Can Quantum Entanglement surpass the speed of light

[Janx]

[MENTION=177]Umbran[/MENTION] and other physics majors will probably have the answers for this. It's in line with some of [MENTION=31216]Bullgrit[/MENTION]'s line of science questions.

As I understand it, Quantum Entanglement is an interesting state where 2 objects become entangled at the quantum level, such that what happens to one happens to the other.

Presumably, the 2 objects are really halves of the same original object and "something" happened to them. Apparently, you can do some things to one half and detect it in the other half.

His Dark Materials by Pullman uses a Quantum Resonator as a communications device. A pretty obvious idea once you hear about the concept. Take one half, induce vibrations in it, listen on the other half.

Thanks to the other threads, we know that through the complex laws of science, that you can't go faster than the speed of light. Even if you are on a really fast rocket and you shoot a rocket from that rocket.

If the reaction transfer of a pair of quantumly entangled objects is instantaneous (big IF), wouldn't that effectively transfer information faster than the speed of light if the two objects were far enough away?

I imagine that one answer is that the 2 objects may appear to be far apart, but exist "very close" in quantum space due to the entanglement (it's the same object, how can it be far away from itself).

What say the science guys?

 

[Cor Azer]

I can't find the link right now, but I had just recently read a newsbyte blurb that mention some science research in quantum entanglement that indicated 'effect' preceding 'cause'.

Grain of salt of course, because I didn't read the paper, so I don't know if the newsblurb was being faithful to the actual science done or just badly summarizing.

If true though, that would be decidely faster than light.

 

[Janx]

If true though, that would be decidely faster than light.

that is the nit-pickety part. assuming my summary of the effect is close enough to true, I see a viewpoint that says we can pass data from Earth to Alpha Centauri faster than light. But I also see a view point that says the data itself, did not travel any distance because the quantum entanglement effect means the 2 objects are effectively the same object.

I have no clue what the official view is.

how easy it is to make a quantum entangled pair?

how easy it it to induce and detect a response in the pair?

If our tech level is sufficient, we can create Quantum Repeaters, a pair of devices with an RJ45 connector on either end that can bridge two locations across any distance. We could basically send ethernet traffic across the ocean to India, without running undesea cables or satellites. (some of you may have no idea how much of a PITA it is to do business over internet with Asia, the speeds are terrible).

We could install one in space probes and get much faster communication with probes that are far away (like Mars and beyond).

If transfer of signal from A to B is instaneous (not affected by distance), aside from the usual setup/teardown to tickle node A and detect in node B which technically has a time cost (like any data protocol does). If we put Node B on another star, technically, we're passing data faster than light (basically, we'd be running at a speed equal to the distance).

I doubt our ability to tickle and detect on a quantumly entangled pair would be any faster than our network speed technologies (gigabit ethernet, wifi under 802.11n or whatever). But that's due to the setup/teardown to get the data into the pair. The real bonus comes if the transfer within the pair has zero latency. At that point, the greater the distance, the greater the pay-off/efficiency.

 

[freyar]

I'm not sure about the report Cor Azer mentioned, but I can tackle the original question. To tell you the truth, this question raises really deep issues about what we mean by "measuring something," issues that no one truly understands, but we fortunately do know the answer to your question. And the answer is that, yes, in an entangled state, the effects of measurement are instantaneous, but that, no, you can't send any information through those instantaneous channels.

Let me give you the canonical thought experiment in simple terms, but I should note that this isn't just a thought experiment any more, as some versions of it have been performed for real. Here goes. Imagine that we create two photons heading in opposite directions with the property that they always have opposite polarizations. If we measure one photon to be polarized up-down, we will always measure the other to be polarized left-right. If we measure one to be polarized along one diagonal, we will measure the other to be polarized along the other diagonal. Well, the trick is, I can measure one photon's polarization on earth, and you can measure the other photon's polarization at alpha Centauri (or wherever, really) at similar enough times that we can't communicate in between. So I don't know if you measure polarization along up-down/left-right axes or along the diagonals, etc. But if we later get on rocket ships and compare notes, we will see that the photons always had opposite polarizations in the cases where we measured the same axes. That's true even though the photons didn't have time to "tell" each other the effects of the first measurement. That's the instantaneous part. But there's also no information passed. If I measure my photon to have up-down polarization, I still don't know what polarization you measure, because I don't know if you measured along a diagonal instead. So it's a subtle issue.

