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Transportation Science

Scientists Teleport Information Between Ions a Meter Apart 220

Posted by Soulskill
from the perfecting-those-heisenberg-compensators dept.
erickhill writes with word that scientists from the University of Maryland have successfully transferred information from one charged atom to another without having it cross the intervening space of about one meter. The academic paper is available in the journal Science, though it requires a subscription to see more than the abstract. Scientists have previously teleported unmolested qubits between photons of light, and between photons and clouds of atoms. But researchers have long sought to teleport qubits between distant atoms. Light's high speed of travel makes photons good transporters of information, but for storing quantum information, atoms are a much better choice because they're easier to hold on to. 'This is a big deal,' comments Myungshik Kim, a quantum physicist at Queen's University Belfast in the United Kingdom. 'To store information as it is in quantum form, you have to have a teleportation scheme available between two stationary qubits. Then you can store them and manipulate them later on.'"
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Scientists Teleport Information Between Ions a Meter Apart

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  • by plnix0 (807376) on Friday January 23, 2009 @08:30PM (#26584207) Homepage
    Right. The abstract:

    Quantum teleportation is the faithful transfer of quantum states between systems, relying on the prior establishment of entanglement and using only classical communication during the transmission. We report teleportation of quantum information between atomic quantum memories separated by about 1 meter. A quantum bit stored in a single trapped ytterbium ion (Yb+) is teleported to a second Yb+ atom with an average fidelity of 90% over a replete set of states. The teleportation protocol is based on the heralded entanglement of the atoms through interference and detection of photons emitted from each atom and guided through optical fibers. This scheme may be used for scalable quantum computation and quantum communication.

    So yes, this is not true "teleportation". It relies on light actually moving from one atom to another through optical fibers.

  • by sarkeizen (106737) on Friday January 23, 2009 @08:37PM (#26584273) Journal

    "teleportation" always seems to lead people to the wrong conclusions. This is about transferring the informational content of a qubit. Which you can't perfectly represent with a classical system. I can see how this as the one commenting physicist claims is a "big deal" when it comes to building quantum computers. But it's not about instantaneous matter transport or superluminal communication.

    I'm not sure what the article meant by ultra secure "quantum communication". Quantum teleportation *is* a quantum communication *channel* but it's unclear what kind of security they are talking about. Perhaps "Quantum Encryption" but that's another term that often sends people down the wrong track.

  • by Anonymous Coward on Friday January 23, 2009 @08:49PM (#26584383)
    If you send a single particle through a slit, you'll get a single spot. If you send many particles through slits, you'll get many spots, just as if you hadn't used entanglement -- they'll be all over the place. Either way, you won't know whether the wavefunction was collapsed by your observation or prior to it by the collapse of an entangled particle's waveform.

    Say particles A and B are entangled, and you are in a position to observe B, but not A. You have no way to know whether A has already been observed, because B will look the same to you either way, unless you already know the state of A.

  • by v1 (525388) on Friday January 23, 2009 @08:56PM (#26584431) Homepage Journal

    you have both a black and red marble and you send one around the world, well when one guy checks and sees that his marble is red, the other guy instantly knows that his marble is black.

    More to the point, the other guy can find out his marble is black, but only if you communicate to him that your marble was red. Thus information was transferred, but you have to communicate by other means to make it meaningful, which defeats the purpose. It's like sending someone an encrypted message over an insecure channel. Great until you realize you now have to send him the key over the same channel. Sure it's encrypted, but the means of making it useful renders it ineffective.

  • by MoellerPlesset2 (1419023) on Friday January 23, 2009 @09:02PM (#26584465)
    Okay, can you clarify for me why exactly you can't? Is it because you can't actually control what state the measured atom, and thus the distant atom, will take?

    Sure, I'll try: A quantum 'entangled' state means that two systems are in an 'undefined' state in the quantum sense, that are interdependent.
    When one is measured, the other one will _instantanously_ adopt whatever state is 'required' to complement the other one. So one 'knows' instantly what the other is doing, so to speak. Which means a sort of information has been transferred at FTL speed.

