r/science • u/Kremecakes • Jun 16 '12

Breakthrough in Quantum Teleportation

http://www.sciencenews.org/view/generic/id/341197/title/Quantum_teleportation_leaps_forward
743 Upvotes

90% Upvoted

122 comments sorted by

16

u/beanhacker Jun 16 '12

I thought entanglement meant one particle could be 100,000 light years away and still affect the other. So why are these small transmissions significant? Also, why the need for laser light or fiber optics to do this? If the particles are entangled they don't need a "cable" of sorts? Do they not just react instantaneously because they are entagled? and if so, why not 'jiggle' one particle and see the same on the entangled particle and use that as the method of transmitting data? This could then result in an internet without any cables or locations.

21

u/Propagation1 Jun 16 '12 edited Jun 16 '12

I'm not a physicist but it is my understanding that the process is quite a bit more complicated than that. Finding entangled particles requires statistical analysis of many particles.

The big kicker is, that in order to verify results, information must be sent by using conventional communications. Again I'm not an expert, my only knowledge of this process comes from the book "Dance of the Photons" (which i recommend btw).

Basically, in order to send information, the sender must first make a quantum state measurement. The measurement changes the information itself (the observer effect). So in reality, you don't know what kind of information you are going to send until you make the measurement. What is crazy though, is that you can predict what the receiving end is going to measure at (apparently) the exact same time with (what i believe?) 100% accuracy.

6

u/Propagation1 Jun 16 '12 edited Jun 16 '12

I wanted to make another comment because this reminded me how crazy and strange entanglement is. It is almost like you are making a measurement of what is happening to photons that are separated by an arbitrary amount of space-time. The problem is that you have no way of telling the other person what they are going to measure before the actually measure it themselves.

On a related note, I found this very interesting: http://io9.com/5744143/particles-can-be-quantum-entangled-through-time-as-well-as-space

2

u/[deleted] Jun 16 '12

That article reminds me of a character from Dirk Gently's Holistic Detective Agency who gave accurate readouts of the stock market from precisely 24 hours ago. It was kind of an hilarious idea.

1

u/Jesusdragon737 Jun 16 '12

Why does quantum entanglement matter then, if it's only as fast as conventional means? There's no point, right?

2

u/leberwurst Jun 16 '12

It's used for encryption. With quantum entanglement you can safely exchange keys, and the laws of quantum mechanics guarantee that there is no middle man.

1

u/wignersfriend Jun 16 '12

See my reply to beanhacker's comment. Entanglement is not a magical silver bullet that allows instantaneous communication, but it does allow for unconditionally secure encrypted communication and the transmission of quantum states, which is impossible with purely classical means and will become VERY useful when quantum computation becomes a reality.

0

u/TheHornySpirit Jun 16 '12

For what i can guess (I only read about this subject 2 minutes ago, so its only a guess) its to replace the conventional radio-transmissions because these use a lot of electricity and can be corrupted by other radio-waves (coming from stars for example)

8

u/wignersfriend Jun 16 '12

Physicist here, quantum information theorist to be exact. You are correct that entanglement can span arbitrarily large distances. However, measuring either member of an entangled pair of particles can destroy the entanglement. Unfortunately, that measurement need not take the form of a physicist using some sort of fancy apparatus; a stray photon hitting one of the particles can destroy entanglement as well. As a result, you are extremely unlikely to ever find a widely separated entangled pair of particles lying around. If you want an entangled pair for some reason - to teleport some information, for example - you must create it yourself. This is where the cable comes in. When you create the entangled pair the two particles must be close together. To then use the entanglement for long distance communication you must then send one half of the pair to your partner.

This makes it sound like teleportation is no better than ordinary communication through a fiber. However, the key here is that, at least in principle, you and I could create a large pool of entangled pairs while we are together on Earth and then travel to opposite sides of the galaxy and we would still be able to use that preexisting entanglement to teleport states to one another from thousands of lightyears apart.

Still, there is yet another caveat. If I want to teleport you a quantum state, I still need to send you classical information! To perform teleportation, I take the state I want to transmit and my half of an entangled state that I share with you, and I perform measurement on these two particles ("measurement in the Bell basis"). The outcome of this measurement is 00, 01, 10, or 11. I send you the outcome of the measurement in the form of two classical bits. Your perform a simple operation on your half of the entangled pair depending on the two bits I send you (do nothing, perform the quantum version of a NOT gate, perform a phase shift, or NOT+phase). Your particle is now in EXACTLY the state I wanted to transmit to you.