This is called an Einstein-Podolsky-Rosen experiment; they wanted to show that quantum mechanics needed to include a "hidden variable" that contains the information about polarization, so there's no need for instantaneous communication. However, it's possible to show (first done by Bell) that measurements in hidden variables theories obey certain inequalities. Experimentally, these inequalities are violated. (I'm oversimplifying a bit.) So quantum mechanics has instantaneous components, but it still obeys the relativistic principle that information can only travel at the speed of light.

 

[Gorgoroth]

cause and effect

has been debunked for a long time now. i.e. it's a macroscopic effect, at best.

 

[Asphere]

<!-- BEGIN TEMPLATE: dbtech_usertag_mention --> His Dark Materials by Pullman uses a Quantum Resonator as a communications device. A pretty obvious idea once you hear about the concept. Take one half, induce vibrations in it, listen on the other half.

The problem here is that no actual information is transferred because you can't affect which state a quantum particle will go into (its random). So once you have prepared an entangled set (A & B) you have no idea which one has spin up and which one has spin down. The only way you can know is to measure one at which point it will randomly be in one of the two states. So if you measure A and it collapses into a spin up state, B will return a spin down state. All an observer measuring B will know is that A made a measurement that randomly yielded a spin up. On top of that, the act of measuring destroys the entanglement.

 

[Umbran]

As I understand it, Quantum Entanglement is an interesting state where 2 objects become entangled at the quantum level, such that what happens to one happens to the other.

I would describe it this way:

Take two particles. Entangle them (don't worry about how for the moment). You know now that some aspect of them (like, say, their spins) are correlated. You don't know what the spin of each particle *is* (they are like Schrodinger's cat, not in one state or the other), but you know the two make a matched pair.

Pick up one particle, and move it a long distance away.

Measure the spin of one of the particles. It spin was indeterminate before, but the act of measuring makes it fall into a single state, so now it is determined.

At some subsequent time, you look at the other particle. It will have taken on the matching state.

You don't get to do what you like to one, and have the same things happen to the other at a distance. You don't get to grab one, shake it around, and have the other at a distance also shake around.

If the reaction transfer of a pair of quantumly entangled objects is instantaneous (big IF), wouldn't that effectively transfer information faster than the speed of light if the two objects were far enough away?

Yes. If it is truly instantaneous (takes ZERO time), then if they are *any* distance apart, it is faster than light.

I imagine that one answer is that the 2 objects may appear to be far apart, but exist "very close" in quantum space due to the entanglement (it's the same object, how can it be far away from itself).

That's one possible explanation. In the science lingo, it is a "hidden variable theory" - while our math says there's some physical distance X between them, there's some variable we don't currently see that defines a shorter distance (Distance through what? Um... well... let's not talk about that just yet.) between them.

The other common view it is that the idea that one particle is in one place, and the other in another place, and that each is localized, is incorrect. The distance between the locations is still large, but there's something about the particles that is spread throughout the entire universe. This is sometimes referred to as "quantum non-locality".

There is a third, somewhat pragmatic, explanation, which is related to the hidden-variable one, which is to say that it isn't that measuring the one causes an action at a distance, but that measuring one increases the amount of information you have, such that it reveals something you didn't know about the other. While seemingly sensible, some of the mathematical results actually argue against this.

While we can experimentally demonstrate entanglement, and produce particles that are entangled, and show the correlation between their measured states, there's pretty much no strong agreement on *how* this happens.

 

[Janx]

This is called an Einstein-Podolsky-Rosen experiment; they wanted to show that quantum mechanics needed to include a "hidden variable" that contains the information about polarization, so there's no need for instantaneous communication. However, it's possible to show (first done by Bell) that measurements in hidden variables theories obey certain inequalities. Experimentally, these inequalities are violated. (I'm oversimplifying a bit.) So quantum mechanics has instantaneous components, but it still obeys the relativistic principle that information can only travel at the speed of light.

my eyes may have glazed over a bit, but thanks for taking the time to supply an example.

