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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."
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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...
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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.
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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.
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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???