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They're using a "clever trick" to approximately evaluate the overall gate from this paper https://arxiv.org/abs/1807.10749 https://arxiv.org/abs/1807.10749 whic

by tagrun 7y ago

They're using a "clever trick" to approximately evaluate the overall gate from this paper https://arxiv.org/abs/1807.10749 https://arxiv.org/abs/1807.10749 which is computationally cheaper than doing a "brute force" simulation (which scales linearly in the number of gates), but it quickly becomes worse as you increase the number of gates. That's basically what it says. It looks like Martinis' group thought a "brute-force" simulation for 54 qubits is impossible, and this appoximate and "clever trick" is the only way to go at this number of qubits, but IBM says that with some different tricks, 54 qubits is still doable (I'm just guessing what they were thinking, and this is the only plausible explanation I can think of). Overall, a discussion which has nothing to do with quantum supremacy really... Whether it is a factor of a million or thousand though, the gap between a quantum computer will increase exponentially as the number of qubits is increased. This is fact, assuming quantum mechanics is correct. Actually, physicists have been trying to deal with this painful fact for quite a long time: it is also the reason why many body physics is so hard computationally and we spent almost a century to develop approximate methods to calculate even the simplest idealistic situations even with hundreds or thousands of atoms using density functional theory, quantum monte carlo etc etc. The whole idea of quantum computation is to turn this difficulty upside down and try to use it into our advantage.

sanxiyn 7y ago
> The gap between a quantum computer will increase exponentially as the number of qubits is increased. This is fact, assuming quantum mechanics is correct. I agree, but then there is no need to prove quantum supremacy after all. This entire business is about whether quantum mechanics is correct or not.
- tagrun 7y ago
  Quantum mechanics is about a 100 years old, and no violation has even been observed in laboratory, particle accelerators or outer space. The quantum theory is the most accurate theory we ever had in the history, tested to less than 1 in a billion precision. Even classical computers rely on it. Physicists don't have doubts about the quantum theory, we know it is possible, the problem is an engineering problem of attaining precise control over quantum systems, which is a very very hard engineering problem but there is nothing in physics which says it can't be achieved. There is still a need, but it is for an entirely different reason: not everyone (people with money and funding agencies, in particular) is physicist.
  - sanxiyn 7y ago
    General relativity is also 100 years old, no violation etc. Still, discovery of gravitational wave was very welcome, because test of general relativity in strong force regime was not very good. Quantum computing is analogous: test of quantum mechanics in "strong computational regime" is scant. You seem knowledgeable, but your comments on current claim of quantum supremacy is akin to, say, when claim of discovery of gravitational wave was made and then disputed, replying, "gravitational wave will be discovered, this is a fact assuming general relativity is correct, general relativity is 100 years old, no physicists doubt the theory" etc. All true, but rather pointless.
    - tagrun 7y ago
      It's a very different thing. I'm not just talking about the age of the theory. I'm talking about the length of the period during which it was tested so many times, to the level of precision that no other theory got tested and stood. General relativity was, and still has never been tested to anywhere near that level of precision, and that many times. And in fact, we still have strong reasons to doubt general relativity because there may or may not be deviations from it observed in galaxies and large scale universe. General relativity may be correct in that scale (with the ad-hoc addition of a cosmological constant) but to be consistent with those observations, one requires the existence of black holes, dark energy and and dark matter, things we never truly observed and don't know for sure exists (although it is our best explanation at this moment). We don't really understand how gravity behaves in very small scales, extremely large scales, or in the presence of very strong energy densities. One thing we know for sure is, general relativity is not the ultimate theory of gravity, it spectacularly fails in very small scales. We would like to stress-test all aspects of general relativity to 1 in a billion precision as well, but we can't. This is basically because gravity is very weak and you can't design all sorts of controlled experiments to test it. The best you can do is to make observations in the vicinity of readily massive things like Earth, Sun or a black hole, which you have no control over. You can't make two black holes, pit them together and see what happens in the lab. A situation very different from the quantum theory. Physicists did expect to observe gravitational waves, and it wasn't a shocker to anyone. The thing that makes is very big deal for physicists is that we now have a whole new way probing things that we couldn't before, in particular things which we don't understand yet, including the violations of general relativity which we do expect to see. We don't expect to see deviations in quantum theory (unless you bring a black hole nearby your quantum computer).
- mlyle 7y ago
  I don't think most critics are doubting quantum mechanics. The question is whether quantum computers can reasonably (as an engineering challenge, as a factor of cost, etc) be scaled and can be adapted to take on important real-world problems better than classical computer systems in practice. We are getting more and more proof points and eliminating a lot of the doubt but this has not been shown yet.
  - tagrun 7y ago
    Actually, we had people who claimed for about four decades that it is fundamentally impossible to have quantum speed up, basically equating it to a perpetual motion machine. We still have such famous people around (who aren't physicists, of course), now a loud minority, and "quantum supremacy" was coined because of/for them. What you're describing is mainly the new generation of people who grew up hearing about quantum computers on the news about experimental realization of small-scale (a few qubits) quantum computers.
    - mlyle 7y ago
      Eh, I haven't really heard them. I'm 40 and have followed this from near the beginning (starting with reading Science News in the late 80's-- even then the criticism was pretty muted). I am not positive we are going to get quantum computers with error correction on boolean qubits that can do all the meaningful tasks we hope quantum computers can do. I think it's more likely than not, but it is not close to happening and may never happen. I am not even 100% certain (but it is very very likely) that it is physically realizable. In my view, this current milestone is kind of contrived. And even if we do, it's not clear what subset of tasks currently performed on classical computers will be superseded by quantum computation. That's perhaps one of the biggest problems: normal computing has had a whole lot of use cases to pay the research and capital costs.
      - tagrun 7y ago
        Well, I'm envious that you didn't have to deal with those people. I am involved on the theory side of the implementation of different kinds of solid state qubits, so you may say I'm biased, but the question really isn't whether we will ever get it or not, the question is when. We already have had exponential growth in single qubit coherence times in the past decade, we have very good entangling gates, and there isn't any fundamental reason why the number of qubits can't be increased. It's not like there is an invisible great barrier ahead of us, and nothing in the physics of these devices say we can't. By the way, they aren't using quantum error correction methods right now, basically because it's not worth it: you need a lot of physical qubits to encode a high quality logical qubits.

