Quantum Key Exchange With an Airplane

submeta writes "Researchers in Munich have successfully performed a quantum key exchange between a moving aircraft and a ground station. Quantum key distribution, which exploits the phenomenon of entanglement, offers theoretically perfect encryption (although it can be vulnerable in practice). This advance is an important step on the way to key exchange with a satellite, which could enable practical usage of the technology."

This post is relevant to my interests

[AikonMGB]

Quantum key exchange via satellite: QEYSSat [qeyssat.com].

A little unclear on entanglement

[msobkow]

Doesn't entanglement also imply faster-than-light communications between the two quantum nodes? Or does the speed of light still apply to the entangled systems?

If the former, this would eliminate the lag for satellite communications, which would be a major breakthrough in global communications, even if the satellites could only link up to a few ground stations because of the limited number of entangled "transmitters" per satellite.

Re:A little unclear on entanglement

[Anonymous Coward]

It does imply faster than light propagation of states, but unfortunately it is not possible to use this effect to communicate information from one place to another. Observing the pair at either end breaks the entanglement immediately (faster than light), but there's no way to tell from one end if the entanglement has been broken, so you could imagine that the universe is talking to itself at faster than light speed, but you can't hear what it's saying.

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[Brad1138]

I love science and (when I have time) have kept a skeptical eye on quantum entanglement. I haven't studied it a lot, but have problems with a number of its claims. You are saying it works and is faster than light, as long as no one observes the outcome... Really? It sounds less legitimate every time I hear about it.

Re:A little unclear on entanglement

[Anonymous Coward]

The mechanism works regardless of if someone observes it or not. The faster than light aspect is just not observable by a person at one end, it can only be noticed when the two ends talk to each other. In other words, the faster than light aspects can not be used in communication, just shows up in patterns that are observed if you compare results from both ends that must be communicated by traditional means.

I haven't studied it a lot, but have problems with a number of its claims.

Considering how difficult the field of quantum mechanics is, and how many people fail to grasp basic principles even with extensive studying of pop-sci level material, maybe a little more humility would be called for until you have a chance to read more about it.

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[Anonymous Coward]

If my understanding of the Dunning-Krueger effect is correct, then humility is not one of the characteristics generally shown by the ignorant.

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[neonKow]

You are simply misunderstanding it. It's not that it works so long as no one is watching. It's that there's no way to harness it to have faster-than-light communication.

You can read entangled bits and they will have the same value on each end. HOWEVER, interacting with the bits in any way, including reading them, will break the entanglement, so you cannot flip a bit on one end and have it flip faster-than-light on the other end.

What you CAN do with this weird property is to use it for secret key exchange se

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[neonKow]

I made a mistake. This article might not have anything to do with quantum entanglement. Everything else applies about how entanglement is supposed to work and how it's not "faster than light."

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[DriedClexler]

I always have to wonder about interpretations of quantum mechanics that are able to believe those ideas at the same time:

"Yes, the information propagates faster than light, but in just the right way that we could never use it for communication" --> isn't that the same thing as "The information doesn't propagate faster than light"?

Re:A little unclear on entanglement

[Skillet5151]

No entanglement phenomenon has ever been shown (or really believed) to be capable of transmitting information faster than light. I promise you'll see it right there in the Slashdot headline if a decent experiment ever seems to show any kind of FTL information transfer.

Re:A little unclear on entanglement

[kasperd]

Doesn't entanglement also imply faster-than-light communications between the two quantum nodes?

No. It's true that you can have entangled particles far apart and measure the state of both of them at the same time. That does mean you instantaneously know what the other end would read. But that does not imply communication, since you don't have any control over what value comes out. And you can't even find out if the other end was measured or not. In such a situation it doesn't really matter which of the two particles is measured first. Measure particle one first, then the other collapses to a state consistent with the measurement of the first. Measure particle two first, then the first collapses to a state consistent with the measurement of the second. Which of the two happened depends on the viewpoint of the observer.