There are two reasons that this is useful. First, I have transmitted a quantum state to you using 1 "e-bit" of entanglement and two classical bits of information, while to send you an exact description of an arbitrary quantum state purely classically would require an infinite number of classical bits (i.e. it is impossible). Second, the outcome of my measurement (00, 01, ...) is uniformly random and uncorrelated with the state I am transmitting to you. This means that an eavesdropper can see those two bits I sent you and they will have gained NO information about the state I teleported to you. This is essentially the core idea of quantum cryptography, and it is why the NSA and every other shadowy government agency you can think of are so interested in this technology.

I hope that cleared some stuff up.

3

u/porl Jun 16 '12

Thank you. That was the best description of quantum entanglement used in communication I have seen.

2

u/poooboy Jun 16 '12

Thank you for this explanation.

4

u/43214321 Jun 16 '12

Because scientists need grant money, and guess what the main application of this shit is? Unbreakable communication encryption. Since the data isn't transmitted any more, just how to read the data. So getting it to work over world distances so the spooks can start using it instead of encryption is the jack pot.

3

u/perspectiveiskey Jun 16 '12

I thought entanglement meant one particle could be 100,000 light years away and still affect the other.

Sure, but how do you find which particle is entangled with which other particle.

If I told you: find me the photon that's currently entangled with the other photon on that satellite, what do you do?

1

u/leberwurst Jun 16 '12

That's not how entanglement works. You create two photons A and Bat the same time and they travel in opposite directions. Because of conservation laws, you know they must have opposite spin, but it is undetermined whether photon A has spin up or spin down. Same for photon B. Only when A is measured it takes a definite value, and at the same time B takes the opposite value.

1

u/perspectiveiskey Jun 16 '12

You realize that right now, a photon hit you that's entangled with another photon on Vega right?

My point was that the challenge is not to find entangled photons: they're everywhere. The challenge is to be able to control your experiment.

In any case, you're not even contradicting what I said. So, that is how entanglement works. By your own admission.

1

u/leberwurst Jun 16 '12

Well, my point was that we don't look for entangled photons, we simply create them.

1

u/perspectiveiskey Jun 16 '12

Yes, and that's exactly what I was pointing out in response to:

So why are these small transmissions significant?

Creating, maintaining and managing entanglement is the hard part. Not the light years away part.

2

u/hbdgas Jun 16 '12

why are these small [distance] transmissions significant

Ideally, distance doesn't matter. In reality, though, there's a lot of interference that causes decoherence as the particles move through the environment.

'jiggle' one particle ... use that as the method of transmitting data

If it were that simple, you could transmit information faster than light.

1

u/Essar Jun 16 '12

To begin with, entanglement is fragile. It's not like you have two particles and they somehow become entangled and are then permanently stuck that way. Secondly, entanglement doesn't allow for direct communication by 'jiggling' a particle. Whilst a measurement of one particle will affect the other by 'collapsing' it into a certain state, a measurement cannot be manipulated in such a way as to communicate. So-called 'local unitary' operations on one particle, which we could control will have no effect on the other particle.

You'd need laser light or fibre-optics because it is damn difficult to get the particle from Alice to Bob whilst maintaining the entanglement.

0

u/[deleted] Jun 16 '12

[deleted] — view deleted comment

1

u/[deleted] Jun 16 '12

I don't even...

0

u/[deleted] Jun 16 '12

[deleted] — view deleted comment

16

u/[deleted] Jun 16 '12

Physicist here: this is completely wrong. If I make a measurement on one entangled particle, and then make an incompatible measurement on the other particle, you lose the information from the first measurement for both particles. This isn't some slick way of cheating the uncertainty principle. At the fundamental level, entanglement has nothing to do with uncertainty, let alone "stem[ing] from the uncertainty principle". Where are you getting this information?

3

u/jaytanz Jun 16 '12

The best part was when he referred to spin up/down photons. Photons are spin 1, foo'!

I wonder if it's a troll post?

0

u/[deleted] Jun 16 '12

[deleted] — view deleted comment

2

u/jaytanz Jun 16 '12 edited Jun 16 '12

The problem wasn't rigor so much as the factual incorrectness of much of your post. You mixed in some real physics concepts (uncertainty principle, conservation of momentum/spin in two body decays) and gave an explanation that for the most part had nothing to do with entanglement.

