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Researchers quantum teleport particle of light six kilometres

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Re: Researchers quantum teleport particle of light six kilometres

#151

Post by lisper »

johncolanduoni wrote: post #145
lisper wrote: post #82

Earlier quoted context omitted.

It is as certain as anything in science. Yes, QM is incomplete, but it places hard constraints on the space of possible completions, and none of them allow FTL communications. FTL would break not only QM and relativity, but causality: it would allow you to send information backwards in time.

> Yes, QM is incomplete, but it places hard constraints on the space of possible completions, and none of them allow FTL communications. This is entirely incorrect. All of the proofs about broken causality (all of which stem from special relativity, since non-relativistic QM doesn't put the speed of light on a pedestal) assume that the laws of special or general relativity hold everywhere. What you're saying is the e…

Yes, I am assuming that relativity holds everywhere. There is zero evidence that this is not the case. This is what it means to be "as certain as anything in science."

Re: Researchers quantum teleport particle of light six kilometres

#152

Post by dragonwriter »

owaislone wrote: post #148
TheRealPomax wrote: post #83

Not quite as impressive when you know, and if you don't you should before opening this link, that "quantum teleportation" is not teleportation. It was a cool-sounding name at the time, but has nothing to do with sci-fi style teleportation: nothing disappears from one place and then shows up in another. "Quantum teleportation" is a process of information duplication using particles that already exist, and have been po…

Isn't that how actual sci-fi teleportation would work behind the scenes as well? Basically, if I wanted to teleport myself from one city to another. I'd actually die and disappear, and an exact copy myself would be created instantly in the other city. It would be teleportation for the rest of the world but for me it would be death.

> Isn't that how actual sci-fi teleportation would work behind the scenes as well?

Depends. Some versions are materialize/dematerialize, some, IIRC, are basically traversable short-lived artificial wormholes.

Re: Researchers quantum teleport particle of light six kilometres

#153

Post by JanezStupar »

arcticfox wrote: post #143
Dylan16807 wrote: post #138

Earlier quoted context omitted.

This is an effect that uses entanglement to move the state of one particle to a different particle. You're describing only entanglement, so that's only half the puzzle. And you're describing entanglement in a misleading way. The most important part of entanglement is that you can't model it as two particles sharing secret data. When you touch one, you affect the output of the other. And this happens faster than light…

Is their any layman's physics explanation of how this works? Obviously you can't transmit information, but one particle can affect the other at distance faster than the speed of light? My sense was that quantum physics basically makes sense if you go really small and have an understanding of how the particles interact. But entanglement is weird, because somehow there's also a mechanism for them to interact at distanc…

Particles are constantly randomly changing their state. Entangled particles change in tandem.

As the GP said - to find out whether a particular change of state was random or a signal, one would need to compare readings from both.

Readings cannot be sent faster than the speed of light. Thus preserving causality.

Re: Researchers quantum teleport particle of light six kilometres

#155

Post by roywiggins »

mechazawa wrote: post #121

Forgive my ignorance as I'm a software engineer and not a physicist. But does this means that in the future FTL communication will be possible. Every time I work with networking one thing always springs to mind. That is that the speed of light is a limiting factor. Will this mean that in the future latency issues will be a thing of the past? http://www.stuartcheshire.org/rants/latency.html

You can't use entanglement to communicate. Quantum teleportation needs a classical channel to work.

> https://en.wikipedia.org/wiki/No-communication_theorem

If you have superluminal communication, signals can travel into your subjective past, which means causality pretty much breaks down entirely. There's no theoretical reason why causality has to hold, but it would make life extremely "interesting"

Re: Researchers quantum teleport particle of light six kilometres

#156

Post by tvmalsv »

arcticfox wrote: post #143
Dylan16807 wrote: post #138

Earlier quoted context omitted.

