4 ms·

Yeah lots of people think decoherence will prevent QC from becoming practically useful. Some researchers seem to think differently, but I never found an explana

Yeah lots of people think decoherence will prevent QC from becoming practically useful. Some researchers seem to think differently, but I never found an explanation for why I should be as optimistic as they are.

ramraj07 4y ago
Which side is Scott Aaranson on? Just curious.
- dataflow 4y ago
  The only link I have handy is this comment from him in 2019 [1]. I feel like I've seen more in the past, but I can't find them now. [1] https://scottaaronson.blog/?p=4372from=timeline#comment-1822765 https://scottaaronson.blog/?p=4372from=timeline#comment-1822...
- mtlmtlmtlmtl 4y ago
  Doesn't he work on QC from the theory side? I assume he has at least some faith if he's spending his time on it. He's on Sean Carroll's podcast talking about this stuff but I can't remember what stance he took on it. I know Stephen Wolfram isn't too excited. But I believe his issue was more that algorithms end with "and then a measurement happens", which might be quite tricky and possibly even cancel out any potential gains. Not sure about the details of that, but it makes superficial sense given the nature of measurement in QM.
  - bowsamic 4y ago
    Why would quantum measurement be a problem in any way? Obviously measurement doesn’t destroy quantum effects, else we would not be able to see QM in the first place. Obviously you have to factor in what the measurement does to your quantum computer, but that’s obviously just part of your design in the first place
    - mtlmtlmtlmtl 4y ago
      I believe his point was more that the quantum measurement could very well end up costing enough time to be an issue. He didn't to into a great amount of detailed as it was mentioned more as a tangent.
      - codeflo 4y ago
        The sceptic arguments I read focus on accidental decoherence during the computation. If that goes well, and classical computers only "catch up" during measurement, wouldn't this mean that the time cost of measurement has to scale super-polynomially? Is that plausible?
        mtlmtlmtlmtl 4y ago
        I got tired of speaking on behalf of a vague memory of Stephen Wolfram speaking that I just looked up the transcript. 6 SW: Yeah, I think… I think it’s not going to be true[that QC gives a speedup], that’s my guess. I think what’s going to happen is, if you take short algorithm for factoring, which is primarily a quantum Fourier transform, that Fourier transform is done beautifully quickly because there are all these threads that are running in parallel. The problem is, every thread is somewhere in a different place in branchial space, that thread, that us observers, we have to corral all those things back together again in order to tell what actually happened, and that’s… So there’s a… Usually in quantum computing one just says, “and then there’s a measurement.” 1:26:49.0 SW: Now, in actuality, when you have an actual device, you have all kinds of issues in making that measurement, all kinds of… How quickly does it decohere, all these kinds of things. There are all these kinds of very practical experimental features, and I think people have generally said, given the formalism of quantum mechanics, it’s like, well, all this quantum stuff happens and then boom, we do a measurement, and the boom, we do a measurement is actually pretty difficult in practice with actual experiments, that people have said, but if we do these experiments well enough, it will become the mathematical idealization that von Neumann and others made about how measurement works, and I don’t think that’s going to be true. I mean, we’re not sure yet, but it seems likely that there will be no way to do… To sort of, if you’re honest about how the measurement works, the measurement takes effort.

ramraj07 4y ago

Which side is Scott Aaranson on? Just curious.

dataflow 4y ago
The only link I have handy is this comment from him in 2019 [1]. I feel like I've seen more in the past, but I can't find them now. [1] https://scottaaronson.blog/?p=4372from=timeline#comment-1822765 https://scottaaronson.blog/?p=4372from=timeline#comment-1822...
mtlmtlmtlmtl 4y ago
Doesn't he work on QC from the theory side? I assume he has at least some faith if he's spending his time on it. He's on Sean Carroll's podcast talking about this stuff but I can't remember what stance he took on it. I know Stephen Wolfram isn't too excited. But I believe his issue was more that algorithms end with "and then a measurement happens", which might be quite tricky and possibly even cancel out any potential gains. Not sure about the details of that, but it makes superficial sense given the nature of measurement in QM.
- bowsamic 4y ago
  Why would quantum measurement be a problem in any way? Obviously measurement doesn’t destroy quantum effects, else we would not be able to see QM in the first place. Obviously you have to factor in what the measurement does to your quantum computer, but that’s obviously just part of your design in the first place
  - mtlmtlmtlmtl 4y ago
    I believe his point was more that the quantum measurement could very well end up costing enough time to be an issue. He didn't to into a great amount of detailed as it was mentioned more as a tangent.
    - codeflo 4y ago
      The sceptic arguments I read focus on accidental decoherence during the computation. If that goes well, and classical computers only "catch up" during measurement, wouldn't this mean that the time cost of measurement has to scale super-polynomially? Is that plausible?
      - mtlmtlmtlmtl 4y ago
        I got tired of speaking on behalf of a vague memory of Stephen Wolfram speaking that I just looked up the transcript. 6 SW: Yeah, I think… I think it’s not going to be true[that QC gives a speedup], that’s my guess. I think what’s going to happen is, if you take short algorithm for factoring, which is primarily a quantum Fourier transform, that Fourier transform is done beautifully quickly because there are all these threads that are running in parallel. The problem is, every thread is somewhere in a different place in branchial space, that thread, that us observers, we have to corral all those things back together again in order to tell what actually happened, and that’s… So there’s a… Usually in quantum computing one just says, “and then there’s a measurement.” 1:26:49.0 SW: Now, in actuality, when you have an actual device, you have all kinds of issues in making that measurement, all kinds of… How quickly does it decohere, all these kinds of things. There are all these kinds of very practical experimental features, and I think people have generally said, given the formalism of quantum mechanics, it’s like, well, all this quantum stuff happens and then boom, we do a measurement, and the boom, we do a measurement is actually pretty difficult in practice with actual experiments, that people have said, but if we do these experiments well enough, it will become the mathematical idealization that von Neumann and others made about how measurement works, and I don’t think that’s going to be true. I mean, we’re not sure yet, but it seems likely that there will be no way to do… To sort of, if you’re honest about how the measurement works, the measurement takes effort.