And actually quantum key exchange does not need entangled particles at all. There are certain optimizations, that could make use of entangled particles. But in plain quantum key exchange, you send a stream of independent particles where the sender knows the state of each particle he sends. The receiver doesn't know the state of the particles as they are received, but may learn something about the state, depending on how they are measured.

I have read of some implementations, that produce a particle in a known state by first producing an entangled pair of particles, and then measuring one of them. By the time the other particle leaves the sender it is no longer part of any entanglement. And the fact that it ever was part of an entanglement is just a minor implementation detail, that doesn't actually impact the protocol.

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[someone1234]

But, this is the same as having a sheet of paper with the same word written on it.

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[emt377]

But, this is the same as having a sheet of paper with the same word written on it.

That's what key exchange is all about.

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[sFurbo]

I think there exists two kinds of quantum cryptography. The one you describe is the one implemented now. It doesn't require entangled particles. IIRC, the other goes something like this:

1. Alice creates two sets of entangled particle, and sends one to Bob.
2. Alice encodes her message into a from that can interact with the entangled particle.
3. Alice lets her message interact with the entangled particles, creating the coded message. It is now no longer readable.
4. Alice sends the coded message to Bob. I

Re:

[kasperd]

I am not sure whether the coded message created in step 3. is entangled with Bobs particles, or whether step 3. breaks the entanglement.

It depends on how you do it. It could come out either way. However the only point in using the second method you describe is, that you can achieve entanglement at that point. I just don't know of any application of that, since usually the data you eventually want to transfer securely, are classical bits.

If you just want to protect classical bits, then the more complicate

Re:

[sFurbo]

I see. While writing my reply, I got the feeling that it had to be something like what you wrote, but I didn't really get a firm grasp of it until your reply. Thank you for that :-)

I think I have the method from some very popular scientific descriptions at least a decade ago, so they probably didn't go into too much detail about entangled data.

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[emt377]

And you can't even find out if the other end was measured or not. In such a situation it doesn't really matter which of the two particles is measured first.

Are you sure about this? Once collapsed the particle will no longer interfere with itself if you pass it through e.g. a double slit. And I always understood that entanglement meant the two particles shared a common wave function, so would collapse together - basically when you measure one the other will take one a single state as well. Otherwise you'd be claiming the entangled particle is not yet collapsed but you'd know what it will collapse to, which is an impossibility (if you know what it will collap

Re:

[kasperd]

Are you sure about this?

Yes.

Once collapsed the particle will no longer interfere with itself if you pass it through e.g. a double slit.

That phenomena is not dependent on entanglement. The self interference requires only a single particle. It doesn't matter if it is or ever was entangled with another particle. AFAIK the self interference requires the particle to be in a superposition of passing through both slits at the same time.

Entanglement is more complicated than just a particle being in a superpositi

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[Aphoxema]

The answer of if it's faster than light is "maybe", but it's reasonably certain that information can't be exchanged faster than light. All entanglement can be used for is to verify something that is already known and you only get on chance to test the condition. If you get it wrong, you waste the entanglement by adding new information to it. It's essentially a one-dial etch-a-sketch.

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[tehcyder]

It's essentially a one-dial etch-a-sketch.

Now THAT's what I call an analogy.

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[gweihir]

No. It could basically be simulated by both particles agreeing beforehand on a state and then reveal that simultaneously. Cannot be used for FTL communication.

There are some quantum-physical effects that indicate that this is not what happens and the two particles really sort-of synchronize in an FTL way in real-time, but my impression of the reasoning was that it requires a bit more assumptions than I am comfortable with, i.e. I think it may turn out to be wrong.

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[_8553454222834292266]

Wouldn't your first paragraph violate bell's theorem?

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[gweihir]

Possibly. I am merely talking about the use for key-exchange.

