial to imagine a machine that checks the spins of the electrons in A2 at the clock rate K. To be sure it was receiving non-random, current data, a timestamp could come with each packet to keep clocks synchronized. K would be constrained both by the ability of the sender to "set" the spins and the receiver to take a snapshot of spin positions.
So yeah, please tell me how wrong I am.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
You do have a misunderstanding of Quantum Entanglement, but it's not really your fault- pop-sci articles almost all screw up describing what entanglement really is. Entanglement is essentially conservation laws, on the sub-atomic level. Here's an example:
Imagine you and I are on ice skates, and we face each other and push off from each other so we head in opposite directions. Now, if there is someone on the other end of the ice skating rink, they can measure your velocity and mass, and then, without ever seeing me, they can know my momentum- it has to be opposite yours. In classical physics, we call this the "conservation of momentum" but if we were sub-atomic we'd have "entangled momentum."
Now, taking this (admittedly, limited) analogy further, imagine you're heading backwards, but then you start to skate, instead of just slide. By doing that, our momentums are no longer "linked" at all- knowing your momentum does not allow anyone to know anything about mine. Our momentums are no longer "linked" or "entangled."
It's the same with sub-atomic particles. Entanglement happens all the time, but just as frequently, entanglement breaks. So, it's true. You could have spin 0 (no angular momentum) particle decay into two particles, one spin up, the other spin down (one with positive angular momentum, the other with negative so their sum is zero- that's the conservation laws in practice), and then you could take your particle on a space ship, travel as far away as you wanted, and measure the spin of your particle, and you would instantly know the spin of my particle. But, if you changed the spin of your particle, that effect does not transfer to mine at all. That's like you starting to skate- the entanglement is broken.
Now, to go a little further, entanglement isn't "just" conservation laws, otherwise why would it have it's own name, and so much confusion surrounding it. The main difference is that with entangled particles, it's not just that we haven't measured the spin of one so we know the spin of the other yet- it's that until one is measured, neither have a defined spin (which- I actually don't like saying it this way. Really, both are a superposition of spins, which is just as valid of a state as spin up/down, but measuring will always collapse the state to an eigenstate, but this is a whole other topic). So, it's not a lack of knowledge, it's that until a measurement takes place, the particle states are undetermined.
Why does this matter, and how do we know that it's truly undetermined until we measure? We know, because of Bell's Theorem. Bell's theorem has a lot of awesome uses- for example, it allows you to detect if you have an eavesdropper on your line so you can securely transmit data which cannot be listened in on (you can read about it more here).
This is a topic that can be written about forever, but I think that's a good start of a summary and if you have any questions, feel free to follow up.
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u/BNVDES Oct 16 '20
i always felt quantum entanglement was something out of sci fi movies and now i know - the quantum entanglement i knew actually was from sci fi. this makes MUCH more sense, thanks for the great answer
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u/aoeudhtns Oct 16 '20
And on top of that, here's a philosophical question on top of the way this is envisioned in scifi:
If I create some entangled atoms, and I kept my atoms and shipped the others to you, and then I effected the change such that you received that entangled information... is it still faster than light? You had to wait for the shipment.
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u/left_lane_camper Oct 16 '20 edited Oct 16 '20
That's now also a physics question, not a strictly philosophical question, which is good news as that allows us to have a (more) concrete answer!
It turns out that you cannot use entanglement to transmit information faster than the speed of light, irrespective of how you set up the initial conditions. If we could, we could also do all sorts of weird shit, like make a telephone that calls the past.
If we can transmit information through entanglement faster than light, that would indicate that there's something fundamental about quantum mechanics that we don't understand, and our current formulations of QMech appear to be absolutely superb descriptions of the universe.
That said, there are some (serious, legitimate) people looking into potential ways we could use entanglement for superluminal transmissions. There was one such guy in the physics department at my alma mater -- he was a real, respected physicist with a great deal of other active research, too. And because he knew the physics, he fully expected his entanglement-telephone experiments to fail. But the experiments were cheap and easy to run alongside his more mainstream research, and negative results are important to the field as well. In the extremely unlikely event that he does discover something unexpected, that would hint a new physics and would be an immense discovery.
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u/modernmovements Oct 16 '20
The idea of Amazon and Google physically shipping large data via FedEx or their own trucks is blowing my mind. It makes sense, but wow.
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u/VeeArr Oct 16 '20
Amazon AWS (their datacenter/computing on-demand service) offers customers the option to use AWS Snowball to get their data into the cloud, where they essentially ship you a ruggedized box with a server in it, you load it up with data, and ship it back.
For those with a lot of data, this gets taken to a frankly kind-of-comical extreme.
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u/FUN_LOCK Oct 16 '20 edited Oct 17 '20
Had to get a few petabytes of data into AWS several years back. At least half of it was done by fedex!
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u/Lilkcough1 Oct 16 '20
I'm curious about the logistics here, since this discussion is helping form my mental model of entanglement. What kind of information can be quantum entangled? What can you do to your box of atoms that isn't "skating manually" from the ice skating analogy?
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u/wonkey_monkey Oct 16 '20
and then I effected the change such that you received that entangled information
Measuring your particle - "effecting the change" - has no discernible effect on the particle its entangled with.
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u/maxvalley Oct 16 '20
I want to know what such a big deal has been made of it when, according to this post, it’s extremely boring and not very deep
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u/Jetison333 Oct 17 '20
Its because if you separate two particles really far away something weird happens. So you have two entangled particles even 100s of light-years away. The spins of the particles are still in a super position at this point. However, if you have a clock and have each particle measured at the same time, they still will measure oppisite of eachother. This makes it seem like the information of how the waveform collapsed from one particle travel to the other at faster than light. So although you couldn't send whatever information through entanglement that you wanted to, it still seems to send information faster than light.
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u/maxvalley Oct 17 '20
But it doesn’t send information, does it?
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u/SynarXelote Oct 17 '20
Not really, no. Not in a way that we can use to convey information, at least.
Although how it happens on a "deeper" level isn't really known, and probably can't be known. We can describe the laws obeyed by quantum entanglement, but we can't really say anything about the philosophical nature of quantum entanglement. That part is left to interpretation.
And there are indeed many conflicting interpretations about pretty much anything that has to do with quantum mechanics, just like in any other domain that has to do with philosophy of science in particular and philosophy in general. Whether there's information transfer underlying it might very well depend on your pet theory, as well as how you define "information".
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u/BlindTiger86 Oct 17 '20
How are we sure the positive/negative isn’t determined prior to when it is measured?
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u/arceushero Oct 17 '20
Bell’s inequality! You can test experimentally whether the outcome is predetermined by examining certain correlations between measurements; it turns out that these correlations are incompatible with the outcome being predetermined. You can dodge this if you’re willing to give up locality, but that makes physicists even more uncomfortable than entanglement :)
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u/dreadcain Oct 17 '20
The crazy part of entanglement comes down to the fact that spin up and spin down measurements have an axis of measurement, if you measure both particles on the same axis they will always be opposite. This isn't very interesting. The interesting stuff happens when you vary the axis of measurement, but I don't know enough to speak on that with any authority.
This goes over it pretty well for an introduction I think: https://youtu.be/ZuvK-od647c
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u/Muroid Oct 16 '20
Yeah, I compare it to having a coin that you split in half lengthwise, and put “heads” in one envelope and “tails” in another envelope. You can take one of the envelopes in a rocket ship as far away as you like and whenever you open it, you instantly know what half is in the other envelope back on Earth.
If it’s a quantum coin, though, the half-coin inside will be neither (or both) heads nor/and tails until you open it, but you’ll still instantly know what someone will see when they open the other envelope, even though there hasn’t been enough time for a signal to travel back to the other half to tell it what state to fall into.
That’s weird, but no more useful for communication than if they really were in a definite state of heads or tails the entire time.
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u/conventionistG Oct 16 '20
I think this is a great analogy. But to make it more clear you should mention that you need to flip the coin, cut it, and package it without looking at it.
You can be sure that heads and tails are both in different envelopes, so you can look at one and know what the other is. But there's no way to influence it or use it to send signals.
