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Science

Baby Steps Toward Quantum Computers 308

Posted by michael
from the ansible-mark-1 dept.
Mz6 writes "In a step toward making ultra-powerful computers, scientists have transferred physical characteristics between atoms by using a phenomenon called entanglement, which Einstein derided as 'spooky action at a distance' before experiments showed it was real. Such 'quantum teleportation' of characteristics had been demonstrated before between beams of light. Teleportation between atoms could someday lie at the heart of powerful quantum computers, which are probably at least a decade away from development. Researchers using lab techniques can create a weird relationship between pairs of tiny particles. After that, the fate of one particle instantly affects the other; if one particle is made to take on a certain set of properties, the other immediately takes on identical or opposite properties, no matter how far away it is and without any apparent physical connection to the first particle." Reader starannihilator adds: "Physics Web provides a good graphic summary of the phenomenon, as well as a good technical article."
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Baby Steps Toward Quantum Computers

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  • Analogue vs Digital (Score:3, Interesting)

    by Nermal6693 (622898) on Thursday June 17, 2004 @01:59AM (#9449437)
    I think (although I'm not certain) I read somewhere that a quantum computer is like an analogue computer - where you're not restricted by 0 and 1. Is that correct?
    • by Anonymous Coward
      It was just a dream, Bender. There's no such thing as two.
    • by LnxAddct (679316) <sgk25@drexel.edu> on Thursday June 17, 2004 @02:12AM (#9449491)
      A quantum computer is completely different. The only thing in common in the binary number system. In a classical computer you have bits, either a 1 or a 0. In quantum computers you have qubits which can be a 1 or a 0 or actaully both values at the same time! This can manifest tself in amazing ways. You can try every solution to a problem instantaneously because instead of having to count throught all of the possible inputs, i.e. going from 0 to 255 with 8 classical bits, in a quantum computer 8 qubits actualy are the values of 0 through 255 all at the same time. The answer is then decomposed or observed forcing the quantum state into a final and complete solution. Some quick info for those who have no idea what qunatum anything is... an observation is essentially defined as any force that forces a quantum state to be amplified into a definitive state. Quantum entanglement occurs when two paritcles intereact for a short period of time (i.e. two photons crossing) and then go off on their own, they can travel to oppisite sides of the universe and whatever happens to one, instantaneously happens to the other. Literally, no moment of time occurs between the change, its quite amazing. If you polarize one photon, the other will instantly be affected. Also if particles A & B are entangled and C & D are entangled then if B entangles with D then A automatically becomes entangled with C. This allows for some truly amazing things. One final note, although quantum entanglement was first observed with laser light(photons), it has since been reproduced with much larger particles including ruby atoms and even bucky balls (google it if you dont know what one is)
      Regards,
      Steve
      • by spacecowboy420 (450426) * <rcasteen@gm[ ].com ['ail' in gap]> on Thursday June 17, 2004 @02:53AM (#9449650)
        OK, maybe I'll sound like a jackass, but I gotta ask anyway. It seems to me that if you can reproduce entangled particles reliably, and you have, lets say two hosts, both with one half of the set of the entangled particles. If you were to manipulate the state of one set, and that immediately affects the state of the entangled partner on the other host, wouldn't that be the effectively TRUE wireless communication. One where the rate of communication is limited only by how fast you could read and process the set of particles that are local? Wouldn't that be as secure as it gets - media to intercept? Sure, there would need to be software to interface with the states based on the input from the hosts - but if you could do this, you could control the mars rover in realtime. Is this where this is headed, or am I confused?
        • by cicadia (231571) on Thursday June 17, 2004 @03:04AM (#9449687)
          Well, you're not a jackass, but it is a bit more complicated than that. Unfortunately, there doesn't seem to be any way to actually transmit information instantaneously with entangled particles. It's true that two entangled particles will undergo the same transitions at the same time, but since you can't predict in advance or control what transition will occur, it doesn't help you send any information to a person looking at the particle at the other end.

          You're right, though, that it's about as secure a communication channel as you can get. It's actually the basis for quantum cryptography -- two people share a set of entangled photons, and they can guarantee that the measurements they make on them will be identical, giving them a shared secret key that no one can intercept. They still have to communicate over regular channels to actually send any real information, though.

      • by Scorillo47 (752445) on Thursday June 17, 2004 @02:56AM (#9449660)
        Note that entanglement is just one approach in building quantum computers, and it is not really the ONLY approach.

        Generally, a quantum computer consists in several quantum systems (for example captured particles, etc). The (quantum) state of these systems varies according to a well-known equation, called the Schrodringer equation. This is a very simple equation that describes the evolution of the system (the derivative of the current vector state) in respect to the current current state & time.

