Researchers quantum teleport particle of light six kilometres • Hacker News Board

Re: Researchers quantum teleport particle of light six kilometres

#181

Post by pqhwan » Wed, Sep 21, 2016, 8:55 PM UTC

roywiggins wrote: post #155

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"

There was a webcomic (or something like it) I saw a while ago that was about a single-buttoned gadget that could always predict whether you will press the button or not (don't remember the exact mechanism, but that was the jist). The story then went on about how the gadget would bring about humanity's collective philosophical collapse, because of its implications on the subject of free will.

I imagine that what you're describing about superluminal communication could be used to make a gadget like this.

I'm also wondering if anybody knows which webcomic I'm referring to. It'd be embarrassing if this turned out to be an XKCD number...

Re: Researchers quantum teleport particle of light six kilometres

#182

Post by cryptarch » Wed, Sep 21, 2016, 8:55 PM UTC

dsacco wrote: post #159

cryptarch wrote: post #150

Earlier quoted context omitted.

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 inte…

Is it possible to know when particles were prematurely untangled? Wouldn't that constitute transmitting information?

How would you tell the difference between a set of qbits entangled with the right other set of qbits, and with a different set of qbits? If an attacker were to intercept the whole stream/set of entangled particles, wouldn't he be able to perform a MITM attack by sending you particles entangled with his particles instead?

The way I mentally model "sending entangled particles and measuring them" is "sending two streams or blocks of exactly inverted random data to two locations, in a format that is impossible to recreate/forge as setting the right spins of particles is not possible". Is it so that this measurement can only be performed once, or can it be measured again later, giving the same results? Does the security of quantum entanglement come from an inability to create entangled particles with specific values, and if so, how does that solve wholesale MITM'ing, were the hole stream is intercepted on the way to one party, and then replaced by a wholy different stream with an attacker-controlled "twin" set/stream?

What I mean is, if one or both streams/sets of entangled particles were intercepted as a whole before arriving at A or B (or replaced before measurement and after arrival), wouldn't that allow for an effective MITM attack? I just find it hard to see how this technology solves the physical access problem.

Re: Researchers quantum teleport particle of light six kilometres

#183

Post by snowwrestler » Wed, Sep 21, 2016, 9:01 PM UTC

retox wrote: post #126

Ciantic wrote: post #118

Earlier quoted context omitted.

It's easier to think these as random number generators seeded with same value, taken six kilometers apart from each other, and then fetching values from them. No data is transmitted when fetching values from these.

Are we witnessing the first steps towards interplanetary turn-based seed driven gaming?!

Maybe!

Re: Researchers quantum teleport particle of light six kilometres

#184

Post by johncolanduoni » Wed, Sep 21, 2016, 9:03 PM UTC

lisper wrote: post #167

johncolanduoni wrote: post #162

Earlier quoted context omitted.

> QM can only be completed within the constraints of SR. This is not true. QM theory in general (without talking about particular Hamiltonians, field or otherwise or not) has no reliance on special relativity; it doesn't even have a reliance on positions in three-dimensional space (physicists who work in quantum information routinely work with QM at this level)! So again, you're already specifying QFT (e.g. by someth…

You're right. Most people don't know the difference between QM and QFT, so I tend to use the terms interchangeably in casual exchanges. You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass. So you can interpret this in two ways, as a prediction that FTL is possible, or a prediction that negative-mass matter does not exist. Nonetheless,…

> You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass.

You only need matter with negative mass if you want to construct one by building a machine. Some models for closed timelike curves that could occur in nature (e.g. the Tipler cylinder with infinite length) do not need negative energy.

> the likelihood of FTL being possible is extremely small, for all reasonable purposes indistinguishable from zero on current scientific knowledge, and the existence of entanglement does not change this one bit

Okay, that I agree with, but this unlikelihood is completely orthogonal to (pre-relativity) QM's compatibility with FTL travel/signaling. That was what I disagreed with.

Re: Researchers quantum teleport particle of light six kilometres

#185

Post by roywiggins » Wed, Sep 21, 2016, 9:10 PM UTC

pqhwan wrote: post #181

roywiggins wrote: post #155

Earlier quoted context omitted.

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"

There was a webcomic (or something like it) I saw a while ago that was about a single-buttoned gadget that could always predict whether you will press the button or not (don't remember the exact mechanism, but that was the jist). The story then went on about how the gadget would bring about humanity's collective philosophical collapse, because of its implications on the subject of free will. I imagine that what you'r…

Sounds like a Ted Chiang story:

http://www.nature.com/nature/journal/v436/n7047/full/436150a...

It sounds more like an SMBC comic than an XKCD one, societal collapse for unexpected reasons is an ongoing theme.

Re: Researchers quantum teleport particle of light six kilometres

#186

Post by lisper » Wed, Sep 21, 2016, 9:14 PM UTC

johncolanduoni wrote: post #184

lisper wrote: post #167

Earlier quoted context omitted.

