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Qbits unstable: May Limit Quantum Computing
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."
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.
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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 disappear, even without external influences. The degredation process is linked to the occurrence of quantum mechanical spontaneous symmetry breaking. In classical physics an equivalent example of this process is spontaneous crystallisation in a solution. At a certain position a crystal is spontaneously formed, as a result of which the fluid structure is broken.
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How do they know it isn't the observation technique that causes the 'instability'. If I read this right, observing the state causes it to disappear.
"Well..here I am..." - Jubal Early
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.
Didn't the Pentium have this problem too?
:-)
3 - 2 = 1.99999999999
sounds like something MS windows can deal with?
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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
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!)
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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 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...
Remember, open source is free as in speech, not free as in bear.
Shouldn't we at least have understood laws of physics that properly govern the quantam world, before we try to use it?
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 football field? The size of a toaster? the size of L.A.? [but of course, it could do the computing of all the 8086's on the planet CONCURRENTLY if only you could cul your answer from all the others it was spewing]
Alternatively, we could use conventional photolithographic techniques but find a way to load, execute and decode results for the entire computation in less then the decay time of a Qbit. Not many useful calculations can be done that fast. not many programmers can parallelize that thoroughly. Its a wierd space/time computing trade off where the shorter the entire computation can be made, the less x has to be in 64+x ECC.
SLASHDOT: news for people who can't concentrate on work or have no life at all and got tired of yelling back at the TV.
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.
My only political goal is to see to it that no political party achieves its goals.
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???
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.
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.
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?
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.
My only political goal is to see to it that no political party achieves its goals.
I'm guessing that by this they mean that this occurs in ion traps, optical cavities, and quantum dots. Nuclear spins could still work since they've been known to last vastly longer than these other methods. The trouble is that it's a pain in the ass to measure and manipulate single atoms. Some technological innovation might still be able to make them more manageable.
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, 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."
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 )
with a few quantum xor gates you could transfer the stored value to a 'fresh' qubit without needing to observe anything.
What I don't know is if any decoherence would get transferred as well. Does decoherence happen gradually, or is it a single event?
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.
"I may be synthetic, but I'm not stupid." -- Bishop 341-B
> "you can't build a computer if state information is going to evaportate in a second or less"
Ever heard of DRAM?
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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 will think of a way to sell such chips to us, and find ways to make em faster.
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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.
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 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, but that's how far off we are. Everyone just sit back and relax for a while on this one....
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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,
o s-ndd112904.php
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
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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.
Assembly is the reverse of disassembly.
If they ever build a quantum grid computer, they should make it 300 qbits long, 50 qbits wide, and 30 qbits high...
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They are not spontaneously losing information. Some dark matter entity is reading the QBits. Reading (copying) QBits destroys them, doesn't it?
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.
Can we get a "-1 Wrong" moderation option?
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?
So you see what had happened was....
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...