Quantum Teleportation Achieved Over 16 km In China 389
Laxori666 writes "Scientists in China have succeeded in teleporting information between photons farther than ever before. They transported quantum information over a free space distance of 16 km (10 miles), much farther than the few hundred meters previously achieved, which brings us closer to transmitting information over long distances without the need for a traditional signal."
Re:Lightspeed limited, not an ansible (Score:2, Informative)
ansible [wikipedia.org]
Next time, define the terms yourself, you insensitive clod.
Re:Wait, does this mean... (Score:3, Informative)
No, even quantum entanglement does not transmit information faster than the speed of light.
Contradictory (Score:3, Informative)
Re:I don't get it (Score:1, Informative)
Yes, whoever wrote the summary has no idea what s/he is talking about. Entanglement transmitted information would violate relativity, since you could send a signal that effectively traveled faster than the speed of light.
Info can't propagate faster than speed of light. (Score:5, Informative)
Don't feel bad, this is a pretty common mistake. People read about non-locality and how what happens to one half of an entangled pair affects the other half instantly no matter how far away it is. There does remain some philosophical debate over what entanglement and non-locality really are, but one thing has been supported very well by both theory and experiment: You can't transmit information or power faster than c. In the case of entangled pairs, actions on one half can have a non-local effect that propagates faster than c, but it's not possible to transmit information or power using that effect. In order to make sense of the results and actually observe the effects of non-locality, you typically need to send additional information classically.
So, this will not lead to lag-less communication over vast distances. What it will lead to is quantum crypto networks. Long distance entanglement swapping or quantum teleportation are one of the key ingredients to building a scalable network.
Re:I don't get it (Score:5, Informative)
- Charlie entangles Particles A+B
- Charlie sends Alice Particle A over fiber
- Charlie sends Bob Particle B over air
- Alice measures A and sends Charlie information about measurement (classic part needed for actual information transfer)
- Charlie sends classic information to Bob
- Bob measures Particle B, combines result with classic information, and voila, Bob can reconstruct the information "sent" by Alice
Clearly no way to transfer information securely or fast, but a proof that entanglement in Particle B for Bob can survive long transfer through air.
Have they now... (Score:2, Informative)
"Scientists in China".
Think I'll be waiting for independent verification of this one then...
Re:Philotics (Score:3, Informative)
That's because in China they just make up results like this to please political bosses.
Re:Wait, does this mean... (Score:5, Informative)
Re:Wait, does this mean... (Score:4, Informative)
Light can travel at the speed of light, things with matter can't.
Re:I don't get it (Score:3, Informative)
As far as I know (very little, please correct me if I'm wrong), you can't neither predict nor influence the outcome of measurements, but you can be sure they will be the same at both ends, unless someone is eavesdropping in the middle. The flow of measures can then be used as a one time pad to encrypt something at one end, transmit it over a conventional channel, and decrypt it at the other end.
Re:Contradictory (Score:2, Informative)
Except each entangled pair is one-use only. You measure the state of one half of it, which instantly sets the same state in the other pair, and then they're no longer entangled (due to you having observed).
Also you can't predict or set the state of half an entangled pair, only measure it, causing the waveform to randomly collapse. The only thing this gets you is secure transmission of a random sequence (many entangled pairs) of states, which you can then use as a one-time pad/key for conventional encryption over a conventional link. If anyone tried to eavesdrop in the middle they interfere in a measurable manner.
Re:Peer Reviewed (Score:3, Informative)
Re:Progress.. (Score:5, Informative)
What a bunch of BS. China has about 300 million "regular" people, that is, decent incomes and they shop for food at grocery stores. China has ONE BILLION desperately poor peasants and workers, whose lives are not getting better at all. "Eating bitterness" is an idiom that they use to describe their lives. They are as docile as cattle. They won't be clamoring for freedom anytime soon.
Oh, and Newsweek is a discredited, partisan source. Didn't anyone get the memo?
Re:Lightspeed limited, not an ansible (Score:4, Informative)
Yes, information can only be transmitted at light speed. (Except [gravity] information [..])
No, that would break the universe. Gravity is also limited by by c. http://en.wikipedia.org/wiki/Speed_of_gravity [wikipedia.org] says: The speed of gravity in general theory of relativity is equal to the speed of light in vacuum, c.