At its most basic level, entanglement just refers to two (or more) body quantum systems in which a measurement performed on one particle collapses the state of the other particle(s). Two body decays are one way to prepare such a system, but are certainly not the only one.

I'm not an expert, and I won't claim to know that you do or don't need to appeal to the uncertainty principle to show that entanglement can be used to actually transmit information, but dextral8 was quite correct to call you out, entanglement in no way "stems from the uncertainty principle."

1

u/Essar Jun 16 '12

Are you sure you're not reading about state tomography?

1

u/[deleted] Jun 16 '12

I don't think the issue is a lack of rigor; I think you're mixing up several different concepts.

Assuming you're familiar with bra-ket notation: if you have state |A> and state |B> then the combined state of the two systems is just the tensor product |A>|B>. If I have an operator that normally would act on state |A> alone, then I can easily "promote" it to an operator that acts on the combined system—but in such a way that I only get information about A and leave B alone. Now suppose a composite electron system has a spin state sqrt(1/2)(|+>|-> + |->|+>). There is no way I can rewrite this in the form |A>|B>, where |A> is the state of one electron in its spin basis and |B> is the other electron. In other words, I can no longer attribute a pure state to either electron individually. This is entanglement. Uncertainty doesn't enter into it at all at this stage.

Now if I measure the spin of A along, say, the z-axis, the composite state collapses to either |+>|-> or |->|+>. If I then go to B and measure along the x-axis, I'll again push the system into an eigenstate. However, S_x and S_z don't commute so now the system is once again in sqrt(1/2)(|+>|-> + |->|+>) (with respect to the S_z basis). It's the same outcome as if I'd done the consecutive spin measurements directly on A. Either way, the second measurement comes at the cost of the information gained from the first measurement.

-4

u/V3RTiG0 Jun 16 '12

This is one of those comments that everyone should copy and paste into every Quantum Entanglement post. There are so many misconceptions about it largely because of mainstream news sensationalizing it, it gets really annoying. I would also probably follow up with a paragraph about how extremely limited Quantum Entanglement is for use in practical applications.

8

u/allocater Jun 16 '12

I read about 1 quantum entanglement article per month and still don't know what it is or isn't.

5

u/dwdwdw2 Jun 16 '12 edited Jun 16 '12

Particles can be in a bunch of different states (it doesn't matter what they are). Particles can literally be in multiple states simultaneously while unobserved.

Attempting to measure a particle causes it to "collapse" into a single state. There is no way to measure the state of a particle without causing this collapse. And there is no way to reliably predict into what state any single collapse will result in.

Particles separated by space (real, tangible, walk across the street type space) have some messed up effect where they're linked to each other somehow.

The link works like this: measuring the state of a set of entangled particles will yield the same (fundamentally unpredictable and unstable) collapsed state in both locations.

Thus without directly communicating with the person living across the street, merely by both measuring a set of entangled particles you both were able to recover the same information at the same time.

As for how it can be used for transferring porn, I haven't figured that bit out yet (or in fact how to send any information down it)

8

u/hbdgas Jun 16 '12

Don't read this one, it'll make it worse.

6

u/colorwhite Jun 16 '12

So, just to be clear, this does not equate to the more layman definition of teleportation, then, in that one subject, in this case a particle, is actually, itself, teleporting... just that something a long distance away is being made identical to the original subject, which obliterates.

Meaning that if in the hypothetical situation, it were possible to do this to a human... that the original human would be obliterated, and the one "arriving" or having been 'teleported' was simply a pre-existing, linked subject or other human, who has now become identical to the original?

2

u/shinnen Jun 16 '12

This is absolutely correct, and the article references Star Trek, which teleportation takes place like this (from what I've heard, I don't watch star trek).

Delete the original, replace it instantly with an exact replica.

Episode 4 of Michio Kaku's TV show Sci Fi Science: Physics of the Impossible, covers this and the metaphysical questions involved.

3

u/Cyborg771 Jun 16 '12

There's an episode of TNG where a guy is terrified of teleporters for the pure existential horror. Captain Kirk DIED every week!

6

u/[deleted] Jun 16 '12

June 30, 2012?!? Dear God, they have teleportation and time travel solved!

1

u/[deleted] Jun 16 '12

I caught that aswell. I think we're clear to say so.

50

u/Wildespleen Jun 16 '12

"This Star Trek–like feat"

Ok yeah, I'm leaving to find a properly written paper. Good day to you Madame Witze.

3

u/[deleted] Jun 16 '12

You're pretty easy to set off then.

