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Science Technology

Qbits unstable: May Limit Quantum Computing 73

Posted by timothy from the that-would-be-a-downer dept.
museumpeace writes "Netherlands Organiztion for Scientific Research provies a human-readable description of research into the stability of Qbits conducted at Leiden University. The bad news: " Much to their surprise they discovered that the coherence tends to spontaneously disappear, even without external influences." The whole story in physicist-readable form is in the June 17 Physical Review Letters by van Wezel, van den Brink, Zaanen [click abstract or huge PDF]. I am not buying any quantum computing startups 'til they nail this matter down...you can't build a computer if state information is going to evaportate in a second or less."
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Qbits unstable: May Limit Quantum Computing More

Qbits unstable: May Limit Quantum Computing

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  • So what? ECC & refresh! (Score:3, Insightful)

    by redelm ( 54142 ) on Monday July 11, 2005 @01:35PM (#13035144) Homepage
    I'm hardly surprised at quantum instability. It's inherent in the beast. But it doesn't much matter iff QC has big enough advantages. Put Error Correcting Code on all the busses and storage cells. Plus checkbits on the calcs as AFAIK is done today on P6/K7+ CPUs.

    The real question is how deep do you need to make the ECC. That depends on error rate, my guess is Hamming 64+8 ECC will do.

    • read the article. The biggest problem, and apparently a surprise to some, is that the instability is inversely proportional to the number of quantized states of particles, electrons or what-have-you that are aggregated to compose the Qbit. Hence all of the cool tricks we know have in our repetoire for minitatureization will work against us. In other words, your 64+8 ECC would not be adequate unless fairly bulky implementations [I know, we need some numbers here] of Qbits were used. An 8086 the size of a

    • Re:So what? ECC & refresh! (Score:5, Informative)

      by NonSequor ( 230139 ) on Monday July 11, 2005 @02:11PM (#13035544) Journal
      Classical ECC techniques won't work for quantum computing but they can be adapted. You can encode a single qubit across five qubits to protect against arbitrary errors (there are infinitely many possible errors) on any single qubit. You can get some protection against some errors that act symmetrically across a set of qubits by using decoherence free subspaces.

      The trouble with just using ECC to refresh constantly is that you have to approximate some of the quantum gates needed to perform the refresh. It's possible to approximate them to an arbitrary accuracy, but you'll still have some error at each refresh and this error will accumulate like error in a classical analog system.

      Decoherence free subspaces don't have this problem since there is no refresh phase for this technique. Basically you take advantage of the fixed points of the noise process and use a subspace spanned by these fixed points. The problem is, this technique only works in situations like sending a bunch of photons through a fiber optic cable that introduces the same error to all the photons.

      Right now, I'm suspecting that we will never see any long term quantum storage. However, if you can perform operations on your qubits fast enough you may be able to get a lot done in a few seconds.

      Research in QECC may still be able to provide us with some new tricks as well.
      • Error-correction in quantum algorithms is actually the key issue in future development of quantum computing. And, not only that, but you have to come up with a correction algorithm where the complexity scales polynomially with the size of the system. Also,

        It is a hard problem - even if we have years of theoretical research, the first succesful experiment that probed the real error correction was done only few months ago (see Nature - Dec 1 2004), or http://www.eurekalert.org/pub_releases/2004-12/ni o s-ndd [eurekalert.org]

    • You completely fail to understand. (Score:1, Informative)

      by Anonymous Coward
      ECC as done on current processors is completely useless with quantum bits. They entire point of quantum computing is that the bits are not in a single state.

      The rules of boolean logic that generate Hamming codes do not apply to qubits.

      There are quantum ECC techniques, but they're different and have their own issues.

      But this "hey guys, it's easy!" snap judgement shows profound ignorance.

    • DRAM! (Score:2)

      by mnmn ( 145599 )
      DRAM uses a clock that refreshes the memory constantly. As a result only one transistor and a capacitor is required per bit. The same can be done for qbits.

      However this will probably add to a similar latency as DRAM does vs SRAM. The only answer I can think of is using plain qbits (not refreshed qbits) in the cache of the CPU to speed it up, and hope most of the hits are while the data is good.

      Even better is the synchronous (async? I forgot) computer which doesnt use a clock at all.

