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Latest Research on Quantum Computing 33

zeristor writes "The The Economist is running a story about the latest progress in Quantum computing. It seems that what has been glossed over in Physics as a minor detail, the decoherence of the superposition of states, is actually quite fundamental to Quantum computing. The decoherence can be measured by something called the Loschmidt echo (is this esoteric or am I just thick? This sounds like a bad episode of Star Trek.) Also goes on to explain how entanglement can be prolonged. All in all very interesting developments."
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Latest Research on Quantum Computing

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  • Come on trolls, you're getting slow. It's the second article down the page. I've never seen a quantum computing article without somebody making the obligatory "It will even run Duke Nukem Forever!" joke.
    • Duke 4ever on quatum computers? That isn't funny. To be funny something has to have at least a bit of realisme in it. Thinking that Duke4ever will be released before QC are obsolete is just ridiculous.
    • The article itself provides all required humor:

      ...bang-bang pulses, at regular intervals can serve not only to suppress decoherence, but also to maintain entanglement...

      ...spontaneous emission, another bizarre quantum effect...


      Of course, most physicists know they need only refer to their email in-boxes to find several offers for products which claim to help maintain entanglement, suppress decoherence, avoid spontaneous emission, and increase their ability to perform bang-bang pulses at regular int
  • by Eevee ( 535658 ) on Monday April 05, 2004 @11:29AM (#8769570)

    We won't know until somebody reads the article and actually understands what it means.

    I found it interesting that something that sounds quite fundamental to quantum physics has been passed over for so long with a 'And then something happens'.

    • I thought this was old news... American computer company(Alientel) supposedly created a storage device based on (I think) the same principle. Something like 12terrasomething. I found this after typing a google search for "Microsoft back engineering alien technology". I did as a joke but what I found was boyh credible and frightening. The guy who founded the company says he learned the concept from a drawing that NONE of the scientists at Bell knew anything about(he worked there at the time..1940's/50's). He
  • by Anonymous Coward on Monday April 05, 2004 @12:12PM (#8770069)
    FOR evidence of the power of simplicity, you need look no further than a computer. Everything it does is based on the manipulation of binary digits, or bits--units of information that can be either 0 or 1. Using logical operations to combine those 0s and 1s allows computers to add, multiply and divide, and from there go on to achieve all the feats of the digital age. But at each step of the complex operations involved, each bit has a definite value.

    The same cannot be said of many properties in quantum physics, such as the spin of an atomic nucleus (loosely speaking, which way it is pointing) or the position of an electron orbiting such a nucleus. At a small scale, such properties can have more than one value at once. In 1994, Peter Shor, a mathematician then at AT&T's Bell Laboratories in New Jersey, realised that a computer that used such quantum properties to represent information could factorise large numbers extremely quickly. This is an important problem, because much of modern cryptography is based on the difficulty of factorising large numbers--so being able to do so quickly would render many modern codes easily breakable. Then, in 1996, a colleague of Dr Shor's at Bell Labs, Lov Grover, showed that such a quantum computer would be able to search through an unsorted database much faster than an ordinary computer--another important application.

    Computer technology

    With these insights, quantum computing, which had first been thought of as a possibility in the early 1980s, became a hot topic of research. It was clear to many physicists that using "qubits"--which, unlike ordinary bits, can exist in a "superposition" of the values 0 and 1 simultaneously--might yield an exponential improvement in computing power. This is because a pair of qubits could be in four different states at once, three qubits in eight, and so forth. What Dr Shor and Dr Grover showed was that the improvement, if the technological hurdles could be overcome, would be not hypothetical, but real, and useful for important problems.

    The technology necessary to manipulate qubits, in their various incarnations, is challenging. So far, nobody has managed to get a quantum computer to perform anything other than the most basic operations. But the field has been gathering pace, and was the topic of much discussion among the scientists gathered in Montreal for the annual March meeting of the American Physical Society, the largest physics conference in the world.

    There are currently several different approaches to quantum computing, all of which rely on fundamentally different technologies, including ultra-cold ions that are cooled by lasers, pulses of laser light, nuclear-magnetic resonance and solid-state devices such as superconducting junctions or quantum dots (which are confined clouds of electrons). What all these technologies have in common is that they can be used to invoke and exploit the bizarre phenomenon of superposition.

    Superposition is not simple. Though a qubit may, for a while, be in a state of superposition between 0 and 1, it must eventually choose between the two. And in even the best quantum computers, that choice, or "decoherence", happens in a fraction of a millisecond. Just how the choice is made, and how to prolong the preceding period of "coherence" that allows quantum computations to be made, constitute a long-unexplained gap at the heart of modern physics. For nearly 80 years, since the inception of quantum theory in the 1920s, most physicists were content to gloss over the process. What is perhaps surprising is that the technological challenge of quantum computing is now a driving force behind efforts to understand the most abstract and philosophical underpinnings of quantum mechanics.