    The reason why this can't actually be used for communication is twofold: One is exactly as you said: Because you can't know which state you'll measure, you can't transfer information through that alone. The second reason is that, an entanglement between two systems occurs only if there's an (unmeasured) interaction between them.

    That means you either separate the two systems from each other (as in the classic example of entangled photons moving apart), or as in this case, by letting them interact with photons - that travel at light speed. Either way though, light speed is the best you can do.
  • by 93 Escort Wagon (326346) on Friday January 23, 2009 @09:02PM (#26584469)

    Any use of the word "instantly" is, quite simply, hype. It's instantaneous like an "instant message" is instantaneous. Not that this isn't a cool discovery; it is. But it's not teleportation and it's not instant communication.

    No, "spooky action at a distance" is indeed instantaneous. It's a quantum phenomena - it's not based on the information being transmitted (which would indeed be limited to the speed of light).

  • Mod patent up. (Score:3, Informative)

    by Hurricane78 (562437) <deleted@@@slashdot...org> on Friday January 23, 2009 @09:06PM (#26584485)

    Yeah. I will try to give a simplified explanation to non-experts (I'm just a curious guy myself):

    First you entangle two particles. Then you let one travel somewhere. (If at bumps into another particle on that way, the particle loses the entanglement.)
    Now if you "measure" the first particle, the "wavefunction" (the entanglement) of both particles collapses in a specific way.

    By measuring that traveled particle, you can get the information on how the other particle got manipulated when it lost the entanglement.
    The nice thing about this is, that it is instantly. There is no measurable delay.

    So you could theoretically entangle a ton of material with another ton of material, and then send the first ton up to some remote planet. (Which of course would take very long. But you could send it at very high speeds which no human could survive too. For example by using a rocket that uses nuclear explosions as propulsion.)
    Say you have defined, that you can use 0.5 kg of material every year for each side, and split the ton in such "blocks". Then you just write the outgoing 0.5 kg block (you collapse the entanglement) over the year, and read the incoming 0.5 kg block at the end of every year. By using a special encoding, you can detect where the data ends, and where the data collapsed trough your measurement. Or you just pipeline the to-be-written data on both sides, and read at the end of every month, week, day, hour, minute, second... whatever is most reasonable. (Making it a buffered transfer of blocks.)

    This would give you a thousand years of infinite-speed (depending on your read rate) communication with the bandwidth of 0.5 kg of material per year (~1,37 g per day). (The amount of bits depends on the material.)

  • by blueg3 (192743) on Friday January 23, 2009 @09:07PM (#26584493)

    You most certainly can measure the propagation time of light over distances of one meter. It takes on the order of 10^-8 seconds for light to travel 1 m, and we have time measurement devices better than ns. (Actually, using clever techniques, you can do way better than meters.)

  • by disputationist (1324927) on Friday January 23, 2009 @09:19PM (#26584569)
    They know it was entangled because they prepared the state way. For example, if you have a spin zero particle that splits up into two particles, and you measure one as spin up, the other must be necessarily spin down, no matter how far away it is, because of the conservation of angular momentum. Or you can think of a neutral particle splitting into positive and negative ones. So I guess it is ultimately the consequence of some conservation law.
  • by mhall119 (1035984) on Friday January 23, 2009 @09:35PM (#26584653) Homepage Journal

    You can't determine if a particle is in a super-position or not, because any measurement of it will instantly collapse the waveform on both particles, and if you collapse yours first you will be unable to receive the information being transmitted by the other. You will need to know that the other entangled particle has already been collapsed, before you read yours, and that information still has to get to you by a conventional method.

  • Re:Mod patent up. (Score:5, Informative)

    by EdZ (755139) on Friday January 23, 2009 @09:36PM (#26584659)
    Oh, if it were that easy. When you collapse the wave function my measuring one 'end' of your hypothetical particle-block, you: have NO WAY of influencing HOW it collapses, and thus cannot send any information to the other 'end'. You cannot determine what spin you will observe, only that the opposite spin will be observed on the other particle.
  • by blueg3 (192743) on Friday January 23, 2009 @09:41PM (#26584695)

    No, it doesn't. View it from the perspective of the two measuring parties. We'll call them Abe and Bob.

