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I just don't understand how Quantum Computing could work. It seems like the premise is QC defies the premise of QM.

meltyness 4y ago
It's about - finding and implementing in hardware an "embedding" which solves certain classes of problems efficiently, especially for which there is no known "efficient" process otherwise. - finding an elegant way of describing those classes of problems and - routes for identifying them. I think [0] Schwartz-Zippel as applied to Polynomial Identity Testing is an instructive example where using noise as a computational resource improves efficiency. [0] https://en.wikipedia.org/wiki/Schwartz%E2%80%93Zippel_lemma https://en.wikipedia.org/wiki/Schwartz%E2%80%93Zippel_lemma
halpmeh 4y ago
QC works by the exploiting statistical properties of an algorithm to infer something about the subject through quantum computation. Writing quantum algorithms is hard because you can rarely write your algorithm in a way that directly deals with the property you want information on. Rather, you need to write an algorithm that yields your desired information as a biproduct of some other computation.
jerf 4y ago
The premise of QC is that QM is essentially correct, to the n'th degree. We know that QM is "wrong" in the sense that it isn't the Grand Unified Theory of Everything because it does not merge properly with Relativity, but we respect the Correspondence Principle to hold for it in the relevant domain. Ignoring the substantial engineering challenges, if QC doesn't work, we would learn something very interesting about our world in how it breaks, because any break would represent a deviation between the predictions of QM and reality, something we desperately need to get to the next level of theory. Contrary to what some people might think, physicists would be giddy over a clean, lab-reproducible, engineering-type break in QM. They are starved for data right now. To be able to push a button and reproduce it is an almost impossibly beautiful dream. Almost for that reason, I expect our problems in QC to remain firmly in the engineering domain, and that no matter how many qubits we manage to deploy they will obstinately perfectly conform to QM theory.
- vba616 4y ago
  I'm completely unqualified to hold an opinion, but it seems so intuitive to me that quantum computing is theoretically sound but impossible in practice because error correction will scale exponentially in difficulty. Experts are clearly more optimistic, especially Scott Aaronson who is often mentioned on HN, but I can't get past a couple of simplistic, non-mathematical objections. One is that it seems already like quantum computing is on a different trajectory than classical computing. If it was an analogous engineering problem, why haven't we already gotten much farther? The other is that it just seems like a matter of symmetry. Quantum computing, even if it's not all powerful, seems like a "cheat code" for reality, and an inability to practically exploit it seems to me like balancing it out. I believe in a vague heuristic that reality has no "thread" that can be pulled to unravel everything. I guess the counter to these vague feelings are that you could've said the same stuff about nuclear fission/fusion and any technology based on quantum theory. So I'm not that confident in my opinion, but I'm still seeking an understandable reason why one should be optimistic. Nothing I've read by experts translates their feelings to my understanding. The sort of situation that seems intuitive to me, I don't see why it would result in any earthshattering violation of theory either, as we'd just approach an asymptote forever. P.S. I have a similar attitude towards practical fusion power for somewhat similar reasons, so that's evidence it's an attitude problem more than anything.
  - K0balt 4y ago
    QC is already a practical computing system that is solving “practically unsolvable” problems. It’s just not highly publicized because much of the work is relevant to state security. The field is a bit farther along than one would infer from contemporary information and publicly published papers. If you think about it even a little, it is almost axiomatic to the intelligence/security world that a machine that could break much known encryption in human relevant timeframes would obviously be being developed under a shroud of secrecy at the bleeding edge. It is far too important to be able to manage the implications of any significant breakthroughs potentially years before public release. QC is not magic and there are effective counter strategies. But standards take time to implement and the well being of society demands a that the impacts of QC be mitigated through control of the tech and the information around it. Fortunately, the tech required so far keeps access to the computing resource somewhat restricted, and organizations that cooperate with governments control it so far. For most people, the important thing to keep in mind is that everything now shielded by many forms of encryption will be trivially readable in the foreseeable future. Truly sensitive data stored long term should be moved to Q hard algorithms and old versions destroyed as possible (easier said than done in many cases)