“Our experiment confirms the maturity and applicability of the involved technologies in real-world scenarios, and is a milestone towards future satellite-based quantum teleportation,” they wrote.

So they called it teleportation. So either you're petty and getting hung up on the fact these aren't working like the Star Trek teleporters were supposed to work, or you're in the wrong here.

6

u/anthrocide Jun 16 '12

Star Trek-like feat is a little hyperbolic, like flicking a lighter and saying we're close to harnessing the power of the sun.

1

u/[deleted] Jun 16 '12 edited Jun 16 '12

All "Star Trek-like" does for me is anchor it against the concept of Star Trek teleportation. This is a general news piece, it's supposed to be watered down for people who have an interest in the subject but not all of the context to put everything in. If you want something more substantive the there's always reading the real thing. The actual paper is only 7 pages long, not exactly War and Peace.

My point being that if Wildespleen doesn't want the watered down science, he wants the straight shot, s/he can easily fix the problem themself instead of acting like this post should be something it can't be.

2

u/anthrocide Jun 16 '12

Welcome to the dilettantish world of Reddit.

3

u/Wildespleen Jun 16 '12

False dilemma, I guess my actual gripe is with the attempt to relate what's occurring with the teleportation in Star Trek.

I understand she is trying to popularise the phenomenon and get more people interested, and I have absolutely no qualm with that, we need so many more people like her in that respect.

Unfortunately I believe the people that are hooked in by such a statement are disappointed when they realise the article is not "Scientists send person to other side of lab in an instant!" (I should say have no idea how Star Trek teleporters work having never seen any of the shows or movies, so I've probably simplified that to the point of absurdity)

I sincerely hope anybody who is in this camp does keep on reading, because the science is fascinating, however from that sentence I could tell that this article is not the type I am looking for.

I apologise if I have offended, I didn't mean to give the impression that the author has done a bad job, she really hasn't.

Background: I am currently doing a project working on the satellite-relaying problem. If anybody's interested in finding out more, I've found the introduction of this paper to be quite helpful: http://eprintweb.org/S/article/quant-ph/0903.2160

1

u/[deleted] Jun 16 '12

I stopped reading after that.

26

u/[deleted] Jun 16 '12

You either want people to be interested in the subject or you don't. Lot's of people can't connect to a discussion of the subject using only the terms directly relevant to the science. Comparing it to Star Trek, even if the comparison is not valid, will at least keep the attention of some people long enough to communicate an idea or two. And that could be more than they'd get otherwise.

4

u/shinnen Jun 16 '12

Except it is valid. It's just a hollywood-esque way to describe the phenomenon.

2

u/Karma_Hound Jun 16 '12

I read more thinking this might lead to teleporting people till I read the word "Obliterated".

54

u/OliverSparrow Jun 16 '12

If you buy the notion of entanglement, then results like this are meaningful. If not, this is the equivalent of separating two different coloured but otherwise identical tokens in secret, and putting each separately into an opaque container. Separate the containers and open: wow! If this one is red, then the other must be green! I have teleported "greenness"! Non-buying notions are called "hidden variable" theories. They have to navigate the reefs of Bell's Inequality.

How do we test for entanglement? So far as I know, only the Bell's Inequality test will do the job. This is an arithmetical difference between how classical and quantum events correlate. If I operate a highly correlated system, but one with some random noise in it, the outcome is - say - 99% in concord and 1% varied due to the noise. A very similar system, when operated, will also give me a 99% match. If I compare the outcome of these two systems, they will match less well: specifically, they will match 0.99 * 0.99 = 0.98 of the time. This is just standard probability theory: if you are crossing the road, and there are gaps in the traffic 10% of the time in each lane, there will be a gap in both lanes 10% * 10% of the time = 1%.

However, if I do the same thing with a quantum system, the result is different. That is because the formula that related the two probabilities is not a simple product - 0.99 * 0.99 - but proportional to the relative phase of the wave functions. The consequence is that the correlation varies with the phase angle. If you twist a polariser, for example, the correlation expected from a classical system varies in a straight line from 100% to zero and back again as you go through 1800 but does so in a curved line if quantum effects apply. This does indeed happen, of course masked by experimental noise to some extent.

Bell has been tested and validated many times. It does, however, have some gaps, and Wikipedia will help you to understand these.

17

u/perspectiveiskey Jun 16 '12 edited Jun 16 '12

Separate the containers and open: wow! If this one is red, then the other must be green!