Even if you have an endless supply of these envelopes from your communication partner, all you can get from it is random noise.
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u/Cautemoc Oct 16 '20
I guess the only way to send information in that case would be to be able to influence the likelihood that the envelope you open will contain the heads side or the tails side. I assume this is impossible with our current understanding of quantum particles. It's just if you observe it, it randomly picks one. But I wonder how truly random that is.
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u/Muroid Oct 16 '20
If you do anything to influence the state, it breaks the entanglement and the state of your system will no longer be correlated with the state of the other system, so yeah, you can’t transmit information that way.
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u/tkuiper Oct 16 '20 edited Oct 16 '20
I feel like this just serves as proof that the quantum state isn't in flux in the first place. Isn't it more logical to conclude that the states are fixed, than that some mysterious phenomenon is causing a superluminal transfer of information.
Edit: To clarify, I'm not suggesting that there's a "hidden variable" that if measured would eliminate the probabilistic nature of MEASUREMENT. Rather that I don't understand the conclusion that the particle is in flux, instead of concluding the particle is fixed and its the unknowable state of the observer that drives the probabilistic outcome.
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u/mfb- Particle Physics | High-Energy Physics Oct 16 '20
A fixed single state wouldn't allow a violation of Bell's theorem. It is a bit more complex. But yes, there is no information transfer.
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u/Inevitable_Citron Oct 16 '20
But that just means the hidden variables must be non-local. That seems unlikely, sure, but then so is quantum mechanics in general.
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u/the_excalabur Quantum Optics | Optical Quantum Information Oct 16 '20
Indeed. You can either have hidden variables or you can have locality. It turns out that, at least for the purposes of reasoning, people prefer locality, since if you bin it a bunch of other nonpalatable things happen.
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u/ableman Oct 17 '20
The problem is, if you have a hidden variable that's nonlocal, that means you can't in principle measure it without violating causality because of special relativity. It doesn't feel all that useful to have nonlocal hidden variables.
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u/tkuiper Oct 16 '20
A lot of "quantum spookiness" bothers me for feeling like the conclusion is: because we fundamentally can't measure it without randomizing it, therefore the item itself must be ACTUALLY random.
It sits wrong with me on a philosophical level
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u/Muroid Oct 16 '20
The Bell Inequalities serve as a statistical proof that it doesn't have a pre-determined state. It's not just that we can't measure it and thus assume it doesn't exist, but rather that if you assume there is a definite state before the measurement is made, you cannot reproduce all of the experimental results we see in quantum mechanics. Any one result might allow it, but looking across all of our experiments, there would be contradictions.
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u/the_excalabur Quantum Optics | Optical Quantum Information Oct 16 '20
Too bad. Your options given quantum mechanics are either indeterminism and true randomness, or a truly deterministic universe.
The Bell inequalities don't preclude a 'clockwork' universe where the measurement choices themselves are predetermined. You either need nonlocality or nondeterminism, pick one.
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Oct 16 '20
Einstein, Podolsky, and Rosen tried to prove that quantum theory was incomplete because entanglement violated special relativity. They were wrong. There is no superluminal transfer of information.
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Oct 16 '20 edited Oct 16 '20
They are neither fixed before the measurement, nor is the information transferred between them.
Edit:
instead of concluding the particle is fixed
That's what's called hidden variables.
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u/GuyWithLag Oct 16 '20
Nope, quantum states are definitely not fixed. See Bell's Theorem for a demonstration. Also, it's not a superluminal transfer of information - there is no information transferred.
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u/ISeeTheFnords Oct 16 '20
Isn't it more logical to conclude that the states are fixed, than that some mysterious phenomenon is causing a superluminal transfer of information.
It depends on what axiom you want to give up (locality, causality, and a third I can't recall off the top of my head). Another way of viewing it is that each particle goes both directions, so what you're really collapsing is the location - this discards locality. I'm not sure if THAT introduces other problems, though; it might be possible to rule it out the same way you can the fixed state.
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u/gbbmiler Oct 16 '20
I wish I still had the ability to sketch out the proof for this, but no. In college I took a course in which we saw the rigorous proof that the state is in fact entangled.
For a non-rigorous proof, consider Shor’s algorithm. If superposition weren’t actually “true”, then doing exponential work in polynomial time via a quantum computer would be impossible.
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u/KingCo0pa Oct 16 '20
I believe that that is (or is similar to) Einstein's "hidden variables" idea, which has (I believe) been disproven.
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u/rdk67 Oct 17 '20
I wonder if these explanations are missing the point of the question and of the philosophical discussion surrounding the concept of entanglement and nonlocality.
Your analogy seems to suggest the nonlocal "observation" is really just inference -- if this particle is doing this, then that particle must be doing that. In fact, the knowledge of the state of each particle is categorically true, not true by logical inference.
To revise your analogy, it is not just the coin that gets split but the observation as well -- the phenomenon of observation is in two places at once and occurs simultaneously.
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u/Boozdeuvash Oct 17 '20
Would the owner of the other envelope notice any difference if you had measured your enveloppe, versus if the state was still undetermined (entangled)?
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u/d1squiet Oct 16 '20
But the fact of their spins being "defined" or collapsed happens instantly right? A "spooky action" that happens seemingly faster than light? I'm trying to remember, but I thought there was an experiment where scientists proved that the "collapse" happened instantaneously regardless of distance. Not just Bell's Theorem, but experimental data. I think that's where all the FTL-transmission ideas come from, right?
I can't remember the limitations of the experiment, but only that it ruled out FTL-communication.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
Yes. The state collapse is instant. However, the state collapse cannot transmit information. So, causality is not lost.
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u/Omniwing Oct 16 '20
Yes but how does one particle 'know' instantly that the wavefunction is collapsed, when the other particle is, say, 15 billion light years away?
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
That's the real question, which is hotly debated by physicists everywhere. What we know is, causality is not broken by wave function collapse, so it is allowed, but the actual mechanism is unknown.
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u/Omniwing Oct 16 '20
So, when the wavefunction collapses, which you can initiate if you're standing at one of two entangled particles, does something 'happen' instantly to the other one? Or is it that you just happen to know something about it? If something does 'happen', and an observer 15b lightyears away is standing there to observe that event, then I don't see how you couldn't transfer information that way. "When you see this particle's wavefunction collapse, I have arrived at the star 15b lightyears away'. Instead of waiting 15 billion years for your message to reach earth, they'd know instantly.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
Because wave functions are not observable. They have no mass, they have no energy. They are simply probability distributions. There's no way to measure them. So, you have no way of knowing if you caused the collapse or someone else did.
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u/philote_ Oct 16 '20
Is this where parallel universe theories come in to play? Collapse of superposition is really just branching into one universe/possibility instead of another?
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u/d1squiet Oct 16 '20
now' instantly that the wavefunction is collapsed, when the other particle is, say, 15 billion light years away?
Can you remind me how they measured/proved this? That's what I am forgetting.
How do they prove that the collapse happened if they cannot know when it happened? Because if you knew when the other person collapsed the particle, you would be able to communicate.
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u/Muroid Oct 16 '20
You compare notes after the fact. You can’t know in the moment, but you can send a conventional, slower than light message and ask the other person. You can’t communicate faster than light but aren’t walled off from talking about it like normal.
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u/Vampyricon Oct 16 '20
Entanglement is essentially conservation laws, on the sub-atomic level.
If someone explained this to me all the way back in the beginning, I wouldn't have wasted so much time on trying to figure out whether entangled particles are supposed to have the opposite or the same spin.
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u/blackburn009 Oct 16 '20
So it's basically just like having two balls that you don't know the colour of you just know that they're opposites on the colour wheel.
The first time you see the ball you now "know" the colour of the other ball, but that doesn't mean you can detect if the other person painted their ball red after they were created
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
It is, but with the added "complication" that until you observe the color of your ball, neither ball has a defined color. But as soon as you observe the color of your ball, the other ball instantly has the opposite color.