        The nice thing about quantum computers is that they operate with multiple simultaneous states, therefore achieving some sort of parallelism. Basically a quantum system can be considered to have a superposition of states - it has two states at once if you want. Some of these states might converge to the same state depending on the hamiltonian or on the external interactions.

        The hard part is that you never know when such a computer stops its calculation since the transformation state is fully reversible and goes on ad infinitum. If you want simply to test if the computer reached the end of the calculation, you will affect the current state. Anywyay, this challenge plus many others (for example the precision of the measurement, etc) makes quantum computing very challenging.

        Still, there is a theoretical possibility that you can get a high degree of parallelism in certain configuration. A classical result from Shor (you can search on Google) shows that one of the classic problems in arithmetic - integer factorization - can be done in a polynomial time on a quantum computer. This simply means that RSA encryption can be potentially broken, irrespective to the length of the key. But we are still safe - so far nobody built a working quantum computer that would carry on simple calculations like factorizing the number 15.

        On the other side, entanglement is an interesting quantum fenomenon which works like this:
        1) First, you have to have a way to build pairs of entangled particles. There are several ways to do this, for example by having any quantum process that generates a pair of photons.
        2) Second, if you modify the vector state of one particle, the vector state of the other one will be equally affected, regardless of the distance between these two particles!

        What's interesting is that entanglement guarantees instantaneous quantum state change therefore contradicting somehow the theory of special relativity. This theory says that events cannot be 100% simultaneous if they occur in different points in space - there is a timing separation based on the particular reference chosen. Practically, no standard matter interaction can be faster than the speed of light.

        But there is an exception here - "collapsing the vector state". If you measure the state of a particle, its state will collapse along one of the measured dimensions (according to certain probabilities). The corresponding entangled particle will suffer a similar change, so if you measure now the state of the this second particle you will see that its vector state has already changed - and you can even perform a partial correlation between the results of the two measurements.

        In conclusion, enanglement guarantees instantaneous "interaction" regardless of the distance between these paired particles (this is why Einstein called it "spooky action at a distance" - because technically it is propagated with infinite speed). Anyway, it has be proven a while back that this does NOT contradict the special theory of relativity since this is not a standard matter interaction, like gravity, etc.

        Going back to computers, entanglement is an interesting approach which might enable new algorithms or new ways to build such computers. But keep in mind that we are in the stone age of quantum computing right now...
        • This simply means that RSA encryption can be potentially broken, irrespective to the length of the key. But we are still safe - so far nobody built a working quantum computer that would carry on simple calculations like factorizing the number 15.

          How do you know this for certain? If the NSA's scientists have cracked this you can be damn sure that they're not going to tell anyone about it.

          HH
          --
    • by jfern (115937)
      With n classical bits, they can be of 2^n possible states.
      With n quantum qubits, they can be any normalized (overall phase doesn't matter) complex vector in 2^n dimensions.
      However, when you measure them, the wave-function will collapse (unless you believe in the many world's multiverse), and you'll get n classical bits.

      Classical information is simply a subset of quantum information.
    • by natmsincome.com (528791) <adinobro@gmail.com> on Thursday June 17, 2004 @02:24AM (#9449530) Homepage
      Not so much Analogue vs Digital but rather Serial vs Parallel.

      In searial you do one instruction per peice of data. In parallel you try EVERY piece of data in one instruction.

      Some problems are trivial in serial but hard in a parallel and other problems are trivial in parallel but hard in serial.

      Simple Example:
      Iterative calculation are great in serial but aren't that good in parallel as you can calcualte the second value till you have the previous value.

      The Famous example:
      The big thing that quantum computers will do is make parallel problems trvial. The big two being simulations and cryptology. Cryptology is only hard because you have to try so many different combinations. Quantum would allow you to try EVERY combination at a single time. This make encryption almost useless at any key length.

      It's also usefull for simulations like ray tracing and vector maths where you have a complex eqation where you just have to run for every possible variable.

      So ever is a single iteration takes 1 hour for a quantum computer instead of 100th of a second for normal computers it will change the world. Breaking a key 2048 bit key will take exactly 1 hour instead of million+ years. Rendering a frame will take 1 hour on a single computer instead of 4 hours on 1000+ computers.

      That being said it would be useless for Word, Excel or Firefox :-)

      Imagine a quantum computer that does 5 Hz out perform a cluster that does 5 TeraHz.
    • by Medevo (526922)
      Somewhat, but you are a little off.

      The best way I have found to think about quantum computations is that digital computers think in 1's and 0's

      Quantum computers allow you to ad "decimal places" to this traditional logic (0.1, 0.2, 0.9, 1.0). As you increase the number of quantum bits, instead of just increasing the number of calculations a second you can do (like with our processors today) you are in fact adding new more "decimal places" by simply looking at the qubits in terms of accuracy. Even a simple
      • by jfern (115937)
        Quantum computers aren't quite as powerful as you make them out to be. At the end of your algorithm, you have to perform a measurement, and each qubit when measured only gives you 1 classical bit.