      I think Intel and AMD

  • Nonsense. (Score:5, Funny)

    by FooAtWFU ( 699187 ) on Monday July 11, 2005 @01:36PM (#13035157) Homepage
    I'm running on a quantum computer right now, and I've not experienced and problems with any instacpqHeIkHBciBhAw 1uU6T1EK22qB9BBhokmNK6Ddv8CzpsgSEm HWn0CQEzPkDZJijN66jc/yy9Z3DBPguo1IqgWpSPMnqXAz4c8W f+2AVHipQWAsqw7QMZ7RO5k6Rr03cSM8d3uM+KdRTBV/q

    ++ATH
    NO CARRIER

    • Re:Nonsense. (Score:3, Insightful)

      by MrScience ( 126570 )
      That's the problem with quantum computers. Their uptime has no negative impact on you... until you start to actually observe their stability.

      • Re:Nonsense. (Score:3, Funny)

        by idontgno ( 624372 )
        root@heisenberg # uname -a
        Quantos heisenberg 2 6
        root@heisenberg # uptime

        uncertainty violation at 0x43c4df30
        kernel panic dumping core

    • Typical slashdot nerd. He has a quantum computer, but still connects via modem!
    • Lol. Poor fool must have been running IE and windows...

    • Re:Nonsense. (Score:2, Funny)

      by Tablizer ( 95088 )
      I'm running on a quantum computer right now, and I've not experienced and problems with any instacpqHeIkHBciBhAw uU6T1EK22qB9BBhokmNK6Ddv8Czps gSEm HWn0CQEzPkDZJijN66jc/yy9Z3D

      You solved it! That is the missing Perl reg-ex code for my masterpeice OS-in-a-wrist-watch! I'm complete now! Thank You Thank You!
  • A quantum computer makes use of the fact that a quantum mechanical system -an electron, an atom or even a larger system such as a superconducting quantum bit - can simultaneously exist in two states. Normally one of the two states disappears as soon as the system comes into contact with the outside world. The coherence then disappears as a result of the decoherence process and the information in a quantum bit is lost.

    Much to their surprise they discovered that the coherence tends to spontaneously disapp

  • TFA says it's related to the size of the qubit (Score:3, Funny)

    by bornyesterday ( 888994 ) on Monday July 11, 2005 @01:42PM (#13035216) Homepage
    Solution: get more cats.
  • Magnetic disk media is unstable. We work around it by encoding extra redundancy as needed.

    IANAQP, but is it not analogous that we can copy quantum state into multiple replicated locations, make the calculations happen multiple times, and compare results to ensure accuracy? This doesn't sound like a showstopper. It just makes it a little harder to design these things.

    • Re:Doesn't sound like such a big deal to me (Score:5, Informative)

      by exp(pi*sqrt(163)) ( 613870 ) on Monday July 11, 2005 @02:06PM (#13035488) Journal
      Qubits are not bits. If a bit is unstable then make lots of bits and use your favorite error correcting code to represent the data. Error correction is a hot topic for error-correcting codes too. But it's very much harder. In particular - the decay of a qubit to decoherence is exponentially rapid. By using error correcting codes you merely extend the decoherence time from something like picoseconds to dozens of picoseconds (those aren't exact numbers BTW, it might be femtoseconds or something else), but the exponential decay eventually wins. Classical systems can remain stable for millennia. (Egyptian hieroglyphs are encodings of classical bits.) Also, every paper I've ever read on quantum error-correcting codes makes assumptions about the form of the influences that causes decoherence. But real systems never fit these models exactly. Any deviation between reality and the model will again result in exponentially fast decay to decoherence. Many physicsts are totally sceptical about quantum computers, at least qubit based ones, for this reason. I personally think the decay of qubits is a showstopper.
    • You can't copy quantum states without destroying the original. Simple replication doesn't work. There are other tricks you can use, but nothing's perfect.

  • No big deal (Score:2, Funny)

    by zappepcs ( 820751 )
    Didn't the Pentium have this problem too?

    3 - 2 = 1.99999999999

    sounds like something MS windows can deal with? :-)
    • > Didn't the Pentium have this problem too?
      > 3 - 2 = 1.99999999999

      Here's a clue, the first one's free: the pentium FDIV bug had nothing to do with subtraction. And a lot of decimal fractions can't be represented perfectly in binary, giving you .99999 all over the place. It's not a Pentium thing when you see that, and in fact, x86's have BCD instructions designed to avoid exactly that sort of precision error.

  • Not necessarily (Score:3, Funny)

    by stienman ( 51024 ) <.adavis. .at. .ubasics.com.> on Monday July 11, 2005 @01:52PM (#13035308) Homepage Journal
    you can't build a computer if state information is going to evaportate in a second or less.

    If your quantum computer can calculate what you need to know within that period of time and still have time left over to read out the state, then I don't care how fast it evaporates.

    I'll still get the cryptokey.