It's about - finding and implementing in hardware an "embedding" which solves certain classes of problems efficiently, especially for which there is no known "efficient" process otherwise. - finding an elegant way of describing those classes of problems and - routes for identifying them. I think [0] Schwartz-Zippel as applied to Polynomial Identity Testing is an instructive example where using noise as a computational resource improves efficiency. [0] https://en.wikipedia.org/wiki/Schwartz%E2%80%93Zippel_lemma https://en.wikipedia.org/wiki/Schwartz%E2%80%93Zippel_lemma

halpmeh 4y ago

QC works by the exploiting statistical properties of an algorithm to infer something about the subject through quantum computation. Writing quantum algorithms is hard because you can rarely write your algorithm in a way that directly deals with the property you want information on. Rather, you need to write an algorithm that yields your desired information as a biproduct of some other computation.

jerf 4y ago

The premise of QC is that QM is essentially correct, to the n'th degree. We know that QM is "wrong" in the sense that it isn't the Grand Unified Theory of Everything because it does not merge properly with Relativity, but we respect the Correspondence Principle to hold for it in the relevant domain. Ignoring the substantial engineering challenges, if QC doesn't work, we would learn something very interesting about our world in how it breaks, because any break would represent a deviation between the predictions of QM and reality, something we desperately need to get to the next level of theory. Contrary to what some people might think, physicists would be giddy over a clean, lab-reproducible, engineering-type break in QM. They are starved for data right now. To be able to push a button and reproduce it is an almost impossibly beautiful dream. Almost for that reason, I expect our problems in QC to remain firmly in the engineering domain, and that no matter how many qubits we manage to deploy they will obstinately perfectly conform to QM theory.

vba616 4y ago
I'm completely unqualified to hold an opinion, but it seems so intuitive to me that quantum computing is theoretically sound but impossible in practice because error correction will scale exponentially in difficulty. Experts are clearly more optimistic, especially Scott Aaronson who is often mentioned on HN, but I can't get past a couple of simplistic, non-mathematical objections. One is that it seems already like quantum computing is on a different trajectory than classical computing. If it was an analogous engineering problem, why haven't we already gotten much farther? The other is that it just seems like a matter of symmetry. Quantum computing, even if it's not all powerful, seems like a "cheat code" for reality, and an inability to practically exploit it seems to me like balancing it out. I believe in a vague heuristic that reality has no "thread" that can be pulled to unravel everything. I guess the counter to these vague feelings are that you could've said the same stuff about nuclear fission/fusion and any technology based on quantum theory. So I'm not that confident in my opinion, but I'm still seeking an understandable reason why one should be optimistic. Nothing I've read by experts translates their feelings to my understanding. The sort of situation that seems intuitive to me, I don't see why it would result in any earthshattering violation of theory either, as we'd just approach an asymptote forever. P.S. I have a similar attitude towards practical fusion power for somewhat similar reasons, so that's evidence it's an attitude problem more than anything.
- K0balt 4y ago
  QC is already a practical computing system that is solving “practically unsolvable” problems. It’s just not highly publicized because much of the work is relevant to state security. The field is a bit farther along than one would infer from contemporary information and publicly published papers. If you think about it even a little, it is almost axiomatic to the intelligence/security world that a machine that could break much known encryption in human relevant timeframes would obviously be being developed under a shroud of secrecy at the bleeding edge. It is far too important to be able to manage the implications of any significant breakthroughs potentially years before public release. QC is not magic and there are effective counter strategies. But standards take time to implement and the well being of society demands a that the impacts of QC be mitigated through control of the tech and the information around it. Fortunately, the tech required so far keeps access to the computing resource somewhat restricted, and organizations that cooperate with governments control it so far. For most people, the important thing to keep in mind is that everything now shielded by many forms of encryption will be trivially readable in the foreseeable future. Truly sensitive data stored long term should be moved to Q hard algorithms and old versions destroyed as possible (easier said than done in many cases)