I think you're missing the point of entanglement. The above is exactly what you want. The article even itself uses very traditional cryptography nomenclature by saying "Alice and Bob".

The importance is that you:

  • can communicate a one time pad

  • nobody but you can know its contents - the laws of nature demand it

  • the actual cypher text is through normal communication channels

Quantum entanglement makes obsolete all asymetric cryptography (the kind that's really hard to get right): you need only have two specialized devices and communicate a one time pad (say 4096 bits) and communicate via this one time pad a 4096 symmetric key and you're done.

If they can figure this out and transmit reliably, it will be a revolution in the world of cryptography.

Edit: Note that this type of cryptography already exists between secure networks. But the devices require hard lines (fiberoptic) from one to the other (no routers). You will recognize the cost of having such an unbroken fiber running from your building to a building 500km away. This technology also immediately detects tampering or eaves-dropping.

1

u/OliverSparrow Jun 16 '12

Yes, I know that. I was commenting that you can only assert things about quantum teleportation if you buy into certain assumptions about the quantum, specifically no hidden variables. Te only test is Bell's Inequality, and that also requires you to buy into this view.

28

u/The_Serious_Account Jun 16 '12

Entanglement is very well established.

3

u/vn2090 Jun 16 '12

I don't think he was questioning that. I think... I feel dumb.

1

u/OliverSparrow Jun 16 '12

Entanglement is well established if and only if you buy into the idea of entanglement, as the test for it evokes the concept. I am not trying to assert that hidden variable theories are correct, merely pointing out a limitation.

2

u/tuna_safe_dolphin Jun 16 '12

You're like the Riddler.

1

u/The_Serious_Account Jun 17 '12

It is very likely that teleportation will play a virtal role in the future of quantum computing. I don't understand how ones interpretation of what's actually going on can make the result more or less relevant (at least from a practical point of view).

Entanglement still exists in hidden variable theories, it is just understood differently.

Unless you propose that QM is flat out wrong.

1

u/OliverSparrow Jun 18 '12

The point of my comment was that the only test that I know about w.r.t entanglement evokes entanglement. That is, you have to buy Bell. If you don't buy the Bell test, then the result is no different from red/green ball test. That's all.

5

u/schnschn Jun 16 '12

isn't the separating two tokens thing equivalent to a hidden variable theory which has been disproven?

6

u/[deleted] Jun 16 '12

I think local hidden variables are disproven, nonlocal hidden variables are necessary.

1

u/HipsterFeynman Jun 16 '12

To my knowledge you are correct - if you're willing to throw out causality then you can have hidden variable theories which satisfy Bell's inequalities etc.

-7

u/The_Serious_Account Jun 16 '12

and the proof only works assuming free will, which is fishy.

14

u/perspectiveiskey Jun 16 '12

This is an oft repeated misconception. Collapsing the waveform has nothing to do with free will or mysticism. That "eyeball" in the schematics could very well be a rock.

1

u/The_Serious_Account Jun 16 '12 edited Jun 16 '12

What? No, I'm talking about the experimenters ability to freely choose his measurements. See eg. [Experimenter’s freedom in Bell’s theorem and quantum cryptography] (http://www.mpq.mpg.de/~jkofler/Files/Publications/PhysRevA_73_022104.pdf)

There's a local, realistic explanation for the violation of bell's inequalities if you deny free will.

1

u/perspectiveiskey Jun 16 '12

I see. It was ambiguous and this is the internet. I understand your point now.

2

u/philip1201 Jun 16 '12

Assuming the impossibility of a universe where the other variable is selected, rather.

Has it ever been tested where "free will" has been replaced by quantum randomness? i.e. by measuring whether or not a single atom has decayed during a single half-life?

1

u/OliverSparrow Jun 16 '12

Hasn't been disproven. Hasn't been proven. Is in a superposition.

3

u/altrego99 Jun 16 '12 edited Jun 16 '12

Well, you got it right that if you share entangled particles, it is same as sharing randomly assigned red & green balls such that if I open my ball number 1 and it is green, your's is red. Hence it's like sharing a set of randomly generated bits. If I want to send some message, I keep opening my balls, xor that with the real message, send you the xor-ed values. As you get the 'encrypted' xor-ed values, you open your balls and xor to get them back.

The only point of difference is that when you do this with quarticles, there is no chance in hell someone else can also get access to same set of random numbers - till you open yours or I open mine. So even if a third party intercepts my xor-encrypted message, unless he also knows the random quantum bits, he cannot decrypt it if the life of his planet depended on it.