        It's been proven that quantum computers are no better than classical computers at sorting (both O(n log n), although they are better at finding something in an unsorted database (Grover's algorithm does O(sqrt(N)), instead of O(N) classically).

        No one has proven that quantum computers are faster than classical com
        • by Medevo (526922) on Thursday June 17, 2004 @02:50AM (#9449644) Homepage
          The limit of computing is, as you say, on the developer's side, no argument here. It its at least partially reasonable that when quantum computers become more available, that ingenious developers will find ways to squeeze out more power.

          Moreover, at the end of the day, you still extract bits from qubits. While one day in the distant future we may be able to interact computers entirely in a quantum environment, but it's a long way off.

          The real potential in quantum computers is the problems of density, power, and heating, that have plagued development of faster CPU's seem to apply on a lesser scale to quantum circuits (not that they don't have there unique problems). At the same time, quantum computers could/would suffer a lot less problems with bandwidth/time delay (light/QE info transfer).

          Traditional MOSFET based transistors, while powerful (look at today's advanced chips) have been around for a while; there is no harm in looking for something new and better.

          Even if quantum computers provided a liner growth curve in processing power to qubits, we could expect a greater throughput in it (due to above stated factors).

          Medevo
    • by Anonymous Coward
      I stand by my prediction that there will never be a quantum computer. It's just a pipe dream of being able to compute all possible combinations simultaneously. It's one of those things that's just not going to happen. Other examples of things that will never happen, no matter how bad we want them:
      • Natalie Portman + grits
      • Cold fusion
      • Time travel
      • Warp drive
      • Linux on the desktop
      • World peace
      • SCO execs get jailtime
      • Quantum computers
      • Teleportation
      • Viable "step 2" in the three step business
    • by wass (72082) on Thursday June 17, 2004 @05:04AM (#9450175)
      Wow, 2 quantum computation articles on /. within two days.

      I mentioned this yesterday as well, but for an idea of what qubits are you can take a look at my currently unfinished Java Quantum Computation applet [jhu.edu]. As of now one can only do single-qubit operations, but eventually I hope to have a demo of quantum teleportation (teleportation of a single qubit, or spinor, that is).

      This applet will give you an idea of what qubits are. Essentially they're a 'spinor' which in quantum-mechanical terms is a 2-element discrete wavefunction. In lay terms, this just means a set of two complex numbers (properly normalized). They are also displayed in a more visible representation, called the 'Bloch Sphere'.

      This applet will let you take any input qubit, and operate on it with 6 different single-qubit quantum gates, and see the resulting qubit.

      Look at the two qubits represented on the Bloch sphere. The yellow vector represents the qubits. The red dot indicates a classical 'zero' and the blue dot indicates classical 'one'. In classical computing any bit can only point exactly to the red or blue dots. In quantum computation a qubit can point anywhere on that sphere.

      [For the mathematically curious, a qubit is 2 complex numbers, which would be 4 independent parameters. However, the sum of the modulus squared of each complex number must be unity, so that constraint leaves only 3 free parameters. Secondly, the entire qubit can be multiplied by any arbitrary phase constant (e^i*gamma) which changes the spinor but not its relative values. Hence, there are only two parameters for each qubit that really matter, so it can be expressed in 2D, mapped nicely to the sphere.]

      In classical computing there are only 2 single-bit gates - Not and Buffer (actually, I never formally studied computer science, so someone please correct me if this isn't true). 'not' flips the bit, 'buffer' keeps the bit unchanged. In quantum computing there are infinitely many single-bit gates, some of the common ones are demonstrated in the applet. Basically, these gates can control how relatively 'one' or 'zero' the bit is by the superposition, as well as change the relative phase.

      Anyway, I should be adding in two-qubit operations soon (like the infamous controlled-not) and hopefully get to something worthwhile.

      So this applet isn't very useful for actual simulation of quantum computation yet, but it will you give an idea of what qubits are and how they can be represented.

  • by Anonymous Coward on Thursday June 17, 2004 @01:59AM (#9449438)
    Isn't this the correlation effect mentioned in the prime intellect story?

    In the PI universe, a Beowulf cluster of these imagines YOU!
  • by Anonymous Coward on Thursday June 17, 2004 @02:00AM (#9449442)
    Just say 20 years from now I am on my quantum fandangle computer that does sub-atomic calculations, what happens when background radiation hits the processor and flips a few 1s and 0s?

    i.e. will my computer crash when there is a solar flare?
    will the new "heatsinks" be lead shields?
    will we need to rotate the shield harmonics? (j/k)

    please... inquiring minds want to know.
    • I'd have to say (not that I actually know) that there would be equal danger now from a solar flare crashing your computer as there will be on a quantum computer. But what the hell do I know? You should go ask Scotty.
    • by jfern (115937) on Thursday June 17, 2004 @02:27AM (#9449546)
      The problem with quantum information is that you can't clone (copy) an arbitrary quantum state, and you can't measure an arbitrary state without destroying the quantum information.