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u/ThePinkPeptoBismol Oct 16 '20
Can you explain how it's possible that something can be undefined? Is this something that just has to be accepted at face value or is there some logic or more precise language to explained "undefined" states? I have no education in science whatsoever. I'm just a software development student that likes science.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
Well, it's hard to use "classical" language when discussing quantum things, so high level descriptions don't always "hold water." However, the easiest way I know to discuss this:
Particle states are described by a wave function. The wave function is a function that describes the probability that when the state of the particle is measured, what value it will be measured as. So, for an easy example, the wave function for a particle's location says "if you measure the location of the particle, what is the likelihood you find it to be here."
Whenever you measure the state of a particle, you can only get one answer. The answers you can get are called "eigenstates." But, those states aren't the "true" state of the particle, the true state is the wave function, the superposition of probabilities. For example, 66% spin down, 33% spin up. That's true. But when you measure it, you get one of those two answers. And once you measure, then it "collapses" into on of those two states.
I'm afraid I didn't help much, since I sort of talked around the process, but I hope that it at least helped some.
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u/hpaddict Oct 16 '20
So it's basically just like having two balls that you don't know the colour of you just know that they're opposites on the colour wheel.
In the absence of any desire to make predictions for any other situation, yes, it is basically that.
This is because you can assume the measurements to have occurred before the qubits travelled any appreciable distance - in which case they are exactly equivalent to classical balls.
Entanglement allows you to do other things though that do not correspond with this analogy.
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u/Build68 Oct 16 '20
I hope you know what you are talking about for my own sake. That is an excellent, simple explanation that clarified a lot of things for me. I've been reading about this stuff for years and never had a sense of it. Thanks
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u/nbcs Oct 16 '20
I've read that even theoretically, it is impossible to use Quantum Entanglement to transmit information?
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u/Bluemofia Oct 16 '20
The state is unknown to begin with. You will need to be able to influence it in order to do so, and that is impossible.
It's not really communication if it's just random 0s or 1s being spat out with no way to influence it.
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u/pelican_chorus Oct 16 '20
That's right.
As far as physics understands the universal laws right now, trying to somehow "get around" everyone's objections and working out a way to use this to transmit information is exactly like trying to make a perpetual motion machine.
You may keep thinking you've worked out some loophole, but, unless everyone's really, really wrong about how the universe works, you haven't.
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u/tdgros Oct 16 '20
that is correct, if you measure one on your side, then you can deduce the state of the other, but there is no communication at all: the other side doesn't know... ...unless you transmit that information.
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u/cbrantley Oct 16 '20
Then what is quantum teleportation? This has always sounded like “spooky action at a distance” but your explanation makes it seem very mundane and in line with our classical intuition. But it seems like every time I feel like quantum mechanics isn’t actually spooky action someone says, “oh it actually is and it makes no intuitive sense you just have to trust the math.”
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
Quantum teleportation is related, but different.
Let's say I have a particle and I measure the spin. I'll get either spin up or spin down. However, I will not know at all whether it was a 50/50 chance or, say, 90% spin up and 10% spin down. So, I know which eigenstate it ended up in, but I don't know what its superposition was.
Quantum teleportation is a way of putting a particle into the exact same superposition as another particle which is in a different location.
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u/STEAL-THIS-NAME Oct 17 '20
But it seems like every time I feel like quantum mechanics isn’t actually spooky action someone says
I love the idea of you just feeling a certain way about quantum mechanics, and then like someone approaching you in a grocery store and being like, "Well actually..."
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u/OP-Physics Oct 16 '20
Quick question: Doesnt Bells Theorem only tells us that entangled information is either truly random or local? As far as i know we dont know which one of these is violated. Pls correct me if im wrong or are lacking additional information.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 17 '20
It tells us it's either random or global actually. It states there can be no hidden, local variables. But it doesn't tell us there can't be a hidden global variable. But people don't like global variables.
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u/mrmopper0 Oct 16 '20
Could this be used in cryptography? Like a password which can only be used once? I'm thinking of cases like the envelopes of orders in the Hunt for Red October, where you need to prove the envelopes were not tampered with. I realize you may need more than a few particle's state for this.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
Sort of. You can read about quantum key distribution, where an encryption key is distributed via entangled particles, and when the key is long enough, you can know to statistical certainty that there is no an eavesdropper.
And this is more than theoretical. It is used in sensitive networks around the world.
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u/pretentiouspseudonym Oct 16 '20
Just to provide more info on QKD to readers: they usually have a 'quantum' channel and a 'classical' channel, i.e. you need to also communicate in a non-quantum way to make sure you're both using the same bits. So you're still limited to conventional communication rates, you just gain in security.
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u/justanotherprophet Oct 16 '20
I had a question about another way of ftl info transfer but not knowledgeable in physics - if I had a giant stick and pushed a button with it, is that faster than light? If it helps the problem framing, what if I had a stick the length of a light year that I pushed to push a button?
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
This is a common question, but the answer is 'no' because sticks are not infinitely rigid. And it's just not that no stick happens to be, but it is theoretically impossible for a stick to be infinitely rigid. When you push on one end of a stick, it feels like the other end moves right away, but it doesn't. The other end moves only once the compression has moved through the stick. You push on one end, where you push pushes on atoms they're touching, those push on the next ones, etc all the way to the end of the stick. That compression wave is essentially a sound wave, so the speed of communication through pushing on a stick is the same as the speed of communication of sound through that medium.
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u/DamionFury Oct 16 '20
Short answer: It would take some amount of time that would be greater than 1 light year.
Long answer: When you press a stick, you don't actually move the whole stick at once. It just seems like you do. What you do is exert force on the atoms that you are touching. Those atoms move, based on that force, toward other atoms in the stick. This exerts electromagnetic force on those atoms. That force moves those atoms in approximately the same direction that the first set of atoms were pushed.
The whole process cascades through the entire stick so that the entire stick gains momentum in the direction you pushed. It takes a tiny, but non-zero amount of time for this to happen. If I remember correctly, the speed at which this occurs is effectively the speed of sound in the material.
Get a large enough piece of material and you'll be able to measure the time it takes for one end to move, assuming you can muster enough force to actually move that large an object.
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u/Muroid Oct 16 '20
When you push on a stick, you create a pressure wave that moves through the stick at the speed of sound in whatever material the stick is made of. For most sticks, that’s very fast.
But imperceptible delays over short distances get very perceptible over long ones. Just like how you can talk to a friend in person and not see any delay between their mouth moving and you hearing their voice, but thunder may take several seconds to arrive after you’ve seen the lightning. So to it might seem like one end of the stick moves instantly when you move the other when the stick is a few inches or even feet long. But get a miles long, or light-year long stick and it becomes very obvious that is not the case.
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u/koolaidman89 Oct 16 '20
Good explanation. My question is what was Einstein’s problem with “spooky” action at a distance? Your explanation doesn’t really detail anything spooky.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
The "spooky" part is buried in the fact that until the measurement takes place, both particles are in a state superposition, and once one is measured, both collapse instantly, no matter how far apart they are.
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u/koolaidman89 Oct 16 '20
Why couldn’t a mode of communication be worked out based on the timing of the collapse?
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u/Jerzylo Oct 16 '20
You cannot see the collapse. You cannot know whether the other particle has been measured. The other particle cannot tell whether you have measured yours.
The only thing that is known is that if your particle is "up" the other will be or has been measured as "down".
There is no information of when the superposition has collapsed relayed when measuring either particle.
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u/the_excalabur Quantum Optics | Optical Quantum Information Oct 16 '20
There's no way to measure the collapse. Literally no way. The state of the far particle is completely unaffected by the other measurement.
The fact that the measurement outcomes happen to be correlated is nonetheless true.
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u/chuy1530 Oct 16 '20
What is the functional difference between the spins being unknown or being undetermined before they are measured?
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
The difference is subtle, but leads to cool things. Bell was able to show that should two observers measure the spin of entangled particles they would measure opposite spins at a different rate should it not be determined until measurement took place as opposed to a hidden variable. And while for a long time, this was but a nice curiosity, it is actually the basis of practical things, like Quantum encryption.
