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."
"Scientists write fake paper for money + prestige" (Score:1, Interesting)
Chinese scientists merely wrote another fake paper, as is the common practice in Chinese academia:
http://www.chinadaily.com.cn/english/doc/2006-03/15/content_536821.htm
http://www.china.org.cn/china/life/2009-02/04/content_17222202.htm
http://search.yahoo.com/search;_ylt=A0oGkm5hk_VLFq4AU84qk6B4?p=as+china+academic+cheating&fr2=sb-top&fr=404_news&sao=0
Re:Wait, does this mean... (Score:2, Interesting)
Re:This would be interesting for production use... (Score:4, Interesting)
Re:Wait, does this mean... (Score:4, Interesting)
Re:Lightspeed limited, not an ansible (Score:4, Interesting)
It happens faster than the speed of light, but it isn't any use without extra information which can only be sent at light speed. You could use it to send secret messages since the state is instantly transferred and cannot be intercepted on the way and then the extra information can be used to get the data.
Re:Lightspeed limited, not an ansible (Score:1, Interesting)
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.)
Re:I don't get it (Score:1, Interesting)
Re:Wait, does this mean... (Score:1, Interesting)
Then can you please explain what TFA is all about?
In effect I hand you a state and you can perform a measurement on it. You will either measure a Zero or a One. You cannot predict which of the two you will measure. And you can perform the measurement only once. However I can prepare my side of the link such as to ensure that you will measure Zero with exactly 15.37586 percent probability. I can now write a learned paper in which I claim that 8 digits worth of "quantum information" have been teleported.
Note that this cannot be used to actually transmit any actual, real, information. Like a phone call or a TV signal or anything anybody actually cares about. Because it's about teleporting a quantum state.
People figured out that this is really uninteresting, boring, and there's no reason to fund it; so they call it "teleporting quantum information" instead, because that keeps the public (and hopefully the funding agencies) interested.
Re:Wait, does this mean... (Score:2, Interesting)
You do all realize that its not actually 'moving' the photon? Two photons are entangled then a change is mimicked at the other side, thus 'information' has been teleported. All this 'speed of light' talk is irrelevant
Re:Wait, does this mean... (Score:3, Interesting)
Information is strongly related to energy (perhaps it would be better to say information is useful energy) and energy and mass look the same if you squint at them.
It's a bit backwards to say that "information can't travel faster than the speed of light", however, as light moves at varying speeds (yes, even in a "vacuum"). It's much better to say "light in a vacuum travels very nearly at the maximum speed of information". It's the speed of infomation that's the primitive here, light is just bounded by it, so using something other than light doesn't help any.
There was a Slashdot article a while back about "teleporting energy", which was really just transmitting information about a system that allowed you to extract energy from it (when in a closed system it would take more energy to get that information than you got out of the system). But, again, that's probably a backwards way of looking at it - energy that's usable in an engineering sense is the direct consequence of information.
Re:Wait, does this mean... (Score:4, Interesting)
Clearly light and time are directly related phenomena. I don't think there's a third phenomenon ("speed of information") capping them both. After all, objects are free to move faster than light in relation to one another. The headlights and tail-lights of your car emit photons at c in opposite directions. The speed difference between them is clearly some approximation of 2c. To suggest otherwise would imply that they are somehow physically linked, which they are not. But your measurements are bounded by time, putting an effective cap on the speed you can observe.
Thats not how relativity actually works. Two objects cannot, in fact move apart from each other faster than the speed of light.
A C
If B sees A and C each moving away from it at nearly the speed of light, that just means that A sees C moving away at even-more-nearly the speed of light. Funky, eh?
Light moves at a speed limited by the impedance of the material it is travelling through. A vacuum is nearly the lowest impedance possible (but you can go slightly lower), but is nowhere near 0. Why does space have such a high impedance that space travel is impractical? The information speed limit would be my guess.
Re:This would be interesting for production use... (Score:5, Interesting)
It works like this. You put a red and a blue shirt in a bag. You and Alice close your eyes. You each take out a shirt and put it in a briefcase. Then you both go on a trip.
When you get to the hotel, you open the briefcase and you have a red shirt. You know Alice's shirt is blue. The next question is, so what?
As you can see from the example, you essentially pre-loaded the answer before you went on the trip. It's not real-time communication when you hand somebody a sealed envelope and walk away.
superluminal communication problems and ??? (Score:3, Interesting)
Measuring at different times doesn't appear to matter (See Wheeler's Delayed Choice experiments). Which is very amazing in itself and an entirely different topic of discussion. The problem is that however you set up your experiment, no practical information is exchanged FTL. Alice could measure the entangled pair at the same interval as Bob, but that doesn't really tell her anything since Bob can't actually cause his entangled particle to have a particular spin, polarization, or whatever they're measuring. It's only interesting after the fact when they compare notes.
So you say well then, instead of using the particles let's use the act of measuring or not to transmit info. If Bob measures his particle he's sending a 0, if he doesn't he's sending a 1. And Alice will see this reflected at her end somehow. But the problem with this, from my understanding, is that everything is going to look random to Alice however she chooses to measure it (or however they agree to ahead of time). Because remember you are looking at individual particles. Again, it's only interesting after the fact when they compare notes.