      However, there still exist quantum error correcting codes that can correct an arbitrary error. Classically, one only gets bit flip errors. In quantum computation, you have to worry about phase flip errors, for instance instead of a|0>+b|1> you have a|0>-b|1>.

      The smallest quantum code that can correct an arbitrary non located (located errors are easier) error on 1 qubit requires 5 qubits. There's a 7 qubit "CSS" code that is important for fault tolerance.

      For fault tolerance, you concatenate a code with itself many times, and if your errors are independent of each other, then by doing all sorts of complicated fault tolerant techniques, you can get fault tolerance. What happens is you get a fault tolerance threshold. If your rate of errors are less than that, you can do arbitrary quantum computation with O(M) qubits in O(N polylog N) time, where O(M) is the qubits required on an error free quantum computer, and O(N) is the time required on an error free quantum computer.
    • by nihilogos (87025) on Thursday June 17, 2004 @02:48AM (#9449636)
      Just say 20 years from now I am on my quantum fandangle computer that does sub-atomic calculations, what happens when background radiation hits the processor and flips a few 1s and 0s?

      Quantum error correction. [qubit.org] is a sub-field of quantum computing concerned with just that, how to effectively perform a quantum computation in the presence of background radiation and other stuff which sub-atomic thingies tend to be quite sensitive to.

      The likelyhood of flipping a few zeros and ones ( and other errors which can afflict quantum bits) is very high, and in reality is more a continuously decay than an instant flip.

      It has been shown, however, that this continuous decay is equivalent to flip errors and phase errors (the other sort of quantum error) occuring with some probability. That probability is 1 in 10 for most of the current experiments, compared to your box in front of you which is more like 1 in 10 billion.

      Fault-tolerant quantum computing is a theory field of research concerned with how good quantum computers have to be before quantum error correction can work. The best results at the moment suggest a probability of error of 1 in 1000 is good enough. The experimenters have a fair ways to go yet.
      • Some of us are working on getting a better result than 1 in 1000. ;) Actually, the important thing is, it depends on what sort of noise you get from your gates.
      • by mcrbids (148650) on Thursday June 17, 2004 @03:15AM (#9449736) Journal
        That probability is 1 in 10 for most of the current experiments, compared to your box in front of you which is more like 1 in 10 billion.

        Would you really think even a e-machine is that error prone?

        Think about it...

        2.5 Ghz * 32 bits/cycle = 80,000,000,000 - that's 80 BILLION bits per second...

        Of course, that's theoretical, there's buffering delays, cache, noops, etc. But, given the theory, there'd be 8 random errors every single second.

        Something doesn't sound quite right, here, especially when you figure the vast majority of computer are sold with no error correction at all on the system memory ?

        I think that 1 in 10 billion is probably quite a few orders of magnitude off....
    • Seriously, ECC correction and redundancy is all that is needed. Even if quantum computers are blistering fast, if getting away with packing 100 individual clusters is available, it will be done. Then, they can all be checked against eachother. Majority rules in a democratic processing fasion.

  • by Naffer (720686)
    So we've got the one atom thing down now. The trick is getting a whole lot of atoms to do it at the same time. If we can convince the porn industry that it would be beneficial to them, We'll be teleporting around the world in less then 5 years. Maybe I should patent teleporting prostitutes.
  • Yes, fast (Score:3, Insightful)

    by Milo of Kroton (780850) <milo.of.kroton ( ... .com minus distr> on Thursday June 17, 2004 @02:05AM (#9449459) Journal
    But what cost? Only government would want new technology this fast, maybe your NSA, that around codebreaking.
    • by tachyonmkg (718196) on Thursday June 17, 2004 @02:28AM (#9449553)
      Only the five richest kings of Europe will be able to afford them.
    • by jfern (115937)
      "We see a worldwide demand for maybe a couple of computers" - IBM
      "640k of memory is enough for anyone" - Gates
    • Only government would want new technology this fast, maybe your NSA, that around codebreaking.

      Only the government would want it? Hell, I'd want it! Who wouldn't?
    • Yeah, right. We'll only need a few quantum computers just like we only need a few computers. Thank you Mr. Watson of IBM.
    • Re:Yes, fast (Score:5, Informative)

      by plaa (29967) <.if.iki. .ta. .nenaksin.opmas.> on Thursday June 17, 2004 @03:58AM (#9449897) Homepage
      Comparing the speed of a quantum computer and classical computer is comparing apples and oranges. Quantum computers work with a totally new set of rules, which allows some applications to make use of quantum properties.