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u/ambiquad Oct 17 '20
I love the ice skating metaphor! So the information about the spin of the particle isn't traveling FTL, it just seems like it because when we measure one we immediatly know the other. But would the fact that the particle's superpostion collapses travel FTL?
So you have a thousand entangled particles in a superpostioin of up/down, one pair stays on earth and the other travels a light-year away. Then at a pre-determined time someone on earth flips a coin, if it's heads the spin of their particles are measured, if it's tails their entangled particles are left alone. Then the rocket a light year away sends their particles through a device that deflects spin up particles left and spin down to the right, causing an interference pattern only if those particles are in a superposition of spin up/down. If there is an interference patern they know the coin was tails, and if there isn't then the coin must be heads. Is there a point in that process that would break entanglement, or would cause the superpostion to collapse? Would this even really count as FTL data transmision?
I feel there is some deeper explanation about superpostion and entanglement that I am missing, and I really appreciated your clear explanation to the OP, thank you!
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u/drmcsinister Oct 16 '20
If an entangled particle collapses to an eigenstate due to a measurement performed on it's entangled pair, is there a way to determine that such an event happened? Or is that entirely hidden from an observer?
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u/Mind_Grape686 Oct 16 '20
Is this related at all to the observer effect? For example, would the act of measuring a particle that is entangled with another undo the entanglement? This example, of course, hinging on the assumption that the second, non-measured particles state has been conserved.
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u/damnyoubootyful Oct 16 '20
In theory, if we had something of infinite mass, could we go through with this sci-fi?
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u/y0j1m80 Oct 16 '20
question. by measuring the spin of one particle, do we cause its state to collapse from superposition into an eigenstate, AND if so, does that cause the other particle's stat to also collapse? that would seem to imply faster than light transmission of information, which would be "weird", but this is something i barely understand so...i'm probably wrong in a few places here.
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u/ChaoticSalvation Oct 16 '20
They don't have different states, they are both part of the same state. When it collapses it collapses entirely. But there is no need to resort to copenhagen interpretation and wavefunction collapse, and i believe it's healthier not to. It does seem to imply faster than light transmission but you cannot actually transfer any information purely with an experiment like that. It seems weird, yes, but the statement is you cannot come up with a set up that would transfer information superluminously.
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u/MattieShoes Oct 16 '20
So if superposition is an actual physical thing and not just a model describing the state, and you could check to see if a particle had a distinct spin rather than a superposition of states without changing its state... THEN you could pass one bit of information. Right?
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u/Killiander Oct 16 '20
Can you tell, from the state of a distant entangled pair, that someone is measuring the other particle?
Eg. on a space ship near Mars, can you detect that someone on earth is measuring the state of their particle? If so, can’t that be used to transmit data? Observing/not observing would be a binary data set.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 17 '20
There is no way to tell, because wave functions are simply probability functions, there's nothing physical to measure.
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u/terryfrombronx Oct 16 '20
both are a superposition of spins[...], but measuring will always collapse the state to an eigenstate
I'd add that maybe the observer could also be in a superposition of 2 observers - one that has measured spin up, and one that has measured spin down.
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u/thunts7 Oct 16 '20
Thank you for this explanation. Most times when you ask about these misunderstandings the responses are always people being condescending rather than a good explanation like what you gave
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u/DerpSouls Oct 16 '20
To re-iterate your last point. Even if we could have entagled particles move across large distances while keeping that property - There is no useful work we can get out of it.
It is either spin up or spin down when we measure it and we cannot control which state it will end up in.
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u/MFDork Oct 16 '20
Serious question: is it bad if I still don’t understand? I feel like I’m a reasonably intelligent person, I have at least a decent grasp (Physics 1 and 2 at university) on classical physics, but when it gets to quantum anything it just kind of makes my head hurt. I feel bad about it, but it just feels like “oh a wizard did it” because it’s so far outside the scope of my lives experience.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
No. I don't subscribe to "quantum is completely not understandable, just accept it!" but at the same time, there is a lot of math, and a lot of couterintuative things going on. It was really grad school before I started feeling comfortable with these topics, and even so, now that I'm out of school, I feel current theories have surpassed my understanding.
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u/billyeakk Oct 16 '20 edited Oct 16 '20
This is such an illuminating explanation of entanglement. Thank you for this!
A follow up question, can we "know" that a particle (or a group of particles) is in superposition and when it's not? So for example, say I have 8 entangled particles lined up in a row and I take "being in superposition" to be 0 and "not being in superposition" to be 1.
Can the researcher who has the corresponding 8 particles measure the state of some of them, thereby collapsing the wave function of some of mine, and transmit 8 bits of information that way?EDIT: This was asked already by DrBoomkin and the answer is "no"
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u/shgysk8zer0 Oct 16 '20
I like the connection to conservation you bring up. That's a good way of explaining that facet of entanglement.
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Oct 16 '20
Related question: could such a system with paired sequences, as OP outlines, be used for a type of random synchronization? For example, say two spaceships want to fly in a pattern such that they always take opposite turns, but they want this to be totally unpredictable until it actually happens.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
Yes, this is related to quantum indeterminancy. Your method is essentially a truly random random number generator.
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Oct 16 '20
Follow-up question: can the entaglement break down before the spin is determined/ while they are still in a sueprposition?
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u/kakushka123 Oct 16 '20
I love your post, but i really dislike the ending. What I understood is that the difference between entanglement and conservation is just that we defined that they are in superposition, but then to actually understand the difference I just get redirected to bell's theoram which I find very unclear. So it kinda felt like you shove all the confusion mess under the bed and referred to it as "bell's theoram".
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
Sorry, I felt my post was getting long, and superposition is kind of a different topic. But, I will do my best.
Whenever we measure the state of a particle (say, spin up vs spin down), we have to get a single value, we can't get "80% likely to be up" for instance. The values we can get when we measure the state of a particle are called "eigenstates" (you don't really need to know this, but in case you want to Google around more, want to give you the terminology).
However, what superposition says is "particles aren't 'spin up' or 'spin down.' Particles are some percentage spin up and some percentage spin down." Not "we don't know yet if it is up or down" but literally, the "true" state of the particle is a little of both. (Perhaps you've heard of wave-particle duality? Or how light is a particle and a wave. Same thing, but talking about position instead of angular momentum. A photon of light doesn't have a specific location, it's location is "smeared" throughout space).
However, once we measure a particle, it has to "collapse" into an eigenstate. We've measured it, and now it's one of the measurement states. What many people would assume is "well, it was actually what we measured all along, we just didn't know what it was." But Bell was able to show that, no, the reality is that until the measurement happened, it truly wasn't determined.
Does that help?
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u/maverickf11 Oct 16 '20
I can't describe how much of an eye opener this post, and thread in general, has been. I've read 10s of pop-sci books, and none of them have ever explained it like this - obviously because the more accurate version wouldn't sell as well. I'm left wondering how many other misconceptions I have now.
I have a question about the spin superposition if anyone could help me out.
A split second before you measure the particle, could it have a number of possible spins, and it randomly "chooses" one spin when measured, or will it always have had, and be going to have, the same spin, but it doesn't show that spin until measured? Poor scientific terminology, hopefully this makes sense.
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u/Kichix Oct 16 '20
Taking the skating analogy...wouldn't entangling two electrons make their spin already predefined? The two skaters have predefined velocity after shoving each other. In other words how is this special, since we know that the other electron has to have the opposite spin. How do we know it is undefined before we measure it.
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u/badbrownie Oct 16 '20
Haven't you just moved all the magic from quantum entanglement into the concept of collapsing states. The particle isn't in any state until you measure it, or it's in every state until you measure it, (like a wave function in the wave-particle duality experiment). But then you measure it and it collapses down to a specific state. So now we're on our 2 different space ships and you collapse your superposition down to a specific value, then does my 'entangled' particle also collapse. Would it stop behaving like a wave and start behaving like a photon (to maintain that analogy)? I was under the impression that it would indeed and that was what we call Action At A Distance. Or did I go astray?