Re:Wait, does this mean... (Score:3, Informative)
If your statement is true, then I'm back to square one on understanding this "entanglement" thingie.
As far as I know, the problem comes down to measurement. The basic idea is that you can entangle two photons (put them both into superposition states) and then move them far apart from each other. At this point you have two photons in an "unknown" state. If you measure one of the photons the superposition will collapse and the other entangled photon will instantly move out of superposition and into the alternate absolute state. This change is instant and does actually "travel" faster than light.
The problem is that you cannot use this mechanism to actually transmit information faster than light because you need some other kind of means to know when to observe your entangled photon. If Bob and Alice have entangled photons, Alice has no way of knowing if Bob has sent her a message using her photon because if she checks it to see if the superposition has collapsed then she will herself cause it to collapse if it hasn't already done so (thereby preventing Bob from sending a message at all).
This means that you're left using some alternative means of communication (radio, etc) which itself is limited by the speed of light. Bob will collapse his photon, send a message to Alice via normal means, at which time she can measure her photon and see the result of Bob's actions on his photon.
At least, that's the way I understand it. No, my uid is not three digits; no, I don't have a degree in Physics; and no, the rest of your post doesn't make any sense at all. Anyone can feel free to correct me.
Re:Quantum teleportation doesn't transmit informat (Score:2, Informative)
Could you elaborate?
In that entanglement is the very basis of quantum communication, I'd say it has a fair bit to do with it.
Re:Lightspeed limited, not an ansible (Score:5, Informative)
Yes, information can only be transmitted at light speed. (Except information pertaining to gravitational fields, which must be transmitted instantly over vast distances in order for planets and moons to stay within stable orbits. Run the numbers for yourself -- see if you can get the planets to stay in orbit when the force points towards where the *current* light-speed gravitational waves say the massive object is.)
Who modded this "interesting"? It is nonsense. The use of the term "force" in the context of gravity indicates that the poster is is talking about classical, Newtonian gravity. And there is no speed-of-light-limit in Newtonian gravity. Neither is there anywhere else in Newtonian mechanics.
You want to do gravity relavtivistically (i.e. correctly, in agreement with actual, modern-level observations) you'll have to use general relativity. Which just so happens to work just fine. You'll find that there's no "force" (or other absolute vector) in there at all. The whole thing is essentially geometry-free, only the differential of any vector ever plays a role. As it should be, in a properly relativistic physics.
Re:Peer Reviewed (Score:5, Informative)
The work was done by Jian-Wei Pan, one of the leaders in the field and a very impressive researcher. You can bet that the result is accurate if his name is behind it. Furthermore, it's being published in Nature Photonics. Besides, the result is impressive, but not ground breaking. Extending the distance of the protocol requires some fancy techniques and a good deal of skill and expertise, but the results aren't surprising.
Re:Philotics (Score:5, Informative)
Half of it nobody understands anyway.
"I think I can safely say that nobody understands quantum mechanics." --Richard Feynman
Re:Peer Reviewed (Score:2, Informative)
however I will be applauding their work with less skepticism when I hear that MIT, Cornell, CMU, etc confirm the results.
Mod the ignorant parent down. If you read TFA, you'll notice that it references the paper as published in the scientific peer-reviewed magazine Nature (once again, the magazine is peer-reviewed and thus only peer-reviewed works are published in it).
http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2010.87.html [nature.com]
Re:Wait, does this mean... (Score:5, Informative)
How did this get moderated up? This poster clearly has no idea what he's talking about.
The whole point of quantum entanglement is that prior to the measurement, there's no basis in which the state is definite. This means it's not just that "you cannot predict which of the two [states] you will measure"; the whole point is that there is no defined classical state the system is in. There's no classical analog for that, so it's really hard (maybe impossible?) to explain without math.
If you don't even know the most basic stuff about quantum mechanics (as is clear from the post), please educate yourself before writing about it or even moderating stuff about it.
Re:Lightspeed limited, not an ansible (Score:2, Informative)
In case it would be possible, I think the effect would be like throwing a big rock in a pond. There would be an abrupt change in the space-time continuum, which would cause gravity waves to ripple out from that place at the speed of light. Since the presence of that new mass is information, the change in space-time will have to travel at light speed or less, or it would violate causality (special relativity).