    Of course, if it's proven that each time you create one it actually forms a micro universe of living creatures and progresses it millions of years before you kill it through apparant neglect, then you're going to have a problem with religious people.

    But you'd still have the key.

    Alternately, you'd have still gotten the message you set the secure channel up for.

    -Adam

  • explanation (Score:3, Funny)

    by hoggoth ( 414195 ) on Monday July 11, 2005 @01:52PM (#13035312) Journal
    > the coherence tends to spontaneously disappear, even without external influences.

    without external influences... from *OUR* universe... (eerie zither music ensues...)



    ( Zorg: Let's mess with their Qbits again.. hee hee)
    ( P'teem: Har!Har! Zorg! I never get tired of screwing up lesser beings!)

  • you can't build a computer if state information is going to evaportate in a second or less.

    Why not? DRAM state information evaporates much quicker, which is why there is DRAM refresh circuitry that cycles through it, reading each byte and writing it back out. Why can't the same thing be done for quantum memory.

    IANAQP, so I apologize if this is a dumb question.
    • You and other's suggest a refresh mech. But this is NOT a conventional memory bit with a definite 1 or 0 state that we are talking about. It is a Qbit. You destroy coherence by sampling or reading a quantized state. Its not clear to me in this case what "refresh" means.

  • Evaporation (Score:5, Funny)

    by masterzora ( 871343 ) on Monday July 11, 2005 @01:57PM (#13035387)
    you can't build a computer if state information is going to evaportate in a second or less. Why not? We Windows users are used to it...

  • I used to play with Qbits when I was a kid (Score:4, Funny)

    by NTT ( 92764 ) on Monday July 11, 2005 @02:17PM (#13035612) Journal
    I always did wonder about the stability of the purple fuzzy guy... I mean how did you know which way was up? Left actually went up and left meanwhile right went up and right and so on. Not to mention that nerve-racking sound when the springy green snake thingy grabbed him was awful. No wonder he is unstable. I would be too.

    Wait... did I read that right???
    • Very unstable. He spoke backwards, fer cryin' out loud.

      Nothing is quite so fun as an isometric game with a 8-way joystick. I picked up an implementation of Marble Madness for my kids' Gameboy, but it's just too darn hard to play. You can map the four diagonal directions to the 4 diagonals of the pad, which is highly-counterintuitive from a "muscle-memory" point of view (at least for someone with 20+ years of playing games with cursor keys), or you can map it to the 4 regular directions and constantly m
  • Let's see, aren't quantum computers supposed to operate at millions or billions or even trillions of operations per second? I'd say just make sure every single qbit gets USED within a second, and then store the next uncertain piece of data in it, for the next part of the overall calculation. That is, the solution to this problem is not to be found in hardware (besides MASSIVE parallelism to fill lots of qbits in minimum time), but in appropriate programming.

    • Re:"In A Second Or Less"? (Score:1, Informative)

      by Anonymous Coward
      The number of operations per second depends on what sort of qubit you're using. Some methods have a potential to go into the terahertz range, but others don't. The thing is, this is talking about elementary operations. It takes a lot of elementary operations to approximate some quantum gates so the "clock speed" is misleading. Also dealing with error correction will also take up some operations.

      The problem with this is that if we don't find a work around, there is an upper bound on how large a quantum comp
  • If qubits can occupy 12 dimensions, and may vibrate amongst all of them at the same time, can you be assured your information is going to remain the same over any given period of time? Can you prove that in any of the other eleven (simplisticly defined) dimensions someone else is not trying the same thing with the exact same qubit?
  • What I want to know though is museumpeace one of the masses who refuses to believe that physicists really are humans or is he a physicist who refuses to believe that he is of the same species as the common masses?

  • Quantum Computing Crash Course (Score:4, Informative)

    by NonSequor ( 230139 ) on Monday July 11, 2005 @03:13PM (#13036199) Journal
    Ok, lots of people still don't know what this stuff is about and I can't say I blame them since I've studied it and still don't get all of it.

    Ok, let's say you have a single qubit. Its state is described by a complex valued unit vector a|0>+b|1>. |0> and |1> is just shorthand for the vectors {1,0} and {0,1}. If you measure the qubit, the probability of getting a 0 is |a|^2 and a 1 is |b|^2.

    You may be asking why it's necessary to have a complex valued vector space. This is because quantum gates are represented by complex valued matrices. This means that you can have a gate that acts differently on sqrt(2)/2(|0>+|1>) and sqrt(2)/2(|0>+i|1>) even though they both have the same chance of coming up as 0 and 1.