      The main property that classical computers lack is that of superposition of states. One can understand this as calculating some result starting with all possible numbers at once, instead of testing each starting value as its own. (In reality it's more complicated than this, of course.)

      Some applications, eg. codebreaking, number crunching and database applications could get a vast boost out of quantum computing. Other applications may not. The most probable places for quantum computers (at first) will probably be number crunching, networking applications (quantum cryptography etc) and database applications.

      For a comparison, searching an unsorted database is classically an O(N) operation, but a quantum computer can do this [wikipedia.org] in time O(sqrt(N)). The best known classical algorithm for factoring a number is exponential, while Shor's algorithm [wikipedia.org] does it in time O((log N)^3) (allowing polynomial-time breaking of RSA).
  • I have a very bad conception of quantum computing as it is, I've somehow confused it with the idea of getting computing power out of the atoms themselves. (which is probably as related to actual quantum computing as star trek is to physics.)

    Still, this is good. A few more angstroms out of electronics means a few more decades of potential improvement.
  • How to choose? (Score:2, Interesting)

    by Shambhu (198415)
    This wiki looks good, and if it isn't too technical, maybe I can find the answer. However, every other article, paper, or discussion that I have seen skips this one question of mine: How is the choice made between all the superimpositions to select ther 'right' answer? Everyone goes to great lengths to explain the superimposition part and its implications for massively parallel computation, but no one ever says how you choose the result! Does anyone have a clue about this?

    • Re:How to choose? (Score:3, Informative)

      by ajayg (122305)
      Good question. In fact, this is one of the trickier problems to solve when coming up with a QC algorithm. The trick is, to use the phenomenon of coherent interference to yield the result that you are looking for. Interference here is basically the same as wave interference. So, after our QC executes an algorithm and finds the solution to a problem for all N inputs simultaneously, we then have to interfere our output result state (which now exists as a coherent superposition of N different outcomes) in such
    • It's my understanding that this is the hardest part of quantum algorithms. It's quite straightforward to just pick one answer and so with fixed probability (based on size of system) answer is right, otherwise it's wrong. IIRC, the beauty of shor's algorithm (factoring in polynomial - linear i think - time on a quantum computer) is that it always returns the correct answer. Unfortunately I don't know how it works so I can't answer that part. i hope someone else can explain that.
    • Re:How to choose? (Score:3, Informative)

      by jfern (115937)
      A typical quantum algorithm puts most of the wavefunction into the state(s) that you want. By applying various quantum unitary gates repeatedly one can do this. It's kind of hard to explain exactly "why". One then measures the state, and with with probability p gets a correct answer. If p> 50%, one can repeat the algorithm a bunch of times to make sure one has the right answer.
    • Sorry to reply again but from wikipedia
      Like all quantum computer algorithms, Shor's algorithm is probabilistic: it gives the correct answer with high probability, and the probability of failure can be decreased by repeating the algorithm.
      And it runs in O((logN)^3) time. So not linear but sublinear unless my math is rustier than i thought.
      • And it runs in O((logN)^3) time. So not linear but sublinear unless my math is rustier than i thought.

        I'm not sure just what you mean by "sublinear," but a few experiments show me that f(x)=ln(x)^3 goes up slightly faster than x does. Could you explain what you mean?

  • by wwest4 (183559) on Thursday June 17, 2004 @02:11AM (#9449484)
    Alice, instantaneously transfers information about the quantum state of a particle to a receiver called Bob. The uncertainty principle means that Alice cannot know the exact state of her particle. However, another feature of quantum mechanics called "entanglement" means that she can teleport the state to Bob.


    Alice: Bob, now that our qubits are entangled, I don't know if mine's spin up down.

    Bob: How 'bout I observe yours for you. How about there?

    Alice: Nope.

    Bob: Here?

    Alice: Closer to this side of the gaussian, Bobby.

    Bob: How about here?

    Alice: OOOOOHHH! You collapsed my wave function DeBroglie!

    Bob: Your qubit is now spin up, in case you were wondering... who's DeBroglie?

    • A quantum particle is speeding down the road in its car. A policeman pulls the particle over, gets out of his vehicle, and walks over to the quantum particle. The policeman asks:

      "Do you have any idea how fast you were going?"

      The particle replies,

      "No, but I know exactly where I am!"

      Ba-dah-bing!