Re:Lightspeed limited, not an ansible (Score:1, Informative)
It wasn't even invented by Card but by Le Guin. It's a well established term used by multiple authors by now.
Re:Wait, does this mean... (Score:3, Informative)
the whole point is that there is no defined classical state the system is in.
Are you sure [stanford.edu] about that [rutgers.edu]?
Re:Lightspeed limited, not an ansible (Score:1, Informative)
I am a physics student. While I have not been studying general relativity yet, I can say
that inflation is a universal thing going on all the time, and for spacetime to expand faster than light,
it has to be very very far away from you (at least now). This distance is called the horizon distance.
Imagine the universe as a baloon being inflated. If you see two points on the surface
close to each other, they don't move away from each other very fast, but if you see one
father away, the points will move faster away from each other.
Putting a mass in, say, in the forward Lagrange point of Jupiter's orbit, is a local thing
and to a very good approximation no inflation is going on in our solar system, or in our galaxy for the matter.
If things move away from you faster than c, it cannot influence you in any way, cause /horizon/ will never get to you because space keeps
the gravitational waves etc etc. moves at or below speed c. It's as if a photon traveling
towards you from beyond this
filling up between you and the photon at a faster rate than the photon can "eat" that extra space.
On an interesting sidenote, any photon traveling in the universe
will actually loose its energy because it has a wavelength. The wavelength gets larger
as the universe expands, and so the frequency, which is proportional to its energy,
goes down.
Re:Wait, does this mean... (Score:4, Informative)
Okay, I'll bite.
So just because you measure the speed between them as c doesn't mean they are each moving at half-c. They are still both moving at c, in opposite directions, for an effective 2c with regards to their eventual position.
No. Your conclusions stem from a fundamental misunderstanding of relativity. It makes no sense to talk about "eventual position" in the way you are, because it requires talking about an absolute time. There is no absolute time. You may have heard this sentence being thrown around before in special relativity, but perhaps you haven't appreciated the full meaning of it.
Let's talk about "eventual position". What you're saying is, we measure the positions of A and C, then wait some time t, then measure their positions again, and, lo and behold, if we divide the distance travelled by the time taken we are left with the impression that A and C are moving apart at 2c. This is true if you measure t and the distance in B's reference frame, but not from A's or C's reference frames, even though these are equally valid.
Once again, there is never one way of looking at things that is just a little bit "truer" than the others, even if your intuition may tell you that, since B's reference frame is at rest, it should provide a less distorted and more objective measurement than A's/C's. Truth is, you could look at the same problem in a different way, where A is at rest. Then B is moving away from it at nearly-the-speed-of-light, and C is moving away at even-more-nearly-the-speed-of-light, at a speed defined by the equation on this page [ucr.edu].
We have no definition of which of the above observations is the "correct" way of looking at things, because they are physically indistinguishable from each other. They are, in fact, the same thing; different realities exist for different observers, which is why the name "relativity" is so fitting.
Here's a better example. The furthest objects in the universe are about 13b light-years away. The light they emitted 13b years ago is getting to us now. Do you think, in the past 13b years, that they haven't moved any further??
Sure, 13b light-years away must mean that a photon arriving on earth right now must have been emitted 13b years ago, right? From our perspective it does. From the photon's perspective, it made the journey in less than the blink of an eye. Does this mean the photon travelled many multiples of the speed of light to get here? No, it just shows, once again, that different realities exist for different observers.
Re:Lightspeed limited, not an ansible (Score:3, Informative)
If gravitational waves could go faster than the speed of light, that would break causality. This means that you could find some reference frame moving at a constant velocity (special relativity) or constant acceleration (general relativity) from/to the source of the gravitational wave, for which you would first detect the gravitational wave, and only later see the event that generated it. Which basically reverses the flow of time. Relativity forbids this (see here for SR: http://en.wikipedia.org/wiki/Relativity_of_simultaneity [wikipedia.org]).
Stretching of space-time (metric expansion of space) is a non-local phenomenon, meaning it falls outside of the scope of SR, but in the domain of GR. It's a very very small effect, that can only be seen at galactic scales. It means space-time is created in between two connected points of space-time, which is not what is happening in your case. Even in expanding space, no signal goes faster than light, and causality is preserved. The light itself keeps moving at c, it only undergoes a red-shift because the space it travels through stretches.