    Each particle has a 50% chance of being in one of two states, + or -. Entanglement means that if Abe's particle is +, Bob's is -, and vice versa.

    Abe measures his particle. Regardless of if his particle is + or -, that doesn't tell him if Bob measured his particle or not. While the values of the measurements are dependent on one another, without information from the other measuring party, the measurer can't tell the difference between the entangled and collapsed states.

  • by getuid() (1305889) on Friday January 23, 2009 @10:34PM (#26585059) Homepage

    Is there any way to know that measurement has taken place at the other end and your local qubit has collapsed?

    Crash course in quantum mechanics, perhaps this explains it: a binary quantum mechanical system is in a linear superposition of states A and B. That is, it is either 100% A, or 100% B, or anything in between; for example 70% A and 30% B.

    Now if you measure, you would only get "pure" results, i.e. purely A or purely B. If the system was pure (i.e. 100% B) before the measurement, you get what it was. If the system was mixed (say, 70-30), and you had the chance to measure the system more than once, then you get A in 70% of the cases, or B in 30%. For example: make 1000 copies of the system, and measure each of them. Roughly 700 (give/take a few) would be A, roughly 300 would be B.

    The biggest problem is that you don't have 1000 exact copies -- unlike with classical information, basic QM forbids cloning of a system. So you basically have one shot, and if you happen to measure B, you'll never know whether it was because of a 100% pure B state, or simply because you "got lucky".

    I mean, I know the answer is you can't communicate instantly, I'm just figuring out why (mostly to help explain to people with roughly my same layman's understanding of physics why instant communication is impossible).

    While the "quantum information" is being transfered instantaneously, the problem is that the quantum state is not transfered 1:1 onto the target. It is ... "twisted". Imagine that like x*A+y*B (-> teleport ->) y*A+x*B. Now you know that the numbers x and y mean the same in both systems -- you just don't know exactly how they would be twisted after the teleportation. There are 4 possibilities how they can be twisted, and all 4 are equally probable, there's nothing you can do to favor the one over the other.

    However, after the teleportation, the guy at the source can tell how they have been twisted (because the teleportation act itself is a measurement, which's result tells him exactly what happened), but the guy at the target does not.

    So at first, even if the guy at the target knows that the atom has been "teleported", he stil doesn't know which one of the 4 twisted flavors of the original atom he got. If he just takes a "wild guess" and tries to measure, he'll get a statistical result which reveals absolutely no information about the actual coefficients.

    The target-guy needs the source-guy to tell him which of the 4 twists occured, or in short: needs an information transfer in order to be able to "untwist" his atom and have an exact copy.

    Again, the important part is that if the target-guy does not "untwist" his atom, but instead decides to go away and measure it anyway, he'll have an overall chance of 50-50 (regardless of the original x and y) to measure either A or B, so there's no information whatsoever that he could gain, not even from repeating the experiment.

    It's the "twist" that makes the twist with teleportation... :-)

  • by Jamu (852752) on Friday January 23, 2009 @10:58PM (#26585199)

    Relativity implies that if information goes from A to B instantaneously for some observers, it also goes from A to B in finite time for some other observers. For all the other observers it goes from A to B in negative finite time, from B to A, in other words. For causality, for A to cause B, then information must always travel from A to B.

    Any instantaneous wavefunction collapse cannot transmit information from distant locations, it must create new information for those locations, i.e. a random value.

  • Re:Mod patent up. (Score:4, Informative)

    by Chandon Seldon (43083) on Friday January 23, 2009 @11:36PM (#26585409) Homepage

    Unfortunately, you can't do either of the things you want to do. Relativity says you can't have synchronized clocks and quantum mechanics doesn't give you any way to know when/if the wave was collapsed.

  • by Giant Electronic Bra (1229876) on Saturday January 24, 2009 @08:03AM (#26587835)

    Here's an illustration of the non-tranmission of information via entanglement.

    Suppose we have a pair of 'magic coins'. Either coin can be flipped and come up either heads or tails, and the other coin will always come up the opposite.