You're right. Most people don't know the difference between QM and QFT, so I tend to use the terms interchangeably in casual exchanges. You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass. So you can interpret this in two ways, as a prediction that FTL is possible, or a prediction that negative-mass matter does not exist. Nonetheless,…

> You're right that GR doesn't rule out closed timelike curves, but to actually construct one you need to have matter with negative mass. You only need matter with negative mass if you want to construct one by building a machine. Some models for closed timelike curves that could occur in nature (e.g. the Tipler cylinder with infinite length) do not need negative energy. > the likelihood of FTL being possible is extre…

OK, fair enough. But just for the record:

> You only need matter with negative mass if you want to construct one by building a machine.

As I'm sure you're aware, things do move faster than light and/or backwards in time on the microscopic level. But that's not what most people care about when they talk about this stuff. They want to know if you can build a time machine. And the answer to that (on current knowledge) is a definitive "no".

Re: Researchers quantum teleport particle of light six kilometres

#187

Post by geoelectric » Wed, Sep 21, 2016, 9:20 PM UTC

semi-extrinsic wrote: post #105

javajosh wrote: post #92

Earlier quoted context omitted.

> Since this process converts quantum information to classical information (a digital or analog signal), we know that it must lose information. Only by making many measurements can we infer statistical properties of the entire quantum state. This is exactly what happens with the two-slit experiment. Wow, I've never heard it explained that way. That's brilliant - did you come up with it? Interference patterns are the…

I can't say I can claim any credit for the idea, no, this is what is you learn once you chase the QM rabbit hole deep enough. But it's maybe not a common explanation in Intro to QM class or in popular science writing? Actually (technical nitpick) it's even more lovely than what you describe: people have done the two slit experiment with verifiably single-photon sources... and you still get interference patterns. Real…

I've been struggling to understand double-slit.

I had thought the pattern represented interference between the probability distributions of where the photon might be found (which themselves follow "wavelength" and produce wave-like behavior). With two slits you get two distinct probability distributions that overlap spatially at the screen, which when added together cause denser/less-dense areas of photon detection and looks like an interference pattern.

That would have explained why streams of single photons "self-interfere" (they wouldn't--it's the underlying probability of their path that interferes with itself, so the pattern would eventually appear at any emission rate) but that seems so simplistic as to be unlikely to be the truth of the matter, especially with delayed choice and quantum erasure effects.

Do you happen to know a good layman's source for a solid explanation of what we know about double slit right now? I'd really like to understand it, if only at a high level.

Re: Researchers quantum teleport particle of light six kilometres

#188

Post by lossolo » Wed, Sep 21, 2016, 9:22 PM UTC

nabla9 wrote: post #93

lossolo wrote: post #62

Earlier quoted context omitted.

It does not work as material thing that is teleported. When you make measurement quantum function collapses and something that we don't know is happening and it's not teleporting anything because state is changed faster than speed of light, and no information or material can travel faster than that. This is a known paradox. Anton Zeilinger (father of quantum entanglement) is describing this in his books in more detai…

Why the assumption that teleportation means faster than light travel?

Because Anton Zeilinger has proven in his experiments that "teleportation" can happen faster than speed of light. It's what Einstein said was "spooky" about "quantum teleportation" that it contradicts theory of relativity.

Re: Researchers quantum teleport particle of light six kilometres

#189

Post by radicaldreamer » Wed, Sep 21, 2016, 9:27 PM UTC

ethanbond wrote: post #178

aji wrote: post #164

Earlier quoted context omitted.

you can't "communicate", but it would be useful for qubits in one quantum computer to be sent to another quantum computer without physically moving the qubit. that was what I meant by wide-area quantum information networks

Oh totally agreed. My response wasn't to your comment, but to a comment implying that aliens are communicating "across" us but never through us via means of quantum teleportation and that's why can't detect them. I agree with you, this is impressive and probably will be very helpful some day.

Thank you! This is definitely one of those things where my understand was inaccurate and this thread has been immensely helpful to improve that.

Re: Researchers quantum teleport particle of light six kilometres

#190

Post by bduerst » Wed, Sep 21, 2016, 9:30 PM UTC

johncolanduoni wrote: post #135

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…

You've compounded their lazy science reporting by making quite a few mistakes yourself in your description: 1. There is no duplication or copying of information, the original particle's state is destroyed. Copying is impossible when dealing with general quantum states[1]. (You may be thinking of measuring an entangled pair to randomly produce a stream of classical bits on each side that is the precise inverse of what…

> The particles when measured will result in the other particle ending up in the opposite of the measured state, and this inversion persists even if we do operations that preserve the quantum state.

This is the part that I don't get - what's the physics mechanism behind this? It sounds very Schrodinger's cat.

It also sounds like a great way to tell if your information has been unknowingly accessed when it's travelling between two supposedly secure locations.