Now the question I am not sure the answer to, is if they were to use a group of photons and either measuring or not measuring the group as a whole. For example, if you think of the classic double slit experiment, doing something to an entangled set of photons to cause their distant pairs to either form a wave-pattern or a blob on a detector. I don't know if this is possible or not, and it sounds like there might actually be some serious debate about this (see Dopfer experiment)
Re:Lightspeed limited, not an ansible (Score:5, Interesting)
Also, please do not just say "Your wrong, GTR says that can't happen", you would be "citing authority" and it really kills the validity of your rebuttal. Sort of like saying "God exists because the Bible says so". Please explain WHY its wrong, as in cite what portion of GTR says it can't happen so I can read it and see where how I went wrong.
According to General Theory or Relativity, as defined in the link you posted, if a mass were to suddenly appear at a location in space-time, say in the forward Lagrange point of Jupiter's orbit, it would take X amount of time before the gravity from that mass would affect the orbits of the other planets in the Solar system. X being equal to time it would take for light to travel from the location of the mass to the rest of the planets in the Solar system.
Have I got it right so far?
But my understanding is that, according to GTR, gravity is caused by the deformation of space-time by a mass. So the mass that suddenly appeared would deform space-time around it, thus imposing a gravitational influence on all objects in range.
Here is what has me going "wait, what?"
Also according to GTR space-time can expand/contract at speeds greater than that of c in a vacuum, as described in the "inflation" theory [wikipedia.org] of the early universe and Alcubierre's [daviddarling.info] "warp drive" theory. Since the mass deforms space by "stretching" it wouldn't that mean that the influence of a mass could affect an object at a distance in less time than it would take light to travel that same distance? Since the "fabric" of space-time could alter faster than light can travel across it.
I'm hoping to get some insight into how I could be wrong, because based on what I know I can't see any reason why it can't happen. It could explain why we haven't detected gravity waves using interferometry, if the gravity wave, a distortion of space-time was moving faster than light it wouldn't be able to affect the phase of the light beams.
Thank you in advance to those who actually provide some useful info to help me improve my understanding.
Re:Lightspeed limited, not an ansible (Score:2, Interesting)
Re:This would be interesting for production use... (Score:5, Interesting)
You and Alice put two shirts in a bag, shake it up, close your eyes, and you each pull out a magic mixed-up shirt which cycles through the color spectrum at random varying speeds (but the same speed on each shirt) until you look at it, at which point it stops cycling on one particular color, and the other stops cycling on the complementary color. You put your shirts in your respective briefcases and go on your trips, and when you get there, you open your briefcase and see your shirt has stopped on red. So now you know that if Alice looks in her briefcase, she will see her shirt has stopped on cyan.
However, the question is again, "so what?"
You don't get to decide whether the shirt is red or blue when you look at it (since the speed it cycles at varies randomly, so you can't very well time it or something), so it's not like you can send a "cyan" to Alice for a "0" and a "red" for a "1". Likewise, when Alice opens her briefcase and sees a cyan shirt, she doesn't even know if you have looked at your shirt or not yet; her shirt might have stopped flashing and just landed on "cyan" by chance when she looked at it (making your shirt stop at "red"), or you may have looked at your shirt and seen "red", making her shirt stop right then too on "cyan".
The only thing that's interesting about these synchronized flashing shirts is the fact that when one stops cycling the other stops at EXACTLY the same time no matter how far away they are. We only know this because when you and Alice do this over and over again and then compare your notes afterward, you always find out that your shirt stopped on one color and hers on the complement. That's interesting because if there was any time delay between one stopping and the other, you would expect the hue-difference between the two shirts to vary with distance: at close distances you'd get close to complimentary colors because they stop at close to the same time, while at larger distances the second shirt would stop slightly later making it slightly off from complementary. And of course if there was no communication between them at all, there would be no correlation between what color you see and what color she sees. But you always see red when Alice sees cyan, and you always see yellow when she sees blue, and you always see green when she sees magenta. Which indicates that anybody looking at either shirt not only stops that shirt but also the other shirt instantaneously.
Which isn't of any practical utility, however, for the reasons described two paragraphs above. But it sure as hell is weird, isn't it?
Re:This would be interesting for production use... (Score:4, Interesting)
The only thing that's interesting about these synchronized flashing shirts is the fact that when one stops cycling the other stops at EXACTLY the same time no matter how far away they are.
In the context of special relativity, what does it mean for two things to happen at EXACTLY the same time?
Re:superluminal communication problems and ??? (Score:3, Interesting)
In retrospect, I should have said measure or not measure in a particular way. So Bob is always measuring groups of photons, and Alice continually shifts the way she measures groups to send a message. You are correct that whoever measures thus ends the entanglement UNLESS they do it in a way that doesn't allow them to get any information. Take for example a variation on the quantum eraser experiment (I chose this one because it has a very intuitive diagram and IMO is a fascinating experiment):
http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser [wikipedia.org]
A person can choose whether to measure the particles in a way that preserves the path information, or one that doesn't. This is where the spooky effect comes in, because if you measure in a way that preserves the which-way path information, the interference disappears, but if you make it such that it's impossible to tell, interference takes place. In that particular case though, without the coincidence counter you can't see anything other than random noise. It's only after the fact when you compare results that it shows up. And needing a coincidence counter is unfortunately part of the delicate nature of these experiments. Also note again this is groups of photons... even though a single photon may be part of an interference pattern, you can't see that interference until you look at a group of them (it just shows up as a random dot until you build up enough of them).
My understanding of the idea behind the Zeilinger/Dopfer proposal is that by shifting the way they measure, they might be able to eliminate the need for a coincidence counter to be able to directly observe an interference pattern or not, indicating how it was being measured at the other end. Which opens up a possibility (albeit remote) for FTL communication.