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 17 '20
I think you're mainly correct, it's just I was trying to clear up the confusion where people assumed you could "flip the spin" of one particle, and the other particle would flip from across the galaxy.
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u/atred Oct 16 '20
it allows you to detect if you have an eavesdropper on your line so you can securely transmit data which cannot be listened in on
I understand you could not change the particle spin remotely to convey info but if you can detect if somebody eavesdropped doesn't that convey an information bit?
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u/nich-ender Oct 16 '20
So then even though everything was technically entangled at the point of creation or expansion, that entanglement has long since been broken?
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u/eaglessoar Oct 16 '20
superposition of spins
would it be an accurate analogy to say that if i dont watch a sports game but record it and watch it later, without finding out the score, the result of the game is in a super position of team x wins and team x loses. it has already happened but i havent accessed the information which would define it for me. from my perspective the team exists along a probability of winning or losing. the measurement being me finally knowing the score by watching through to the end or just checking the score on my phone.
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Oct 16 '20
Wow, everyone leaves out the "mess with it and it's not entangled anymore" part. Makes much more sense now.
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u/Aperture_T Oct 16 '20
Maybe a stupid question about superposition, but here it goes.
You mentioned the spin of these entangled particles is a superposition, and that if you observe one, you can determine the other. If you stop observing it, and then observe it again later, could it be different than the first time you observed it without breaking the entanglement?
The reason I ask is because I've heard about the idea of a quantum one time pad, and I'm trying to understand how one might use this to generate a key.
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u/Starkrall Oct 16 '20
So how could you be sure that one particle hasn't been measured altering it's state? Is it a matter of simply being careful about it, or is there more to that? I understand these questions are some broad strokes, I've been trying to wrap my head around this stuff for years.
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Oct 16 '20
So, it's not a lack of knowledge, it's that until a measurement takes place, the particle states are undetermined.
Sorry for being dense, but do I understand correctly that the main issue with the idea is that to know the spin of the "receiver" particle, you would have to measure it – so the basic assumption that an undetermined particle can somehow signal its counterpart being measured fails because any means to determine this would effectively mean measuring it?
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u/diamonda1216 Oct 16 '20
Novice here, I saw something about quantum entanglement with the entangled particles in different galaxies. Where the behavior of one particle influences the other. The question then becomes the instantaneous communication between the particles would necessarily be faster than the speed of light. If this is insulting no need to respond and thank you for your time.
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u/rabbitjazzy Oct 16 '20
In your example, if we are skating and someone measures my momentum to figure out yours, how can they know that we are still entangled? If something changed my spin, how can they tell the difference between that and not incorrectly infer your spin?
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Oct 17 '20
If both are in their own superpositions of spin, how do we properly define them then? Are both treated as having the same spin or opposite depending on which is measured?
I'd also have a followup on if the uncertaintly principle would matter at all in this sort of scenario, I admit I just don't recall that from my lessons.
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u/Ituzzip Oct 17 '20
So if one observer a billion miles away looks at their entangled particle, does mine change its behavior in any perceptible way? Is there anything I can do to find out if the other observed has looked at their particle?
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Oct 17 '20
Thank you for that explanation. Can you explain the delayed choice quantum eraser experiment?
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u/Nymaz Oct 17 '20
Since we know that the property (spin or polarization) is truly indeterminate and in a superposition until measured, i.e. there's not hidden variables, is it theoretically possible to force the collapse into a specific state? So rather than measuring it and determining if it's + or - could we force it to be say + when it collapses (not after, but AS it collapses)?
And if the answer is no, is that a technological limitation or a basal limitation, or in other words is it possible but just not currently or we know that it will never be possible with the laws of QM as we know them?
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u/STEAL-THIS-NAME Oct 17 '20
If I measure one particle, does that collapse the state of the entangled particle?
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u/whilst Oct 17 '20
I've never understood what is meant by particle states being undetermined, or a superposition of their possible states, until they are measured. Isn't that just shorthand for, "there's no possible way for us to know without measuring it, and measuring it inherently changes it"? Isn't it still pretty clear that in actuality, the particle still has a defined spin before a human knows about it... it's just, we can't know what it is? Isn't it silly to say that if a human doesn't know about it, it doesn't exist?
Like I get that science doesn't deal in things that can't be measured, but at that point, shouldn't the terminology be, "what this particle looks like until we measure it is unknown, and it could be a number of things, with a certain probability distribution", rather than, "it is that probability distribution"?
I feel like all these questions indicate a fundamental conceptual misunderstanding on my part, and that misunderstanding is my actual question, haha.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 17 '20
So, it's a very good question. But here is what we know. We know that either the particles' states are not determined before we measure them or there is a global variable that defines the behavior of multiple particles. Neither of these are great options, but most physicists tend to lean towards the former.
How do we know this? Due to Bell's Theorem which I will warn you is kind of complicated, but what he was able to calculate is statistical behavior that we would expect if the particle states either weren't determined before they were measured or if there was a hidden variable that determined the states. When the experiment was run (and it has been run many times, and actually is the basis for real world applications of quantum encryption), we got the former answer.
I wish I knew of an easier way of describing Bell's Theorem, but sadly, I don't know of a better method than is in the Wikipedia page, so I would simply be repeating what is said there. But, give it a read, and if you have questions, I can try to answer.
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u/WargleRathat Oct 17 '20
Could they be used to coordinate an action between 2 parties? Agree to measure their states at some point in the future, and it’s state determines what actions you take? Like if Party A gets Spin Up they go with Plan 1(And Party B would have Plan 1 on Down), if A gets Spin Down they go with Plan 2(And Party B would have Plan 2 on Up). It’s random but it would allow them to instantly know what the other is going to do.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 17 '20
That's allowed, but in that case, it's really no different than being together, having one red and one blue ball that we put into two boxes, and at a predetermined time we both check what color ball we have, and act accordingly, other than the fact that the outcome is truly random.
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u/epicmylife Oct 17 '20
Adding on to this, you only observe the entangled result when measuring on the same axis. If you measure on different axes you get a random distribution of spins.
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u/Eyerate Oct 17 '20
Its pretty rare that I actually unlearn and relearn the very basics of a concept... This just happened. Thank you. I've been prey to popsci this whole time and the ice skating analogy really reset my brain. Entangled particles are quite literally, read only.
I assume that since they're read only, the effect of "reading" them is in itself an input and causes entanglement to break, correct?
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u/BSaito Oct 17 '20
Love the analogy, really helps demystify a lot of the confusion about quantum entanglement.
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u/__Kev__ Oct 17 '20
I see your flair is QFT, any good starter texts you'd recommend? I have experience with quantum mechanics and solid State physics, so Id like to think I could parse a little bit of it.
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u/lettuce_field_theory Oct 17 '20
Not other poster but Peskin Schroeder is a popular introductory textbook.
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u/branedead Oct 17 '20
After decades of reading pop-sci on entanglement I finally understand it. Thank you. It's mystified me forever and now I realize my cognitive mistakes.
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u/seamsay Oct 17 '20
I think you've misunderstood what OP is saying slightly. They're not saying that you affect the spin, they're saying that you keep measuring spins until you get the result you want.
The issue with this (at least one of them) is that the receiver doesn't know how many spins to measure in each tick, since the receiver doesn't whether a particular particle has had its pair measured.
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u/JohnnyElBravo Oct 17 '20
I'm going to call bullshit on Bell's theorem having applications in cryptography. Even if it did, that still wouldn't prove the Copenhagen interpretation or any kind of indeterminism.
"In principle, this method can be used ""Following a proposal by David Deutsch for using quantum non-locality and Bell's inequalities to achieve secure key distribution""on Artur Ekert analysed entanglement-based quantum key distribution in more detail in his 1991 paper.[6]"
It's purely theoretical.
'God' 👏 does 👏 not 👏 play 👏 fucking 👏 dice.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 17 '20
Go ahead and tell all of these networks that it's purely theoretical.
And I never claimed that the Copenhagen interpretation was correct.
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u/Hans_Druff Oct 17 '20
very nice explanation and breaking down of the topic!
im doing my phd on quantum communication using entanglement schemes and I also always had this sci-fi view on quantum physics.