    If you have a qubit in an unknown state you have no way of determining what a and b are. If you measure a qubit and it comes up as 0 then it's in the state |0> and if it's 1 then it's in the state |1>. You can also measure the qubit with respect to other bases. For example you can measure it with respect to |+>=sqrt(2)/2(|0>+|1>) and |->=sqrt(2)/2(|0>-|1>). The probability of getting |+> is equal to the absolute value of the square of the projection of the state vector onto |+>. If the result comes out as |+> then the qubit is in the state |+>.

    You can't copy qubits without destroying the original. However, you can entangle qubits together so that their values are dependent on eachother. Understanding the entanglement between qubits in a quantum algorithm is of critical importance and it really makes quantum algorithms a lot harder to understand than classical algorithms.

    Systems of two qubits are represented by vector spaces spanned by |00>,|01>,|10>, and |11>. Larger systems are represented similarly. Gates acting on multiple qubits are represented by unitary matrices (basically they map unit vectors to unit vectors). There are infinitely many quantum gates, but they can be approximated to infinite accuracy by using a handful of single qubit gates and CNOT gates. CNOT maps |00> to |00>, |01> to |01>, |10> to |11> and |11> to |10>.

    I hope that at least some of you can follow all that.
    • I hope that at least some of you can follow all that.

      Yes. Those of us that already understood qbits generally followed it.

      Those who did not already understand qbits were lost by the first | character.

      -

  • Oh god. (Score:1, Offtopic)

    by ravenspear ( 756059 )
    The quality of spelling/grammar on /. is so far down in the toilet that it needs to be flushed.

    "Netherlands Organiztion for Scientific Research provies a human-readable description of research into the stability of Qbits conducted at Leiden University. The bad news: " Much to their surprise they discovered that the coherence tends to spontaneously disappear, even without external influences." The whole story in physicist-readable form is in the June 17 Physical Review Letters by van Wezel, van den Brink,
  • Isn't that the same as the already known spontaneous quantum entanglement collapsing ? (i.e. one particle doesn't interfere in 2 slits anymore, although neither two entangled particles were measured )
  • If it weren't for "quantum mechanical spontaneous symmetry breaking", the universe would be one big Bose-Einstein condensate, if not still stuck inside a singularity. Q.C. is a great idea, but God seems to insist on rolling dice anyway.
  • > "you can't build a computer if state information is going to evaportate in a second or less"

    Ever heard of DRAM?
  • I am not a Quantum physicist, but surely if its stable for a second that should be long enough to copy it across onto conventional storage? You only need to look at Schrodinger's cat once to know its alive, anymore seems redundant.
  • Just use hamming codes and periodically correct the errors. It wouldn't be the first time we've had memory that needed to be refreshed on an interval.

  • vacuum tubes (Score:2, Insightful)

    by KurdtX ( 207196 )
    So what? Before vacuum tubes there really wasn't any way to save the state information on a magnetic charge (or whatever those things held) reliably, and then after years and years of using those, we got good and have been making the space to store a bit ever smaller.

    This is still experimental, so of course it's not consumer ready; ENIAC [wikipedia.org] was built in 1946, and we're not even there yet. I'm sure there are folks on Slashdot who will never get to use a quantum computer first-hand, which sounds depressing,
  • I suspect that any truly significant quantum computing is unworkable. Just as Bell's Inequality & the EPR paradox allow for action at a distance, but not for any meaningful transmission of information across time, I suspect that the apparently magical results from quantum computing will evaporate as we approach. My bet is that the problems of implementation will turn out to be fundamental and insurmountable.

  • Noah's Arq... (Score:3, Funny)

    by CptNerd ( 455084 ) <adiseker@lexonia.net> on Tuesday July 12, 2005 @02:17AM (#13039995) Homepage

    If they ever build a quantum grid computer, they should make it 300 qbits long, 50 qbits wide, and 30 qbits high...

  • Nearly all our computers are running DRAM, which suffers the same problem: bits disappear within a fraction of a second, unless refreshed periodically. It's just a technical problem which can/will be overcome if the payoff is high enough.

  • Quantum physics predicts the probability of something happening and quantum particles are also highly unstable. I think that by measuring the qubits they were probably changing the state hence leading to the data being lost. So...they need to figure out how to read it and not lose the data, or read and write the data at the same time to make sure it isn't lost. Or maybe I'm entirely wrong about this?
  • If you look at the time scale formula they give, it has a factor of absolute temperature in the denominator -- so you ought to be able to set up a problem and solve it (the whole idea is to compute fast!) before decoherence if you bring your ensemble down to, say, microkelvins...

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