  • This... (Score:3, Interesting)

    by Cyno01 (573917) <Cyno01@hotmail.com> on Thursday June 17, 2004 @02:11AM (#9449487) Homepage
    Sounds more like the basis for instantanious comunication (read too much OSC). If we ever invented non reltivistic FTL or spread far enough that we'd need instantanious communication it would probably be based on this.
    • nah, you cant use it for coms, you have to transport the information about the first entangled particle to the second entangled particle using normal communication mechanisms.
    • Re:This... (Score:2, Funny)

      by Mr. Roadkill (731328)

      Sounds more like the basis for instantanious comunication (read too much OSC). If we ever invented non reltivistic FTL or spread far enough that we'd need instantanious communication it would probably be based on this.

      Actually, even if we never develop FTL transportation, FTL communication could be very, very useful.

      Find stars with earth-like planets, send probes containing quantum-entangled data comms gear and pretty well documented interfaces, and invite them to offworld their call centres to India.

    • Re:This... (Score:3, Informative)

      Actually no. There are 2 steps to the process: the 'teleportation' one (collapsing the remote state) and the 'turtle' one (tell the other party what result you measured so that he can rotate his collapsed state to the right one). The second phase is the actual information transmission and it's slower-than-light. Also, you can't skip it by 'guessing', as the possible values for the collapsed state do not even form an orthogonal state[*]. Sorry.

      [*] for 2-state particles (simplest case), the measurement of th
    • Re:This... (Score:3, Informative)

      by NonSequor (230139)
      Nope, doesn't work that way. In order to make this work you also need a classical (ie slower than light) communication channel. In quantum teleportation, one person interacts a qubit with one half of an entangled pair of qubits, performs a measurement, and then sends that measurement to the other person. The other person then performs an action on their half of the entangled pair that transforms it into the same quantum state as the original qubit. The original qubit is altered in this process and each enta
  • by achurch (201270) on Thursday June 17, 2004 @02:11AM (#9449489) Homepage

    Can someone explain why this can't be used for FTL communication? The folks at Cornell [cornell.edu] seem pretty convinced [cornell.edu] that FTL communication is impossible, but from my reading of the article, in this experiment the first particle is forced into a known state, so (IANANuclearPhysicist but) it seems to me that if the state of the second particle can be measured (even if that measurement causes the state to change), communication has been accomplished. What am I missing?

    • by wwest4 (183559) on Thursday June 17, 2004 @02:17AM (#9449506)
      Because Alice can't know the state of the information she's sending. If she does, then the superposition collapses.

      It's not intuitive, but the "collapse of the wave function" metaphor fits observation.
    • by jettoblack (683831) on Thursday June 17, 2004 @02:31AM (#9449562)
      What you're thinking of doing is creating an entangled pair, and keeping one particle on Earth, and keepting the other on a spaceship. Then by changing the state of the Earth particle, you could affect the state of the spaceship particle. Right?

      The problem is, we have no way to choose what state the particles will go into when we observe one. Its a random outcome, and you can't acheive any communication if the output is just random noise.

      Furthermore, from the spaceship's viewpoint, how do you tell if your particle's state has changed due to an incoming transmission? The only way to know would be to observe it. But, we don't know if that particle had been observed by Earth yet. If it had, then we just disturbed the state that Earth had set. If it hadn't, then we just forced it (and Earth's particle) to a random state. True, the Earth's particle will now be set to the same random value, but random values are still uselss for communication.

      For it to work, you'd need a second channel of information, which could transmit some kind of key to decoding the random states into data. Of course, this channel of information would have to go FTL too, so its a Catch-22...
      • What you're thinking of doing is creating an entangled pair, and keeping one particle on Earth, and keepting the other on a spaceship. Then by changing the state of the Earth particle, you could affect the state of the spaceship particle. Right?

        Yup, exactly.

        The problem is, we have no way to choose what state the particles will go into when we observe one. Its a random outcome, and you can't acheive any communication if the output is just random noise.

        But I thought that's exactly what this experiment

        • But I thought that's exactly what this experiment accomplished. The Physics Web article and diagram certainly suggest that they're teleporting a known state, via the use of a third particle to influence one side of the pair; am I reading them wrong?

          IANAQP, but my interpretation of their diagram: Basically, you start with your source particle S and destination particle D. The goal in quantum teleportation is to copy the exact state from S to D. They suggest doing this by creating an entangled pair, A and

          • So...what about if we had a large bucket of pre-entangled particles?

            We could take these with us somewhere, then perform the collisions and teleport states, thus communicating a limited amount of data back at FTL speeds.

            Now I realise there are current limits on how long entanglement lasts, plus all sort of quantum error correction required, but essentially it *could* be feasible, no?
  • Need 3 particles (Score:3, Interesting)

    by miyako (632510) <miyakoNO@SPAMgmail.com> on Thursday June 17, 2004 @02:14AM (#9449499) Homepage Journal
    I am not a physicist, or a physics student, or even an arm chair physicist, but from what I understand, creating a quantum gate requires (at least?) 3 particle entanglement, which is quite a bit more difficult than 2 particle enganglement. Can anyone better versed in the subject confirm or refute this?
    • I am not a physicist, or a physics student, or even an arm chair physicist, but from what I understand, creating a quantum gate requires (at least?) 3 particle entanglement, which is quite a bit more difficult than 2 particle enganglement. Can anyone better versed in the subject confirm or refute this?