    Now, suppose 2 people meet in New York and agree that they will meet again in Oslo if Amy's coin comes up heads and Bill's coin comes up tails, or they will meet in Sidney if Bill's coin comes up heads and Amy's coin comes up tails. Then Amy goes to Peking and flips her coin. It comes up heads, so she meets Bill in Oslo.

    The information, which city they will meet in, was AGREED ON BEFORE HAND, it wasn't 'transmitted' by the flip of the coins. The information was in Amy's head when she went to Peking, it traveled by a classical channel governed by relativistic limitations.

    This can be seen explicitly if you assume that Amy and Bill DIDN'T agree on which face of the coins meant Oslo or Sidney. In that case when Bill and Amy flip their coins they DO know that their opposite number's coin came up the other way, but neither of them knows which city to go to! In other words, no information was conveyed between them BY the flip of the coins.

  • by sfazzio (1227616) on Saturday January 24, 2009 @09:16AM (#26588325)
    A completely valid arguement-- until 1964:
    http://en.wikipedia.org/wiki/Bell's_theorem [wikipedia.org]
  • by The_Wilschon (782534) on Saturday January 24, 2009 @10:33AM (#26588921) Homepage
    You can't send quantum information faster than light either. You can cause change to propagate faster than light, but no actual information is conveyed by that change. It is a subtle, but important, distinction.
  • by ThreeGigs (239452) on Saturday January 24, 2009 @10:47AM (#26589011)

    Wrong.

    The only way to tell if your waveform has collapsed is by measuring it. And measuring it collapses it.
    Thus *every* time you check to see if yours has been collapsed, it will always show as collapsed.

    Go look at yourself in a mirror.
    Now close your eyes.
    Now, you cannot tell when your reflection has opened its eyes, unless you open yours, and if you open yours, the reflection will have its eyes open.

    Same with the waveforms. No matter what, if you look, it'll be collapsed, and you can't tell if it was collapsed before you looked at it, or because you looked at it.

  • Mod Parent Funny (Score:1, Informative)

    by Anonymous Coward on Saturday January 24, 2009 @02:31PM (#26591107)

    n/t

  • But they DON'T know (Score:3, Informative)

    by Giant Electronic Bra (1229876) on Saturday January 24, 2009 @07:16PM (#26594009)

    At no point can either Amy or Bill determine whether or not the other coin has been flipped. All they can say for sure is that WHEN IT IS, it will come up a certain way. Maybe it already has been flipped, maybe it hasn't. The only way to find out would be using a classical communications channel.

    There is a CORRELATION between the two 'coins' with entanglement, but there is NO causality. Flipping one coin does NOT cause the other one to flip, this has actually been verified by various iterations of experiments testing Bell's hypothesis. The logic is a bit trickier and my simple analogy isn't good enough to explain it, but in actual quantum mechanics if one flip caused the other, then certain experiments could be devised which would have different results than if the two flips are merely 'coincidence'.

    This is one of the amazing things about QM. There is actually NO causality anywhere in QM, only a distribution of the probabilities in a particular special type of function space. Causality is an emergent phenomenon which only exists at the 'macroscopic' level. Even then it isn't absolute. I believe it was Stephen Hawking who once quipped that Cthulhu could materialize in the middle of the Pacific Ocean at any moment and no law of physics would be broken.

    There are a LOT of other interesting ideas which come out of this, like questions about the meaning of entropy, which leads into questions like the Anthropic Principle.

  • by franl (50139) on Saturday January 24, 2009 @07:19PM (#26594039)

    If Bill or Amy flip a coin, then instantly the other one KNOWS the other person flipped a coin at that point in time.

    Nobody has said that. How can Bill's measurement force Amy to make her measurement at the same time? That's not possible. Especially since "at the same time" has no meaning for spacelike-separated events (cf. the Relativity of Simultaneity [wikipedia.org]).

    This is what happens: Bill measures a random value. Amy measures a random value. The two values are both random, but 100% correlated with each other. Bill knows what value Amy measures and vice versa, but no information has been transmitted, because the values are random.

  • by sootman (158191) on Sunday January 25, 2009 @12:11AM (#26595813) Homepage Journal

    I have two basket balls, one has a cat inside - I don't know which one.

    The heavier one. Duh. :-)

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