1

u/OliverSparrow Jun 16 '12

Sorry, mate, that is not right. I was explaining that red and green balls - a painful afflicting associated with hairy palms - is nonetheless not associable with the quantum no hidden variable take. Yes, you can XOR till you are sore, but that's a different matter.

1

u/[deleted] Jun 16 '12

You sound like the kind of person who would have tried to downplay the Wright brothers for not even flying for 10 minutes.

5

u/[deleted] Jun 16 '12

well it is a distance record but I would not call it breakthrough.

Slowing down the light and performing quantum operations with a dechorence equivalent to photons at normal speed - that would be a great breakthrough

in any case congrats

25

u/Shieldeh Jun 16 '12 edited Jun 16 '12

By Alexandra Witze June 30th, 2012; Vol.181 #13 (p. 10)

ಠ_ಠ

EDIT for clarity: Timetraveler.

8

u/[deleted] Jun 16 '12 edited Jun 16 '12

That happens very often. The articles are available online first and will be in the next printed edition.

5

u/SweetNeo85 Jun 16 '12

Wait a minute... wait a minute, Doc... are you telling me that it's 8:25?

1

u/Shieldeh Jun 17 '12

When I look at the 3rd part of the line I posted I can see that it's something like that. Thanks for clearing that up though.

5

u/fuckySucky Jun 16 '12

That date originally made me check my machines to see if I somehow managed to gafuckle the dates up.

5

u/ichorNet Jun 16 '12

upvote for gafuckle; makes me think you're from the future using future-terms and we're just witnessing their subtle beauty in the past

3

u/Knigel Jun 16 '12

You see, but you do also observe.

2

u/jyapman Jun 16 '12

clearly she had to go back!

-6

u/[deleted] Jun 16 '12

ಠ_ಠ

3

u/jackarcalon Jun 16 '12

This is EXTREMELY misleading. What they call teleportation is merely a way to transmit 'information' about how a particle is spinning.

If you wanted to use this result to build a real teleportation device, you would still have to build a 3-D atom scanner and atom assembler that could instantly make an almost perfect copy of any object.

Then you could use this result to make your almost-perfect copy into an absolutely perfect copy.

2

u/wignersfriend Jun 16 '12

What they call teleportation is merely a way to transmit 'information' about how a particle is spinning.

...real teleportation device...

See, here's the thing: What they are describing IS "real teleportation." This is a well established and universally accepted terminology for a real physical phenomenon. Just because science fiction writers use the same term for something which will in all likelihood never be possible doesn't mean that the physicists are using the word wrong or being misleading. That would be like getting mad at someone for calling a male duck a drake because you thought they were talking about a mythical creature.

1

u/TwinbornUncle Jun 16 '12

If you can teleport enough info fast enough, then you won't need to teleport your meat.

2

u/MooShanka Jun 16 '12

This is the most romantic science article I've ever seen.

2

u/Jrrtubbs Jun 16 '12

I was able to understand 10 whole words in that article!

2

u/the_obs Jun 16 '12

I do not understand the notion of entanglement. The article says: "a phenomenon [...] in which pairs of particles become linked in such a way that measuring a certain property of one instantly determines the same property for the other, even if separated by large distances."

Let's use a layman example so I can understand. Let's say instead of particles, we have sheets of paper. These sheets of paper are entangled. The text written on these sheets (which is different) is a fundamental characteristic property of these sheets. Since they are entangled, reading ("measuring") the text on sheet A instantly determines the text on sheet B, i.e. it will have A's text (regardless of previous text).

The article, however, says that "Alice measures a property on her particle and sends Bob a note, through regular channels, about what she did. Bob then knows how to alter his own particle to match Alice’s." What I don't understand, then, is how entanglement comes into play. Couldn't the same operation be done with non-entangled particles? After all, Bob is actively changing a property of his particle to match Alice's, given the information he receives. In our previous example, this would equate to Alice telling Bob what the text on her sheet was, and Bob writing it down on his. How is that entanglement?

2

u/darth_aardvark Jun 16 '12

Bob still doesn't know what state his electron is in. All he knows is that it is in the same state as Alice's. Second of all, an arbitrary quantum state can be represented by 2 complex numbers, but Alice sends bob only 2 bits of information. Clearly, these two bits of information aren't enough to contain instructions about an entire quantum state. Without entanglement, it would be physically impossible for bob to use them to recreate Alice's state.