    Echoes of the future

    Until a qubit interacts with the macroscopic world, which follows the classical laws of physics, it behaves according to the laws of quantum mechanics, which are well understood, at least by physicists. However, the interaction with the classical world--decoherence-

    • Though a qubit may, for a while, be in a state of superposition between 0 and 1, it must eventually choose between the two.

      Just a quick layman question on the above. It is my understanding that subatomic particles exist in two or more states until an intrusive event happens (such as observation) and it is that event that forces the particle to "choose". Is this correct or am I mis-reading?
      • If I remember this correctly the particle's wavefunction is either already in one of its eigenvalues, or not.

        If it's not it is undecided prior to observation in what energy state it is, but it will collapse into one of the eigenvalues.

        The particle must be in an eigenvalue to produce a standing wave solution to the Schrodinger equation, or else the particle would vanish.

        So yes, in layman terms, the particle chooses one energy state when you observe it.
      • Yeah, that's one way to interpret it. But trying to understand what's actually happening on the quantum level really gets metaphysical and violates common sense.

        Another, in some ways more rational way to explain it, is that a qubit is in both states, and that the qubit is always in both states. It's just that *you* are also in both states, and when a "meaurement" happens that gives a single value for the qubit what happens is that you in one state see one result for the qubit, and you in another state see
        • The most rational explanation I've come up with so far is that we are actually existing inside of a simulated reality. If you were going to simulate a universe but only populate it with a couple of billion people, you would do much better to only pay attention to what each individual sees. That way, you could ignore the virtually infinite processing and memory requirements, and just have a processor per person, and interconnections between people in order to make the experience consistent. So Quantum Phy
          • while (sig==sig) sig=!sig; ::cringe::
            I just thought of an actual use for that code. Assume sig points to an audio port. The output tone would then indicate the load on the system. Owwwwww, owwww, pain, pain, lol.

            As for Quantum weirdness, it kinda makes sense to me. It's hard to describe what I picture, but I'll take a stab at it. Our entire perceived reality would just be a single point or surface in a much larger system or reality. When it seems a quantum value has more than one value, it really has both
  • by junkmail ( 99106 ) on Monday April 05, 2004 @12:22PM (#8770159)
    Ok, lets consider two dice to be our collections of qubits. They can each hold the superposition of the numbers 1 - 6. Shaking and throwing the dice cause the superposition of the values and decoherence happens when they come to rest.

    The question is, what do we use as 'bang-bang' pulses in order to keep the dice from decohering until we can coerce them into making our point?

    1. ???
    2. Go shoot craps
    3. Profit!
  • by jo42 ( 227475 ) on Monday April 05, 2004 @12:31PM (#8770254) Homepage

    Why don't you send me a quantum echo from the future when this is all running nicely...
    • The Loschmidt echo is what you hear in the state of decoherence. Typicaly it is similar to human voice (quite like yours, but very remote and slow)saying "and make it double, will you"
  • ....to understand quantum mechanics, apparently. "Quantum" is not a proper noun, not even in the context of quantum mechanics!

    -psy
  • by exp(pi*sqrt(163)) ( 613870 ) on Monday April 05, 2004 @05:14PM (#8773243) Journal
    what has been glossed over in Physics as a minor detail, the decoherence of the superposition of states, is actually quite fundamental to Quantum computing
    This has never been a minor detail in quantum computing. People who think quantum computers won't go anywhere (like me) have been arguing that decoherence will kill any quantum computer with more than a handfull of bits. On the other hand, most (maybe even nearly all) papers I've seen on quantum computing recently have been about using error-correcting codes to fight decoherence.
    • On the other hand, most (maybe even nearly all) papers I've seen on quantum computing recently have been about using error-correcting codes to fight decoherence.

      By, like recently, do you mean since 1995? [arxiv.org] Quantum error correction (or more generally, the theory of fault-tolreant quantum computation) is one of the most surprising discoveries of the last decade. The next few years are the years where these ideas finally get tried out in the lab. The coming of the quantum machines has begun.

  • Eigenvalues (Score:3, Informative)

    by manganese4 ( 726568 ) on Tuesday April 06, 2004 @01:19AM (#8776996)
    A number of threads have mentioned eigenvalues/eigenstates and how a system is represented by them.

    Eigenvalues and eigenstates have meaning when in terms of an operator which represents the perturbation to or observation of the system.

    Every operator has a characteristic set of eigenvectors. Every quantum system is described by a wavefunction and prior to a pertubation/observation this wavefunction can be described as a linear combonation of the eigenvectors of the operator.

    Following a perpurbation/observation decribed by the operator, the quantum system will be described by a one and only one eigenvector of the operator.

    Of course, the probability of a particular eigenvector being chosen is represented by the square of the eigenvalue of the eigenvector.

    for more see wikipedia [wikipedia.org]
    • Looking around on the web I found this [qubit.org] rather good site on Quantum Computing.
      This Loschmidt echo thing seems to be buried quite deep, I have not found it referenced in my Quantum Mechanics books.

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