      Disclaimer: I'm a physics student, not a physicist. However, it is my poorly educated thought is that they could use a Fredkin Gate [arxiv.org].

      Dogg

  • We hope to be able to use this for computing, but we know it could be used for communication even better. All we have to do is develop better, cheaper tools for manipulating & reading the particals.

    Unfortunatly, so far it only seems to work with pairs, we can't seem to get multiples going, so use is limited. but let's try this from the military point of view: In theory, we could build 'ansibles' (to steal from Orson Scott Card) that operate in pairs. Every ship and command unit could have one, the oth

    • It is currently generally believed that there is no way to manipulate the particles in such a fashion as to allow usable information transfer at a FTL speed.

      However, it can be used in conjunction with a classical communication channel to provide uneavesdroppable encryption.
  • by mcc (14761) <amcclure@purdue.edu> on Thursday June 17, 2004 @02:19AM (#9449514) Homepage
    Is the idea here basically just that this means that they'll be able to transmit information between qubits without the qubits having to be right next to each other?

    Does this mean they might finally break that 7-qubit barrier that quantum computers up until this point had seemed to have been limited to?

    I really don't get exactly what's going on. I ASSUME the news doesn't mean that they've find a way to transmit information instantaneously using QE.
    • The point of Quantum Entanglement is that two particles will assume the same quantum state no matter where they are in space. Change one, the other will change, no matter the distance separating them. While the effect is not well understood, it has been demonstrated with photons, and now atoms. This does not directly solve the 7-qubit barrier, but any advancements aren't going to hurt.

      If this whole thing of instantaneous communication seems odd, it should. While we are yet to find anything near a economica
  • by www.fuckingdie.com (759660) on Thursday June 17, 2004 @02:24AM (#9449532) Homepage
    What happens when quantum computers, which are able to use quantum teleportation, start to exert influence directly over the matter that makes up say a Human Brain for example. Or to make matters worse the brain accidentally starts to exert control over the computer.

    "We are sorry - the application you were running has crashed because you were thinking unhappy thoughts."

    or

    "You have 60 seconds to close and save all thoughts before your brain will be automatically restarted"

    Can we say sasser-"cranial edition"

  • Electrogravity (Score:2, Interesting)

    If it is FTL communication, then we've stumbled into the area of electrogravity.
    FTL is not an impossibility; it just stands in relation to relativistic physics as it stands in relation to classic physics.

    As many know, around a black hole there is a very strong gravitational field. This field has the property of bending the dimension of time itself. We can therefore state that time is not linear, and that a hypothetical theory of electrogravity would be entirely four-dimensional. This would mean that as far
    • Which would lead us to the conclustion that we, indeed, have a destiny which we cannot change... The buddhists would be scientifically proven to be correct, and christians (with the causal evil vs good struggle) to be proven wrong...
  • This can not be used for faster than light communication. No "information" is exchanged in the "teleportation" it is just that one can "copy" a quantum mechanical state from one place to another, which of course is crucial for building quantum computers. For more explanation on the difference between entangelment and FTL communication see for example see a discussion of the EPR Paradox [brainyencyclopedia.com].
    • No "information" is exchanged in the "teleportation" it is just that one can "copy" a quantum mechanical state from one place to another

      Not quite.

      You're correct that quantum teleportation will transfer a quantum wavefunction from one point to another. But it cannot 'copy' the wavefunction. In order to send the wavefunction, the original wavefunction must be destroyed during the process.

      Sorry, fanout is strictly prohibited in quantum computing.

  • Okay, so this is probably incorrect, but it is a train of thought. With the state of quantum encryption being that if a third party observes the key in transit, it is apparent, and the key is useless, would this have a potential application to break this encryption.

    Using this method, the duplicated particles could be observed, leaving the original particles in the encryption stream relatively unmolested. Yes, it would be impractical and the equipment needed would be very distinctive and difficult to hide, but it raises the possibility.

    • Nope, there's a theorem called the no-cloning theorem that says that you can not copy an arbitrary quantum state. There's no way to start with a state |v> and get |v> |v>, which would mean I could perform destructive measurements on one |v> and be left with |v>.

      This follows from 2 facts
      1. Quantum measurements can be replaced by quantum gates
      2. Quantum gates preserve the inner product of two states.
    • No because measuring the second entangled prticle has the same affect on the first as directly measuring it. Otherwise one could get around Heisenberg's uncertainty principle which afaik appears to be a law of nature and not something that one can avoid.