The thing is that quantum key-exchange is completely irrelevant and useless, as both particles need to be at the same point at some time in the past to get entangled. Instead you could just generate a key at that time and transport it to both locations. Security should be about as difficult, but transport will be far easier, as there is no need to keep particles in an entangled state. For the key-exchange scenario, it is also completely irrelevant

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[WaffleMonster]

With quantum entanglement you can securely transmit a one-time-pad (secure because you'll know if there is a man in the middle intercepting it), and you absolutely do not get more secure than using a one-time-pad.

Does entanglement include some kind of cosmic identity filter to validate who you are taking to? ...so really it is capable of detecting no such thing.

All this does is allow people who already have a preexisting trust relationship via classic means to accumulate OTP.

The problem with lack of effective binding between classic and quantum channel leaves me wondering what the practically useful aspects of the exercise is.

If something is that sensitive you can't use crypto then would not prearranging a few TB of

Re:

[gweihir]

You miss that you have to transport the entangled particles. That is about as insecure as transporting the pad in the first place. Maybe you are referring to some other form of "quantum crypto" that uses polarization, but not entanglement. In that system, single photons are sent. It also has been broken several times by now.

Re:A little unclear on entanglement

[camperdave]

No. It's kind of like flipping a coin. If you know one side is heads, you automatically know that the other side is tails no matter how far apart the sides of the coin are. Quantum Entangling photons is kind of like splitting the coin in half.

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[Kartu]

Nope: http://en.wikipedia.org/wiki/No-communication_theorem [wikipedia.org]

From this it is argued that, statistically, Bob cannot tell the difference between what Alice did and a random measurement (or whether she did anything at all).

I can't state I fully understand it though.

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[ikaruga]

I'm more curious to know how entanglement is even relevant to this study. I'm just an engineer, not a physicist, but from my understand of the article, they basically used an almost standard laser communication. However, in this case with two slightly different beams(different polarization) for transferring the encryption key. The reason they called this "quantum" was simply because they were reading the key signals by directly measuring the photon state due to polarization. That is important because, if so

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[__aaltlg1547]

The speed of light applies because the information travels with the entangled particles.

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[azav]

Yes, if you consider distance to be a factor. No, if you consider this two entangled systems to really be collapsed into one system but in >1 location.

I wonder if we will be able to have more than a pair of items entangled at one time?

Assuming we are only using binary systems, envision this:
Entangle several items, say 7, and add one more and then attach these to a reader the tabulates the results from the first 7 when the result of the fifth goes from 0 to 1. After tabulation, this can be passed on to an

I will believe it...

[kiriath]

When Sheldon is excited about it.

Re:

[Anonymous Coward]

This is a joke, right? Nobody here actually watches this show, right? If i'm wrong, man I need to stop reading all your guy's comments. Wow.

BB84 protocol does not use entanglement

[Anonymous Coward]

This experiment implemented the BB84 protocol with attenuated coherent laser pulses (with a view to use decoy-states to close the photon number splitting attack on attenuated coherent states). The ideal implementation would use single photons (which are highly non-classical states). The protocol does not utilize entanglement. The E91 protocol was the first QKD protocol to propose using entanglement for this purpose.

Solution without a problem

[Mathinker]

QKD is a solution to a non-problem. Even if the current public key algorithms would be broken (via a revolutionary advance in the the field of quantum computing, or in some other, yet unknown way), there exist backup algorithms for which there are no known quantum algorithms which break them.

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[fatphil]

It's also in some ways a non-solution to a non-problem, as it has the issue of being unable to distinguish evesdropping from noise. And as noise always happens, you can never be sure some of it isn't actually evesdropping.

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[Anonymous Coward]

The security assumption is that all noise is assumed to be due to eavesdropping. The noise level (e.g. quantum bit error rate) is used to determine the level of privacy amplification required to factor out any possible third party correlations with the final key. The tolerable error rate depends on the exact QKD protocol, e.g. for BB84 with perfect single photon sources it is at least 11% depending on the coding used in the privacy amplification step. Hence QKD in the presence of noise is feasible.