I suspect the "hidden variable" is something data communications/computer science people who develop low-level protocols for passing data solve with agreed upon patterns that can be identified.

For instance, Morse code is really a binary protocol (dots and dashes). We define a dot to be an asserted signal with a duration between X1 and X2. We define a Dash to have a duration between X2+Z and X2+Z+D where the values of Z and D and ensure that a Dash's duration does not overlap with a dot's duration.

At that point, the reciever simply listens for the signal to assert itself, measures the duration, and determines if it was a dot or a dash.

Note, this example doesn't account for static on the line where a dashed assertion on the line is interrupted by noise, appearing to be a deassertion, and thus could be mistaken for dots or dashes, instead of a singal dash. But the protocol worked for the telegraph, because the assertion signal was strong enough that any variance was still within the range of "off" or "on"

Once a protocol like this is established, the device is built (photons, magic rocks, beats me) and the 2 nodes are deployed.

At Node A, I assert a signal (up-down polarized photons I guess), and on Node B, you listen for diagonal photons (once again, I guess from your explanation). You keep track of all the . and - you see and in what order, and parse that into the message.

For bi-directional communication, it is more than likely that these Quantum Repeaters have pairs of pairs. Node A has a a quantum-half wired up to detect a signal, and it has a different quantum-half wired up to be tickled. Node B has the corresponding halves. In my original example of the boxes with an RJ45 connector, that's an 8 wire connector, with so many wires for recieving, so many for sending, and a ground.

In the Quantum Repeater, the "ground" wire could instead be used as a timing pulse so that the reciever can synchronize what it's recieving on the other lines.

I don't know how quantum physicists think about this stuff, but once you say you have a QM box that can induce X number of states or values from the input node to the output node, the Electrical Engineers and Computer Science guys have expertise to turn that into a communication protocol.

Now if you instead were referring to a wierd behavior where dots and dashes start coming out before I send them as [MENTION=6674889]Gorgoroth[/MENTION] might have been talking about, that sounds cool, but I'd have to know more before I could discern if that's a problem or not.

And of course, I have no idea if QM technology exists where you can tickle Object A and detect the tickling in Object B. By "tickle" I mean do whatever you got to do to get that effect. Shine a light on it, lick it, send an electrical current into it, whatever.

 

[Asphere]

<!-- BEGIN TEMPLATE: dbtech_usertag_mention --> I don't know how quantum physicists think about this stuff, but once you say you have a QM box that can induce X number of states or values from the input node to the output node, the Electrical Engineers and Computer Science guys have expertise to turn that into a communication protocol.

But the person inducing the values at the input node has no idea which values he induced until after he induced them.

 

[Janx]

The problem here is that no actual information is transferred because you can't affect which state a quantum particle will go into (its random). So once you have prepared an entangled set (A & B) you have no idea which one has spin up and which one has spin down. The only way you can know is to measure one at which point it will randomly be in one of the two states. So if you measure A and it collapses into a spin up state, B will return a spin down state. All an observer measuring B will know is that A made a measurement that randomly yielded a spin up. On top of that, the act of measuring destroys the entanglement.

thats a bummer that it gets wrecked. Disregarding the destruction problem, if all we're getting for info is both items have the same spin (or opposite spin, whatever), that's a cool effect, but from a communications perspective, not quite sufficient for transferring data (well, we could do a pony-express fuel rod system).

I was hoping for something where you could make item A spin left, then right, then left again, and item B would copy it. Get that, and you have yourself an FTL telegraph in the making.

If all we really have is "here, take this rock and go west, look at it next week." and it'll be spinning right or left, but not at my control, then I'm not really able to pass anything through the connection.

if a magnetic charge, spin RPM, light, or some vibration could be sent, that's the ticket.

I know physics guys aren't stupid, so they know that too. But if they're befuddled over some "how do I know it's time to listen" problem, we got that solved in the computer science department.