So basically we just have a bunch of theories that tell us how the universe works, and those theories seem to hold up during experiments. They don't tell us why there is an upper speed limit, only that because the speed of light is constant and limited, no information can move faster, or causality would break, and the universe would be an even stranger place.
To really know why this is so, and what exactly causes metric expansion of space, we need to find a working model of quantum gravity. GR doesn't seem to work very well at quantum scales. Several candidates exist, but they don't produce enough predictions to allow for conclusive testing. There are indications that the continuous space-time breaks down into a fractal pattern of small units of space-time (strings, loops, pentachoron depending on the theory) that form ever changing interconnections, a bit like water molecules in a drop moving around without the overall shape changing, but this in 4 or more dimensions. Since this all occurs at the Planck scale (about 10^20 times smaller that the diameter of a proton), and basically is the foundation of all space-time and thus reality, that makes it very hard to perform experiments that tell us anything more about it.
On the quantum level we have the same problem: we have complicated field theories (quantum chromodynamics) that tell us how particles interact, but they don't tell us why they do so, or why they even exist with the mass/charge/color they have.
One day we might find some unified theory that will answer all this, and from which everything will emerge naturally, but until then we'll have to do with what seems to work (SR/Newton for normal scales, GR for galactic scales and large masses, QM/QCD for quantum scales).
Re:Wait, does this mean... (Score:3, Informative)
No, what you say is the current theory. The experimental results are precisely what the grandparent poster described: that you cannot predict which of the two states you will measure.
The experimental results agree completely with the current theory: you cannot predict which of the two states you will get from the measurement. If that was all the grandparent had said, I wouldn't had bothered answering.
The problem is, he seems to think the entangled state is such that you can fix probabilities in one end with a measurement, and then the measurement in the other end would depend on these probabilities. This is complete bunk, regardless what interpretation you subscribe to. No one serious ever believed that, and no one would publish a paper saying they transmitted information that way. (Well, maybe a crackpot.)
And we know it since Goedel that all interesting, non-trivial frameworks of formal logic have an infinite number of questions to which we'll never know the answer (within that framework). The quantum state of particles may be one such phenomenon in our universe.
There's nothing special in QM to suggest that (any more than any other physical theory). And there are no serious doubts that the mathematical formalism of quantum entanglement I mentioned is complete and self-consistent -- there are serious disagreements regarding its interpretation, not the formalism itself. And no one ever suggested that the equivalent of a "Godel sentence" that's undecidable inside it exists: the formalism doesn't have enough in it to do arbitrary arithmetic.
Re:Lightspeed limited, not an ansible (Score:2, Informative)
If gravitational waves moved at instant speed, LIGO (our current detector) would indeed not be able to detect it, since it would instantaneously compress or expand space everywhere. This would also cause big problems. Energy would radiate out of the observable universe faster than the observable boundary expands (at light speed). Meaning conservation of energy would be violated big time. The first law is a cornerstone of physics, and has never been known to be violated. In cases where it seemed to be broken in some past experiments, some interesting phenomena were discovered that explained why it actually wasn't.
Also, I'm not at all certain that causality wouldn't be violated in this case. There might be some other way to detect a passing gravity wave (besides laser interferometry) that we just don't know about yet. Maybe some change in the rate of collisions between a particle beam and virtual particles from the fluctuating zero point energy. It wouldn't even have to be technically possible to measure it, so long as the effect would be physically real.
There can be several reasons why we didn't detect any yet. For one, gravitational effects are very weak and our sensors have limited sensitivity. Combined with this is the fact that gravitational radiation follows an inverse square law (its amplitude is inversely proportional to the square of the distance from the source). This means that only powerful sources like collapsing or colliding stars, or closely orbiting black holes or neutron stars can be detected if they're sufficiently close to us. The chances of measuring such an event with the current LIGO installation were estimated as only 1 in 6 by 2010, so it's quite possible such an event just didn't happen yet. LIGO2 will be 10 times more sensitive, and is expected to detect multiple events weekly. But we'll have to wait until 2014 for it to become operational.
It may turn out we don't detect anything, which may mean our detectors don't work, or our theories are wrong. The latter would actually be a very interesting result, since it would provide new insight into gravitation (whereas detection would just reaffirm our current theories). I'm still convinced that we will detect the waves eventually, and that they will be moving at light speed as GR predicts.
Regards.