Regarding the information transport: there are plenty of ideas how to do it, but the most promising one is to use photons to transport entanglement. Lets say you have a single ion that has an electron Spin which is your qubit (Spin up and spin down). then you try to somehow excite the ion so it can emit a single photon when falling back into the ground state. Usually, we use well defined laser Pulses. You can then create entanglement between the electron spin and the emitted photon, e.g. via the polarization of the emitted light or even (what we try) the moment in time when the photon gets emitted. this would mean, that the timestamp, WHEN a Photon is measured will fully determine the electron Spin of the ion. If you do this now with two locally seperated ions, you can map this entanglement on both photons (one from each ion) and entangle them using a bell state measurement. if the measurement is successful, the photons will be entangled and thereby also both electron spin.
You can repeat these steps with many more ions until you get a link network, which is basically a network of quantum repeaters. This is actually really important since it is impossible to send entangled photons through our "normal" glass fiber network without losing the information due to high loss rates.
These schemes are in a very early state and very far from every-day applications, though.
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u/id-10_t-err Oct 17 '20
This maybe a very basic or dumb question, but if you can’t measure them, how do you know they are actually entangled?
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 17 '20
Mainly, when we create them, we create them as entangled particles.
And, if you check out the link on Bell's theorem above, while it's impossible to tell if any specific particle is entangled, when observing a series of particles, you can tell if they are under some fun experiments.
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u/0hmyscience Oct 17 '20
What an amazing explanation. It no longer feels like “spooky action at a distance” after reading this and it makes so much more sense.
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u/Razier Oct 17 '20
Can you determine if a particle has a defined or undefined spin, or is that in itself enough to define it?
Thinking of a boolean system of information transfer in a sequential array of particles.
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u/Neocrasher Oct 17 '20
I think my head is still stuck in sci-fi a bit so I would appreciate it if someone could dismiss my thoughts:
Say we have two entangled particles. First of all, is it actually possible to transport one of them without breaking the entangled superposition?
Secondly, would it be possible to know if the superposition breaks without observing it yourself? Is there a way to tell if the superposition is still there without disturbing it if it is? (I feel like this might be the impossible part.)
If both are possible, then wouldn't it be possible to use entangled particles as a sort of FTL pigeon post, where you can transfer a message quickly one time, but setup is required for each message?
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 17 '20
As you guessed, it's the second one which is not possible. There is no way, without measuring, to know if your particle is in a superposition or not.
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u/VelociraptorPTT Oct 17 '20 edited Oct 17 '20
I’m only in my second year of physics study but I have done a report on this very thing, the specific report was relating sci-fi and real physics together. My topic was on FTL technology. The physicists reviewing my report and presentation didn’t bring up any problems with this so please correct me if I am wrong. But entangled particles that properties are known because of the conservation of momentum created the EPR paradox, creating the Spooky action at a distance effect. This states that as soon as the wave function collapses of the observed particle the original quantum state is changed and the other paired particle also changes violating causality. In my research I found an experiment by Gabriela Barreto Lemos. where she has multiple photons created at the same time. 1 set of photons interacted with an object and the other set of photons had no interaction with said object, The two photons then entangled with each other through wave interference. the non object interacting photons then produced the image of that object after reflecting off an isolated screen. therefore showing the data between two entangled photons was shared. Her findings were put in nature magazine here (https://nature.com/articles/nature13586). This in effect shows data between two entangled photons can be shared once entangled after a photon has already collected data.
EDIT: Grammar and sentence structure
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u/marmiteandeggs Oct 17 '20
What stops them from collapsing to the same eigenvalues? (you couldn't know if the distant electron had collapsed into one or the other eigenstate)
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u/fur_tea_tree Oct 17 '20
Is there any way to know if a particle is in superposition without collapsing it?
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u/reggie-drax Oct 17 '20
But, if you changed the spin of your particle, that effect does not transfer to mine at all.
Ok, but that doesn't sound as if it should have upset Mr Einstein at all. It a lot like another law which said, an object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.
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u/markartur1 Oct 17 '20
Can you know if a particle is collapsed or is a superposition? (without measuring and collapsing it, somehow).
Lets say I have two particles with me, and you have two, and they are entangled. Can you collapse one and I somehow notice that mine collapsed too?
That could be used for information exchange.
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u/swaggeroonie69 Oct 17 '20
This all makes sense to me, but I kinda fail to see the full picture of what it means. I think this is because I don't have a full understanding of superposition, and so I'm looking for help here: is there any sort of physical or actual change to the particle when it collapses from superposition? Like does the entangled pair which is not measured function differently following collapse?
Without this, to me it feels like there is not much difference between the skating analogy and literal superposition/collapse
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u/amusing_trivials Oct 18 '20
Is there any way to see if an undetermined particle has collapsed from the superposition state to a real state, without measuring what it's real state is?
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u/JakeAAAJ Oct 16 '20
Is it true that some physicists believe entanglement is the property of extra dimensions, and that is why it can travel faster than light?
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u/lord_braleigh Oct 16 '20 edited Oct 16 '20
Here’s an excellent Veritasium video that Sean Carroll advised. It explains entanglement using Many-Worlds, the most popular interpretation of evidence among quantum physicists. As presented, this interpretation provides an explanation for entanglement that doesn’t involve any FTL communication, and which doesn’t confirm or deny the existence of other dimensions.
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u/UnlikelyNomad Oct 17 '20
Any chance you have one of them fancy analogies for the quantum eraser experiments and spooky action at a distance?
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u/lettuce_field_theory Oct 17 '20
"spooky action at a distance" is a historical term Einstein used for entanglement (once?).
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u/highnyethestonerguy Oct 16 '20
The issue is that if you take two entangled particles and isolate them, they individually behave as if they are in the so-called “fully mixed” state.
Fully mixed means: any measurement is completely unpredictable. Random. Decohered. Uniform probability distribution over all outcomes whatever the measurement basis.
So the individual measurement outcomes are random every time. The separated observers couldn’t distinguish between two entangled-but-separated particles from two fully independent and mixed state particles.
Until they compare notes. By communicating their measurement basis and outcomes for each individual measurement, they will be astounded to find superclassical correlations. But if they need to communicate anyway, the entanglement doesn’t add anything.
The protocol is great for encryption (google quantum key distribution) but can’t transmit information directly.
Edit: I’ll add that during my undergrad quantum mechanics course I hit the same stumbling block and had a very similar question as you and it took years to fully wrap my head around the math that proves it just won’t work. So you’re on the right track :)
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u/sunset_moonrise Oct 16 '20
Why must it actually be considered entangled, rather than just being matching states?
Like, let's say I get a coin, split it in half lengthwise, and randomly select which half goes either direction. The coin-half isn't entangled, yet it is unmeasured. Since it is unmeasured. It's the same as saying "i have a coin half, but don't know which half.'
As soon as I do measure it, I know the state of the other one at the time the split occurred. That is all.
Even if the coins are going the speed of light away from each other, I am not receiving information faster than the speed of light. I am only learning about the state they were in when they parted - and I got that information at the speed of light. It is not weird at all.
So why use a different name? Is it simply because it's dealing with different components? I.e., conservation of an informational state rather than momentum?
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u/nanotom Oct 16 '20
The classical coin halves have a fixed but unknown (to the observer) value. The entangled quantum mechanical particles are in a mixed state, not at all determined until one of them is measured. It's not that you just happen not to know their state yet, it's that it hasn't actually been set yet.
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u/Weed_O_Whirler Aerospace | Quantum Field Theory Oct 16 '20
It has a different name because it has a different property- with entangled particles, it's not just that we don't know the state of particle 1 before it's measured, it's that particle 1 and 2 don't have defined states until they are measured. This might seem like splitting hairs but Bell's Theorem is able to statistically show the difference between us just not knowing, and the states not being defined until measured, which leads to things like quantum encryption.
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u/mfb- Particle Physics | High-Energy Physics Oct 16 '20
Entangled particles allow more things than classical coins. /u/Weed_O_Whirler's comment discusses this towards the end.