This is an effect that uses entanglement to move the state of one particle to a different particle. You're describing only entanglement, so that's only half the puzzle. And you're describing entanglement in a misleading way. The most important part of entanglement is that you can't model it as two particles sharing secret data. When you touch one, you affect the output of the other. And this happens faster than light…

Is their any layman's physics explanation of how this works? Obviously you can't transmit information, but one particle can affect the other at distance faster than the speed of light? My sense was that quantum physics basically makes sense if you go really small and have an understanding of how the particles interact. But entanglement is weird, because somehow there's also a mechanism for them to interact at distanc…

While I don't have an explanation for you, you aren't the only one to think it's weird. Einstein called it "spooky action at a distance".

Re: Researchers quantum teleport particle of light six kilometres

#157

Post by johncolanduoni »

lisper wrote: post #151
johncolanduoni wrote: post #145

Earlier quoted context omitted.

> Yes, QM is incomplete, but it places hard constraints on the space of possible completions, and none of them allow FTL communications. This is entirely incorrect. All of the proofs about broken causality (all of which stem from special relativity, since non-relativistic QM doesn't put the speed of light on a pedestal) assume that the laws of special or general relativity hold everywhere. What you're saying is the e…

Yes, I am assuming that relativity holds everywhere. There is zero evidence that this is not the case. This is what it means to be "as certain as anything in science."

That's not a "hard constraint on the space of possible completions". That's assuming the answer to begin with.

Re: Researchers quantum teleport particle of light six kilometres

#159

Post by dsacco »

cryptarch wrote: post #150
roywiggins wrote: post #149

Earlier quoted context omitted.

My understanding is that it's snooping-evident, at least in theory. If someone takes a peek at the entangled bits in transit, the entanglement will be broken and when A and B compare notes, their states will no longer be correlated. This is good for key distribution because if someone snoops a key, you know the key is compromised and throw it away. If you have a secure way to distribute your OTP I'm not sure there'd…

But if you have a secure way of getting your quantum bits to Bob, couldn't you get your OTP there in the same way? I mean, yes, you could detect it if your stream of entangled bits is intercepted while it is going, but you couldn't detect it if it was intercepted from the get go and MITM'd.

>>But if you have a secure way of getting your quantum bits to Bob, couldn't you get your OTP there in the same way?

No, because you do not choose the information to be verified on either side. You're not choosing a key and "encoding" it into "quantum bits", you're measuring the quantum state of two entangled particles.

If the quantum state matches, you have provably secure transmission. If the transmission were intercepted, the act of intercepting it would cause the measurement verification to fail due to the observer effect. The parties would be able to see that the key was intercepted because the particles were prematurely disentangled.

This is quantum key distribution, which is a subset of (and often misleadingly conflated with) quantum cryptography. It is not possible to use quantum cryptography to send provably secure arbitrary data of your choosing, it's only possible to generate and distribute provably secure keys and use those for classical cryptographic communication.

The reason why it's important to call it quantum key distribution is because it makes it clearer that this only solves one problem in a greater cryptosystem. Key distribution is a significant problem in cryptography, and provably secure key distribution is a great leap forward, but it's not the only problem.

Re: Researchers quantum teleport particle of light six kilometres

#160

Post by lisper »

johncolanduoni wrote: post #157
lisper wrote: post #151

Earlier quoted context omitted.

Yes, I am assuming that relativity holds everywhere. There is zero evidence that this is not the case. This is what it means to be "as certain as anything in science."

That's not a "hard constraint on the space of possible completions". That's assuming the answer to begin with.

No, it isn't. The mathematical formalism of QM is fully compatible with SR. QM can only be completed within the constraints of SR. To get FTL you would not have to complete QM, you would have to overthrow it and SR and GR and replace them all with something completely different. Not just that, but whatever you replaced it with would still have to account for the observed invariance in the speed of light in all reference frames.

QM and entanglement are really red herrings with respect to the FTL question. The impossibility of FTL is a direct logical consequence of the fact that there exists a universal speed reference in space-time. Entanglement only appears to violate this because people don't understand what entanglement actually is and how it works. When you make a measurement on one member of an EPR pair, nothing happens to the other particle. It's still in exactly the same state it was in before the measurement. So there is absolutely no reason whatsoever to believe that FTL could be possible, and a lot of reasons to believe that it's not. Granted, this is not "absolutely certain" (nothing in science ever is) but it's as certain as it gets.

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