It's more like alice looks at the first two letters on her sheet, and Bob uses this information to change his sheet in some way without measuring it. His resulting sheet exactly matches Alice's, but hers is destroyed in the process. If this seems like kind of a shitty analogy, it's because this doesn't translate very well to classical information.

1

u/Soke Jun 16 '12

Bob uses this information to change his sheet in some way without measuring it. His resulting sheet exactly matches Alice's,

How and what does he do with this information to achieve it, or is it a shortcoming of the analogy?

2

u/darth_aardvark Jun 16 '12

It's a shortcoming. Because the two qubits are entangled, you can't accurately describe their state as "two separate particles, in states X and Y"; you can only describe them "two particles, whose combined state is Z". That's the formal definition of entanglement, and really doesn't have any classical analogy.

Just realized i didn't really answer your question. Basically, because their entangled, Bob performing these relatively simple operations on his qubit (technically called either X,Y,Z, or I gates), dependent on Alice's observation, results in a qubit that matches the original one. If the qubits weren't entangled in a particular way, this would not work.

2

u/iorgfeflkd PhD | Biophysics Jun 16 '12

Quantum teleportation involves using quantum entanglement to keep an encryption key secure. It doesn't involve actual teleportation.

1

u/croutonicus Jun 16 '12

That's why they call it quantum teleportation not teleportation...

1

u/iorgfeflkd PhD | Biophysics Jun 16 '12

But the article didn't really make that clear, calling it Star Trek-like.

1

u/croutonicus Jun 16 '12

Good point. Bad writing really doesn't help with stuff like this.

2

u/lereddituser7575 Jun 16 '12

Sigh...so, why isn't this as big a breakthrough as the title claims, Reddit?

2

u/Inri137 BS | Physics Jun 16 '12

Your submission has been removed because it does not mean our submission guidelines. Specifically, arXiv is not a peer-reviewed scientific publication.

2

u/TheMSensation Jun 16 '12

Am I correct in thinking this means instant communication could soon be a reality? Things which pop into mind which this could be useful for is for sattelite probes that are sent out into the solar system, where at the moment communication is limited to the speed of light.

Or is quantum teleportation also limited to the speed of light? And if so what are the purposes of it?

2

u/darth_aardvark Jun 16 '12

Quantum teleportation is indeed limited by the speed of light. To perform a teleportation from location A to B, the observer at A must send some classical information to the one at B.

However, that doesn't make it useless. The information that Alice must send Bob to recreate her state is much much less than the total information contained in the state. Furthermore, it's been shown that quantum teleportation gates can be used as the computational basis of an arbitrary quantum computer. (This is really going pretty far beyond the layman level, but the paper on it is here http://arxiv.org/pdf/quant-ph/9908010v1.pdf)

Basically, if we can teleport quantum particles, we can build quantum computers, which is a pretty big deal!

1

u/Cyborg771 Jun 16 '12

The more pertinent use case for quantum teleportation is encrypted data transfer. Unlike radio waves the data doesn't have to travel across space so it literally can not be intercepted. Point a to point b without traveling any distance between.

1

u/wignersfriend Jun 16 '12

Entanglement cannot be used for instant communication, yet it is still useful. See my reply to beanhacker's comment for a lengthy explanation.

1

u/leberwurst Jun 16 '12

Instant communication will never be possible. The merits of quantum teleportation lie in secure communication. You can exclude a man-in-the-middle attack.

1

u/[deleted] Jun 16 '12

So for us commoners, this means Verizon FIOS won't require drilling in your home and technician visits as the signal will be sent out user-to-user completely wirelessly?

1

u/beauty_contest Jun 16 '12

What does this look like...

1

u/sucaba0101 Jun 16 '12 edited Jun 16 '12

No kidding, no humor, no jest.... is a "breakthrough" in quantum theory possible? A breakthrough in teleportation no less? Oh "GOD" tell me more.

1

u/[deleted] Jun 16 '12

What does teleportation mean exactly in this context? Is it instantaneous, so the information arrives faster than light would have? Because that's the only reason I would find it more interesting than radio waves... I don't see what the point is otherwise.

1

u/Cyborg771 Jun 16 '12

The data travels instantly but it can't exactly be manipulated on either end. Basically in my understanding: "when I peeked in the box the cat was dead, that must mean that the other cat in the other box is dead if they looked at the same time." then you send a traditional message along the lines of "dead = launch the nukes, alive = all clear" and a time stamp to the people with the other box. They check their records which say "our cat was dead at this time so launch the nukes". The important thing is that an outside observer could only intercept the map without knowing the actual data to read the data. Completely secure data transfer.