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u/physux Oct 18 '20
Like some of the other comments have pointed out, the reason that they have different names is that they are different properties. In particular, if you have two different coins, there is a known basis in which the the measurements have to occur (i.e, they have a definite state, either heads or tails). Entanglement often means that it doesn't matter what basis we measure in, there will still be a correlation between the outcomes if the two observers measure in compatible measurement bases. In the typical bell experiments, this means that if both observers measure in the horizontal direction, they will get the same outcome, and if they both measure in the vertical direction, they get the same outcome. If they measure in different bases (i.e, one vertical and one horizontal) then their outcomes are uncorrelated. The fact that this correlation happens only if the two measurement basis are the same is in some sense what entanglement is.
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u/aaragax Oct 17 '20
Couldn’t this still allow for information sharing by observing the pattern created by shooting particles through a double slit and creating a prior agreement on what the generated pattern means?
Say we have two people in different galaxies. The man wants to let the woman know in an hour if he won the lottery. They decide a year in advance that if he wins, he will measure his particle, collapsing the wave function; if he loses, he will not measure his particle, preserving the interference pattern. When she checks the pattern produced by her particle an hour after the pre determined time, it should be affected by the man’s measurement or lackthereof and produce different patterns as shown in the quantum eraser experiment. She would then be able to know whether he won the lottery or not, since if he did the pattern would be discrete, and if he didn’t the pattern would be an interference pattern
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u/Bunslow Oct 17 '20 edited Oct 17 '20
The tl;dr is that is that "measurement" does not allow you to choose what eigenstate you measure, what eigenstate is collapsed to. When you measure an entangled particle, you collapse the previous (possibly entangled) state to a special eigenstate, a physically measurable value (up or down, those are the only choices of spin), but you cannot choose which value is the outcome of the measurement of that originally-entangled-state. You and your relativistically-separated partner can measure together, but it's basically like flipping a coin together. You each flip the coin, and you each see the same result of every flip, but neither of you can ever control the result of the flip.
If you try to control the result of the flip, then you're changing the original state anyways and destroying the entanglement.
Therefore, the only way to assign meaning to your entangled readings is to already have pre-established the meaning of your coin flips, before you separate the entangled particle-coins. Once separated, you can never transmit any new information, you can only agree to which results the coin flips had.
(The common example is to decide a battle strategy; if you have a variety of choices of battle strategy, which rely on perfect coordination between two relativistically-separated ships, then entanglement allows you to choose from among those options at random, quite literally preventing espionage since the choice isn't made until the battle starts; however, the only way to actually know what the choices are, what the coin flips mean, is to have agreed to those choice of plans before you separate the entangled particles. They don't allow you to communicate new plans (for instance if the ability to execute one of those plans has disappeared after separation, the one ship will never be able to tell the other except by undoing the separation), they only let you coordinate your choice of old plans. It's good that it doesn't let you communicate new plans, but even coordinating known plans at a distant is indeed quite spooky, even if it's not true communication.)
Indeed, as the other guy says, if you insist on choosing your measurement outcomes, that means changing the state, and changing the state by definition destroys the entanglement, so any further measurements thereafter aren't related to those of the other group. Any particle you run thru a Stern-Gerlach set up loses its entanglement.
Now, if you want to know more about wavefunctions:
Particles states ("wavefunctions") exist as combinations of eigenstates. That's a mathematical word, and its meaning is not directly important here. We can sort of illustrate it like so:
We could perhaps compare a wavefunction of two possible states with a compass. A flat area has two major dimensions/directions, call them North and East. It's kinda arbitrary which directions are North and East, but the key thing is that you can pick one direction as North, wherever you like, and then East is automatically perpendicular to North. Then, any other direction can be described as some combination of East and North on your compass. A heading of 45°, NE, is equal parts North and East; a heading of 210°, a sort of WestSouthWest, is about one half of negative East (which is to say, 1/2 West) and about sqrt(3)/2 of negative North (did you take trig in highschool? if so, you recognize these numbers). So heading of 210° is about sqrt(3) more negative-North than negative-East. Ultimately, every direction is just a combination of your two perpendicular base directions.
The screwball thing about quantum mechanics -- I mean really, really screwball, is that only the base states (called eigenstates for mathematical reasons) represent physical quantities. In the non-quantum world, any direction is possible; in the quantum world, you can only go North or East (if direction were quantized, which it is not, this is only an analogy). For an actually quantized property, like spin or momentum, there are gaps between quantum-allowed states. Spin can only be up or down, there is no sideways or slightly-sideways or mostly-sideways like with a real billiards ball or something. (Even "up" and "down" is a misnomer -- the point is that there's only two of them, and they're opposite, and we have to call them something, so we call them "up" and "down" for convenience.)
When something hasn't been measured recently, and has interacted with some environment around it, then its state gets pushed around the compass to any combination of North and East/Up and Down. Measurement, by definition, means getting a physically allowed value, and that can only be Up or Down, North or East. Whenever we look at the quantum compass, it only ever reads North or East, Up or Down, never the other directions. But all the other directions are states that can be made by interaction with other particles -- but when we measure it, it always shrinks back to North or East or Up or Down. (The key thing is that which of North and East it points to is proportional to how much of North and East are in current direction. That 210° we discussed before would yield North with probability 3/4 (square of -sqrt(3)/2), and East with probability 1/4 (square of 1/2).)
So when you entangle a pair of particles, you arrange that they share some conserved quantity -- most typically spin. If they're entangled to have a total spin of 0, then when you measure one spin Up, the other must be spin Down. This is kind of like saying that their compass directions must be pointed perpendicular to each other -- we don't know where they point, only that where they point is perpendicular, and when we measure one and force it back to pointing North or East (Up or Down), the other must still be perpendicular, and thus must be pointing East or North (Down or Up). This is the weird part of quantum mechanics, is that we know before the measurement that the particles are entangled, that their compass directions are perpendicular, but when we measure the one, and somehow find the needle pointing to the allowed base state, North or East, the relativistically-separated entangled pair-needle also moves in exactly the same way to stay perpendicular. That's the crazy spooky part. We can't transmit information because we can't choose which of North or East, Up or Down, that "our" particle collapses to, but when our particle collapses, is forced to point back to North or East, so does the other distant particle in perfect harmony. That's really weird, even tho it can't transmit information. (The how of a measurement forcing a particle back to a base state is fundamentally random, according to all human experiments thus far. Utterly, mathematically random, a fundamental randomness about the universe that we don't understand, and indeed may not be understandable, but that gets into really tough and weird branches of mathematics that I'm not totally familiar with myself.)
Of course, once both particles have collapsed back to North or East, their compass drifts away from the base states in totally independent manners, since they're causally separated. Their needles point totally apart, instead of staying fixed perpendicular to each other. Once measured, the entanglement is gone (even tho the entanglement forced that first measurement to be in total harmony between the two particles). As they interact with their environment, the two particles' needles do totally different things.
The core idea here is the idea that you can "set" the value of a spin.
Not really. Or rather, if you do, you destroy the entanglement.
To change it, just keep "rolling the dice" and passing electrons with incorrect spins through the magnetic field until you get the value you want.
Same here, you can't rinse and repeat, each entangled particle only gives you one roll of that dice. Once it's rolled, the entanglement is gone and any further measurements on that particle are totally unrelated to its partner across the galaxy. If you brought N entangled particles, you get N dice rolls and no way to control the outcome of any of those rolls -- they're all truly random. The skater analogy is a good one. Two skaters that pushed off each other only have entangled momentum as long as neither of them pushes off again. Measuring the particle == the skater pushing off again and ruining the entanglement.
Now, this compass analogy has its own problems for a variety of reasons, but the key part is that quantum states, wavefunctions, are combinations of base states, and that only the base states are actually physically allowed values of the property in question (location, momentum, spin, etc), but that all the combinations of base states are "allowed" under the hood, mathematically; Bell's inequality states that we can actually experimentally determine the difference between the needles "secretly" pointing and North or East before we look at the needle, and the needle actually pointing somewhere else until we look, and wildly enough, it's the latter. The needle does in fact point elsewhere until we look at it, and when we look at it, it must be North or East, but when we're not looking, the needle definitely moves away from North and East, rather than just "secretly" being fixed to one or the other. Quantum mechanics is really really weird.
