6 ms·
Two-qubit silicon quantum processor with operation fidelity exceeding 99%
- tines 5y ago99% doesn't seem very high. Does that mean that if you do 100 operations, 1 of them will fail?
- klodolph 5y agoThe "Quantum threshold theorem" is what you care about here: https://en.wikipedia.org/wiki/Quantum_threshold_theorem https://en.wikipedia.org/wiki/Quantum_threshold_theorem Basically, if the error rate on an individual gate is low enough, you can use the gate to construct larger (less efficient) gates with arbitrarily low amounts of error. 99%, as you say, is a bit low and you'd need impractically large circuits to use these gates. You want error correction with classical computers too, it just works differently.
- fsh 5y agoRegular CPUs don't have error correction. The logic gates simply don't make mistakes, even after trillions of operations. The reason is that the gates are digital. Manufacturing imperfections and noise don't matter, as long as the signal levels stay within their bounds. On the other hand, current quantum computers are analog. Each quantum gate will get the coefficients of its output states a little bit wrong, and after a few tens of operations only noise is left in the qubits. In principle, quantum error correction could be used to measure and compensate for these errors. But none of the technologies demonstrated so far are anywhere near good enough for this.
- version_five 5y agoNot my area but you hear about but flips due to cosmic rays, and I've heard of mainframes that run everything on two cpus as a form or error correction. I think your point stands, just pointing out that there are occasional errors in cpus
- klodolph 5y ago> The logic gates simply don't make mistakes, even after trillions of operations. The error rates are not zero. Low, but not zero. Trillions of operations is what, a split-second of runtime on a five-year-old GPU? "High-reliability" systems invariably use some form of error correction or error detection. You can do this at different levels of abstraction. At a low level, you can build redundant gates. At a company I used to work for, this was a product we sold--it would synthesize ICs from an HDL and incorporate error correction. (This particular feature forced the company to get ITAR export licenses.) At a different company I worked for, we did our error correction at a high level using software. We encountered hardware errors on a regular basis. I'm not even talking about ECC--I'm talking about CPU errors. The only reason why you can think that imperfections and noise don't matter is because there isn't much noise in your environment and you aren't dealing with enough data that you'll notice any errors. Deal with a large enough amount of data, enough CPUs, enough RAM, and error becomes a certainty. The "quantum computers are analog" line is at best profoundly misleading. If your definition of "analog" extends to quantum computers, then I'd say that digital computers are also analog. Which is not an incorrect thing to say.
- fsh 5y agoSure, if your CPU is controlling an airplane, it should better be redundant. Still, the CPUs inside your laptop, smartphone, or even compute server do their jobs pretty well without any error correction. The transistors inside a digital CPU are of course analog devices. However, the digital interpretation of the logic levels means that analog errors do not propagate between gates. If the voltage fluctuations are small enough, the probability of triggering a logic gate incorrectly becomes astronomically small. Analog computers (such as all current quantum computers) do not have this feature. Each operation adds a little bit of error to the quantity that is being processed. This could be the voltage in an electronic analog computer, or the state coefficients in a quantum computer.
- klodolph 5y ago> Each operation adds a little bit of error to the quantity that is being processed. This could be the voltage in an electronic analog computer, or the state coefficients in a quantum computer. This brings us back to the Quantum threshold theorem, which says that it's possible to design a quantum computer so that you can perform as many operations as you like without accumulating additional error, as long as individual gates in the quantum computer have a bounded amount of error. This is one of the reasons why thinking of a quantum computer as "analog" will lead you astray. Quantum computers are more like classical digital computers than like analog computers.
- anonymousiam 5y agoSEUs are more of a problem in space. CPUs used for space missions mitigate the errors in a few different ways. In the past, the feature size on the CPU substrate was large enough to absorb most cosmic radiation without incident. As the feature sizes have shrunk and the old foundries are no longer producing the old chips, many missions have adopted "multiple voting" architectures. Satellites often use "triple voted" processors. The Space Shuttle had a system with five votes. I'm not a quantum computing expert, but I wonder if a SEU would even matter in some parts of a quantum system. The bits are in an undefined state during operation anyway.
- abdullahkhalids 5y agoThey do. They have physical error correction in the form of majority-vote. Whichever way the majority of electrons go, is what the digital signal will be. Then the total number of electrons is above a threshold such that you basically get less than a handful of errors in years of operations if that.
- scythe 5y agoRealistic thresholds kick in around 99.9%, IIRC. So this is good, but not quite enough.
- awillen 5y agoThird on the front page and no comments about how this is a total misrepresentation and/or a huge leap? C'mon folks - this is where I come for due diligence on these kinds of things.
- nh23423fefe 5y agoSpam, for want of content, is no virtue.
- awillen 5y agoThis begs the age-old question - whose content is less valuable: the person who posts the substanceless comment or the person who replies to it with an equally substanceless comment pointing out that it's a substanceless comment? Ethicists may never come to a consensus (at least not until they can agree on who's worse between the person who ccs a huge number of people who should be bcced or the person who replies all saying that they should have bcced everyone and asking everyone not to reply all).
- lcnPylGDnU4H9OF 5y agoMaybe they're both undesirable.
- moralestapia 5y agoGood leap forward but much more than 99% is needed, at least 5-6 nines. Thing is that, as you add more qubits, "operation fidelity" (as they call it) goes down exponentially, maybe even superexponentially (?), so it's very hard to keep it within an acceptable range on larger systems.
- awillen 5y agoThat is a helpful summary - thanks!
- pyinstallwoes 5y agoSo is quantum uncertainty a feature or a bug?
- sva_ 5y agoConsider your 2-bit RSA keys to be 99% compromised.
- rafale 5y agoIf you could reduce the search space by 99%, it will still be too large.
- zauguin 5y agoIf you reduce the search space of 2 bit keys by 99% then there there are only 0.04 keys left. That's too small to find a single key, not too large.
- jwilk 5y agoThere are actually no 2-bit RSA keys. The smallest possible key is n=10, e=3. Conveniently, d=3 too, i.e. encryption and decryption is the same operation. I guess you can think of it as a ROT-13 of the RSA world.
- zauguin 5y agoThen all possible keys fit into the 0.04 bit remaining keyspace.
- alpineidyll3 5y agoAssuming the logic is gate free lol. For real to call two qubits a 'processor' has to be a new low. These reviewers should be in science prison.
- deleted 5y ago[deleted]
- DebtDeflation 5y agoIs it able to factor 21 without precompilation and other tricks?
- fsh 5y agoOnly if they figure out how to encode 21 with two bits.
- messe 5y agoThat’s trivial[1]: 11 = 21 10 = 7 01 = 3 00 = FileNotFound [1]: https://thedailywtf.com/articles/What_Is_Truth_0x3f_ https://thedailywtf.com/articles/What_Is_Truth_0x3f_
- bdamm 5y agoI know you're joking, but, this is not how Shor's algorithm works.
- messe 5y agoSure it is; with the right isomorphism. (Edit: I have a mathematical physics degree; everything I’ve said here is sarcastic/jokingly. The person I’m replying to is right, this is not how Shor’s algorithm works although I’d hope that was obvious from my original tongue in cheek comment)
- Maursault 5y agoThank you for this.
- upofadown 5y agoThose are physical bits. It is nowhere close to even a single logical bit that could do stuff like that. They are showing us what they can do. Now someone has to figure out how to turn it into something practical. I think the current goal is the factorization of 15. There is no reason to suggest insane levels of challenge like 21.
- KSPAtlas 5y agoI thought that said cubit for a second and got confused.