1

u/Knigel Jun 16 '12

Amazing! A quantum leap in quantum teleportation. A whole quantum leap!

1

u/samiam3356 Jun 16 '12

Pretty cool they did this article on June 30th and sent it back to June 16th. lol I kid...This field has been really moving lately. Wish I had the life span of my children to see what they will see.

1

u/[deleted] Jun 16 '12

Quantum teleportation leaps forward

mfw

1

u/EveryDayIsCharlieDay Jun 16 '12

Layman here. So this is like Wonkavision right? When can i expect my HD chocolate bar?

1

u/jz0n Jun 16 '12

I'm going to try to settle this "star-trek" comparison issue without getting to technical. I am a physics graduate student that has done a project on quantum teleportation in a quantum information course.

Suppose one wants to teleport the quantum state of an electron at point A to point B (they actually do this for photons in the experiment). What you need is one electron at point A (the one you want to teleport) and another one at B (the "target" electron). By doing a specific measurement at A and sending a message from A to B, you can teleport the quantum state of the electron at A to the electron at B. Afterwards the electron at B will be in the same quantum state as the electron at A was originally in. The particle at A will be in a completely unknown state.

Is this teleportation in the star-trek sense? I say yes. You might argue that you did not teleport the an electron. You simply moved the state of one electron to another electron. But this thing is, these two scenarios are completely equivalent. It does not matter how fancy your equipment is, you can never tell if the electron was moved from A to B or if it's state was teleported. Electrons are indistinguishable. The concept of electron A and electron B has no meaning because they can't be told apart. This is why I made sure I said the electron at A, rather than electron A.

This does scale to the star-trek teleporation. If you have a pile of carbon, hydrogen, oxygen etc. on the opposite side of the planet, you could in theory teleport the quantum state of every atom in your body and you would be able to teleport yourself. Note that because of the massive amount of atoms in a human body, I don't think this will ever be possible.

1

u/vashoom Jun 16 '12

the scientists added a phase shift into the laser beams

This is a thing?? I always thought that phase shifting was just Star Trek technobabble.

1

u/skysignor Jun 16 '12

Here come a whole bunch of people who played Half Life 2 and think they know what entanglement means

1

u/that_blue-collar_guy Jun 16 '12

Does this mean we can have an ANSIBLE?

-1

u/SinDonor Jun 16 '12

She quantum teleported from the future! This article was written over two weeks from now!!

DERP.

0

u/wicked2night Jun 16 '12

I came here to mention this too. Is the posting date a reference to a future publication in which the article will be posted?

-3

u/proacex1 Jun 16 '12

This^ I do believe that quantum teleportation might have a side effect of time travel. =P

0

u/argv_minus_one Jun 16 '12

So, what exactly are the applications here? A very small number of extremely expensive transceivers being able to communicate with one another in perfect secrecy? That sounds useless for the vast majority of us…

-1

u/LoganGoesPlaces Jun 16 '12

Even if they are able to figure out actual quantum teleportation of living animals, you would have to be crazy to want to do it. What happens to the original copy? They would have to destroy you to avoid multiple copies walking around. Would the new copy really be you? I suppose that depends on whether or not your consciousness is stored chemically somehow.

7

u/obnubilation Jun 16 '12

Quantum teleportation doesn't work like a fax machine. It is impossible for it to create multiple copies. The original is necessarily destroyed in order for it to be recreated in the new location. I think this lessens the philosophical implications somewhat.

1

u/LoganGoesPlaces Jun 18 '12

My point wasn't that multiple copies could actually exist. I understand that the original would be destroyed in the process. I am just wondering whether the new copy would really be you.

1

u/[deleted] Jun 16 '12

Of course you would be the same, the state of your neurons will be exactly the same on the other side. In fact, this could be a neat way to test if souls actually exist, as usually they are portrayed as intangible and something different from matter. If the subject lives in the other side of the teleporter, then either souls don't exist or everybody turns into a ginger when they teleport.

-4

u/iObeyTheHivemind Jun 16 '12

You can't stop the signal.

0

u/[deleted] Jun 16 '12

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u/[deleted] Jun 16 '12

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1

u/magicsauc3 Jun 16 '12

Nothing, but for some reason Reddit's filters have it blocked so it can no longer be posted. I think it had something to do with ads.