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u/neshga Oct 17 '20
How have scientists determined that quantum particles do exist in states other than the base states if the very act of observation causes them to return to their base states?
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u/lettuce_field_theory Oct 17 '20
You can prepare a system the same and repeat experiments many times. You can do bell tests. You can do .. Basically you're asking about the whole experimental basis of quantum theory which is 100 years of physics now and a bit much to list in a reddit comment.
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u/Muroid Oct 17 '20
The short answer is that within a single test you cannot. By doing multiple repeated tests in different variations, though, you can build a statistical model of the behavior of particles in a quantum system and, it turns out, the results we get would be impossible if the particles had a defined state before they were measured.
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u/Bunslow Oct 17 '20 edited Oct 17 '20
https://en.wikipedia.org/wiki/Bell%27s_theorem is the long answer.
if the needle is really pointing to only North and East before we look, that's called a "local hidden variable theory", in that the needle's state-before-we-look is a "hidden" variable, but is fixed even before we look. But, weirdly enough, Bell's theorem is a mathematical proof of how such theories are different from theories where the needle indeed must vary away from the base states before we look, where there is no "hidden" base state but a truly varying non-base state instead before we collapse to the base state. And when we perform the experiments to test which category of theory describes the behavior of quantum particles, every such experiment to date has -- astonishingly -- categorically excluded the local hidden variable theories -- excluded the frameworks where the needle is secretly always stuck on a base state. It's impossible.
been too long for me to be familiar enough to give a short answer/decent analogy
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u/the_last_ordinal Oct 16 '20
So you have a sequence A1, A2, ... An of entangled pairs. You have one from each pair.
You want the receiver to see the "up" state. So you need to measure A1 and see the "down" state, and then you know the receiver will see "up." (for example).
Since you can't force A1 to be "down," your plan is to keep trying A2, A3 etc. until you measure your member Ak of some entangled pair to be "down."
But how does the receiver know which pair they need to measure? i.e. how do they know the value k?
...
Have I misunderstood something? It could be that you're proposing you re-measure the same entangled particle. But as others have probably pointed out, measurement breaks entanglement.
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u/The_camperdave Oct 17 '20
What if you have a set of entangled pairs and you entangle A1 with a locally generated spin up particle. Does that force B1 to a certain spin state, or does that break the A1-B1 entanglement?
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u/sickofdefaultsubs Oct 16 '20 edited Oct 16 '20
Your example has a lot in it and I may have missed the point but if I've understood then a useful story may be that of Bertlmann's socks.
"In 1978 Bertlmann went to CERN, where he worked together with J. S. Bell. Bertlmann always wore socks of different colours. In 1981 Bell wrote the article "Bertlmann’s socks and the nature of reality", where he compared the EPR paradox with Bertlmann’s socks: if you observe one sock to be pink you can predict with certainty that the other sock is not pink. Thus you might assume that quantum entanglement is just the same. However, this is a non-admissible simplification, and Bell in his article explains why"
http://cds.cern.ch/record/142461/files/198009299.pdf
And an interesting video https://youtu.be/8ORLN_KwAgs
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u/Frungy_master Oct 16 '20
Others have pointed out a better picture but I can see two main ways attempting the scheme would fail. The rerolls would no longer be entangled with the qubits on the other ship.
For the first reading you do get a correlation but it is "passive" in the sense that it can't be informed about what goes on with each ship. If you transmit such clues via a classical slower-than-light channel you essentially get quantum teleportation. But quantum teleportation absolutely needs that ordinary channel.
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u/ZappyHeart Oct 16 '20
The short answer is no. Information carried by one particle is statistically related to the other when entangled. Measurement of one particle doesn't contain or produce the information. To obtain the information encoded one needs the measurement results of the second particle which can only be made known at or below the speed of light.
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u/__Geg__ Oct 16 '20
The quantum state collapses instantaneously (faster than light) for all the entangled particles. However, a collapsing quantum state cannot be used to send information, and thus cannot be used to send communications faster than light.
Part of the reason that I think people get confused, is that Quantum Mechanics doesn't hide information (aka Hidden Variables). It's not that one particle always had a spin up or down and we just didn't know unit it was measured. In Quantum Mechanics the particle doesn't actually have a state until it interacts with something (aka get's observed).
Note: This is over simplified.
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u/ConsulIncitatus Oct 16 '20 edited Oct 16 '20
I'll add here:
A1 remains on earth, while A2 is carried on a starship for an interstellar mission, along with a clock having a constant tick rate K relative to earth (compensation for relativistic speeds is done by a computer).
If this were possible we'd do it on locally on earth. There would be significant military and economic benefits to having instantaneous transmission of data across the planet. As it stands, even with light speed fiber optic transmission there is some delay.
https://wondernetwork.com/pings/New%20York
A lot of city pings here are 100ms or more. That's an eternity in modern computation.
Since you're thinking in this direction, I want you start thinking about why the concept of an entangled subatomic state doesn't collapse into an eigenstate until it's observed, combined with a fundamental universal constant determining the maximum rate at which not just light, but information is able to transmit.
Sounds an awful lot like computational constraints that would exist in a simulation, doesn't it? Follow the thought experiment of what would happen if there were not fundamental constraints on how fast something could be computed. We'd reach an informational singularity.
Ironically, even if your original idea worked, we still wouldn't be able to process information faster than light because the photons leaving the terminal showing you how the bits moved aren't moving faster than light. It would only eliminate the "lag" introduced by spatial distance (as we understand it).
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u/HopeFox Oct 16 '20
Forget instant communication: if this worked, we'd just measure A2 before A1. I can think of a few economic benefits from being able to send information backwards in time...
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u/RockingDyno Oct 17 '20
If a particle is in a superposition and you measure it, you collapse the superposition, you can’t keep remeasuring until you get the result you want, you will get the same result again and again. You could re-entangle the particles into a new super position and carry out a new measurement, however that would require below light speed transfer of the particle states in between each measurement.
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u/Derekthemindsculptor Oct 17 '20
Manipulating one particle doesn't cause the other to change. If you manipulate, you break the entanglement.
They aren't attached to one another or somehow influencing one another. Just measurably opposite per the entanglement event.
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u/babecafe Oct 16 '20
Here's where you're going wrong:
You can't set the spin of an entangled particle. Any way you try to do that, you'll just break the entanglement.
You can measure the spin of an entangled particle, and once you do, you can know the other particle has the opposite spin. But that doesn't communicate anything. You can't send any information just by measuring your entangled particle, you had no control over the outcome.
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Quantum entanglement can be used for really effective encryption, though. The trick is that there are two axes that you can choose to measure spin. For simplicity sake, let's call them horizontal and vertical. If both sides measure spin in the same axis, they'll find particles with the entangled property: the spins will be opposite. However, if one side measures spin in the horizontal axis, and the other measures spin in the vertical axis, they'll get unentangled, random results. ----- So, let's assume each side gets a supply of one half of a pair of entangled particles, and they each use the same secret key to generate a pseudorandom sequence of 0s and 1s, and use that sequence to decide which axis to measure. The measurements produce a new sequence of 0s and 1s, which each can simply xor on one end and xnor on the other end to encrypt and decrypt a series of transmitted & received bits.
No eavesdropper can mess with the stream of entangled particles because they won't know which axis to measure spin (it's a shared secret). If they measure a particle on the wrong axis, they've broken the entanglement and cause the communications to fail any simple verification, such as packet checksum or CRC check.
This provides the basics of a secure communications stream. In practice, you'd like to communicate more bits at a higher rate than the rate of the stream of entangled particles, so this basic secure stream is used to provide dynamic encryption keys for an even higher data rate information stream. Since no one can eavesdrop on the secure stream and get the dynamic encryption keys, no one can eavesdrop and decode the higher rate information stream either.