      Within special relativity, causality can be preserved by forbidding information from travelling faster than the speed of light this does not mean A cannot communicate w/ B FTL but that no useful information to an outside party can be passed (i.e., 1 cannot
  • by Anonymous Coward
    Can you imagine playing Unreal Tournament at a ping of 0? and having a Inernetlink with and unlimited speed? [well depends on the put and get on the link ion] You could probably syncronize what ever you want in just a few s.

    kindest regards,
    mo
  • For giving the popular press one of the most annoyingly overused quotes ever. PS: spooky first post at a distance.
  • by Komi (89040) on Thursday June 17, 2004 @03:01AM (#9449679) Homepage
    I know this is slightly off topic, but what physically is spin, and how do you measure it? These experiments always talk about how this property called spin can be entangled with other particles.

    IANAP, and in the high level articles I've read, I've never seen spin discussed to anymore depth beyond just that it's a property of fundamental particles. I know that force particles have integer spin (and thus ignore the exclusion principal), and matter particles have half integer spin (and have to obey the exclusion principal), but I don't know what that means physically, or how you measure it. Does it have to do with angular momentum? From a macro world of physics, to measure the angular momentum of something, you can apply a torque and see how quickly it accelerates. I also know that you can measure the charge and mass of a particle by seeing what sort of spiral it makes in a cloud chamber. Is measuring spin related to either of these techniques at all? Thanks for the help!

    Komi

    • The following 3 things are equivalent
      A qubit
      The spin of an electron
      The polarization of a photon

      They are equivalent that they can each be representated by a 2 dimensional complex vector, where you don't care about the overall phase (and 0 isn't allowed).

      Every played around with polarized lens filters? You have a horizontally polarized lens followed by a vertically polarized lengs, and no light goes through.

      You add one that is polarized at 45 degrees, and suddenly 1/8th of your orginal light is going

      You c
      • Actually now that you mention this, I've seen this analogy before. But that is how you measure the polarization of light. How do you measure the spin of a particle? Is there an equivelent polarized lense for spin? If I want to go to a lab and actually measure the spin of a photon, how do I do that? What tools do I use?
  • by elhedran (768858) on Thursday June 17, 2004 @03:12AM (#9449722)
    Normally I am not so pedantic but the poster repeatedly misrepresented what is happening in entanglement.

    4 times in the post it was said that the particles teleport or communicate, they don't.

    Its more like the particles are using the same day planner to decide what to do next.

    Think of it like to processes running the same code. if they have the same inputs, they will have the same outputs. It doesn't mean they communicate or teleport.

    The reason it bugs me so much when people talk as if the particles interact after they have been entangled is it leads someone sooner or later to start asking why we can't use that to beat the speed of light for communication, or a dozen other things that have nothing to do with entanglement.
  • by Anonymous Writer (746272) on Thursday June 17, 2004 @03:20AM (#9449752)
    Too bad I can't bloody understand any of it!
  • "Researchers using lab techniques can create a weird relationship between pairs of tiny particles. After that, the fate of one particle instantly affects the other; if one particle is made to take on a certain set of properties, the other immediately takes on identical or opposite properties, no matter how far away it is and without any apparent physical connection to the first particle." ---- Can it always be told beforehand (whether true for all cases) if the other will take identical or opposite propert
  • by Barkmullz (594479) on Thursday June 17, 2004 @03:34AM (#9449814)

    Having scientist using words like "spooky" and "weird" cannot be a good thing...

  • by jandersen (462034) on Thursday June 17, 2004 @03:35AM (#9449818)
    Well, actually I don't, but that's another matter.

    However, it seems that every time somebody mentions something about 'quantum' people around here go into Batman and Star Trek Mode.

    1. This whole thing is still very much in the early days of fundamental research. Think Babbage or Archimedes or something similar. I suspect that much of the hype about 'quantum computing' is simply a magical mantra that produces funding.

    2. There still is no such thing as teleportation, not even theoretically. Entaglement only means that you can get two objects to behave 'in step' even at a distance, but so far it has always involved that they start out together, ie. physically close to each other. Teleportation on the other hand is normally thought of as transporting mass from one point of space to another, sort of magically, without passing through the space and time that seperate the two points. There really isn't much chance of that ever making even theoretical sense.
  • by dcw3 (649211) on Thursday June 17, 2004 @06:39AM (#9450500) Journal
    So does this mean that all the future Windows Quanta PCs will go blue screen at the same time?

    I'm kidding...well, sorta.
  • by master_p (608214) on Thursday June 17, 2004 @07:36AM (#9450729)
    Others said that measurement of an entangled particle will make it loose its state (collapse of superposition), but how are we going to get information out of the quantum computer ? can we use the same way to successfully read the quantum state for communication ?

    After all, transmission of information in a computer circuit is no different than communication.

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