by tagrun 7y ago

sanxiyn 7y ago

> The gap between a quantum computer will increase exponentially as the number of qubits is increased. This is fact, assuming quantum mechanics is correct. I agree, but then there is no need to prove quantum supremacy after all. This entire business is about whether quantum mechanics is correct or not.

tagrun 7y ago
Quantum mechanics is about a 100 years old, and no violation has even been observed in laboratory, particle accelerators or outer space. The quantum theory is the most accurate theory we ever had in the history, tested to less than 1 in a billion precision. Even classical computers rely on it. Physicists don't have doubts about the quantum theory, we know it is possible, the problem is an engineering problem of attaining precise control over quantum systems, which is a very very hard engineering problem but there is nothing in physics which says it can't be achieved. There is still a need, but it is for an entirely different reason: not everyone (people with money and funding agencies, in particular) is physicist.
- sanxiyn 7y ago
  General relativity is also 100 years old, no violation etc. Still, discovery of gravitational wave was very welcome, because test of general relativity in strong force regime was not very good. Quantum computing is analogous: test of quantum mechanics in "strong computational regime" is scant. You seem knowledgeable, but your comments on current claim of quantum supremacy is akin to, say, when claim of discovery of gravitational wave was made and then disputed, replying, "gravitational wave will be discovered, this is a fact assuming general relativity is correct, general relativity is 100 years old, no physicists doubt the theory" etc. All true, but rather pointless.
  - tagrun 7y ago
    It's a very different thing. I'm not just talking about the age of the theory. I'm talking about the length of the period during which it was tested so many times, to the level of precision that no other theory got tested and stood. General relativity was, and still has never been tested to anywhere near that level of precision, and that many times. And in fact, we still have strong reasons to doubt general relativity because there may or may not be deviations from it observed in galaxies and large scale universe. General relativity may be correct in that scale (with the ad-hoc addition of a cosmological constant) but to be consistent with those observations, one requires the existence of black holes, dark energy and and dark matter, things we never truly observed and don't know for sure exists (although it is our best explanation at this moment). We don't really understand how gravity behaves in very small scales, extremely large scales, or in the presence of very strong energy densities. One thing we know for sure is, general relativity is not the ultimate theory of gravity, it spectacularly fails in very small scales. We would like to stress-test all aspects of general relativity to 1 in a billion precision as well, but we can't. This is basically because gravity is very weak and you can't design all sorts of controlled experiments to test it. The best you can do is to make observations in the vicinity of readily massive things like Earth, Sun or a black hole, which you have no control over. You can't make two black holes, pit them together and see what happens in the lab. A situation very different from the quantum theory. Physicists did expect to observe gravitational waves, and it wasn't a shocker to anyone. The thing that makes is very big deal for physicists is that we now have a whole new way probing things that we couldn't before, in particular things which we don't understand yet, including the violations of general relativity which we do expect to see. We don't expect to see deviations in quantum theory (unless you bring a black hole nearby your quantum computer).
mlyle 7y ago
I don't think most critics are doubting quantum mechanics. The question is whether quantum computers can reasonably (as an engineering challenge, as a factor of cost, etc) be scaled and can be adapted to take on important real-world problems better than classical computer systems in practice. We are getting more and more proof points and eliminating a lot of the doubt but this has not been shown yet.
- tagrun 7y ago
  Actually, we had people who claimed for about four decades that it is fundamentally impossible to have quantum speed up, basically equating it to a perpetual motion machine. We still have such famous people around (who aren't physicists, of course), now a loud minority, and "quantum supremacy" was coined because of/for them. What you're describing is mainly the new generation of people who grew up hearing about quantum computers on the news about experimental realization of small-scale (a few qubits) quantum computers.
  - mlyle 7y ago
    Eh, I haven't really heard them. I'm 40 and have followed this from near the beginning (starting with reading Science News in the late 80's-- even then the criticism was pretty muted). I am not positive we are going to get quantum computers with error correction on boolean qubits that can do all the meaningful tasks we hope quantum computers can do. I think it's more likely than not, but it is not close to happening and may never happen. I am not even 100% certain (but it is very very likely) that it is physically realizable. In my view, this current milestone is kind of contrived. And even if we do, it's not clear what subset of tasks currently performed on classical computers will be superseded by quantum computation. That's perhaps one of the biggest problems: normal computing has had a whole lot of use cases to pay the research and capital costs.
    - tagrun 7y ago
      Well, I'm envious that you didn't have to deal with those people. I am involved on the theory side of the implementation of different kinds of solid state qubits, so you may say I'm biased, but the question really isn't whether we will ever get it or not, the question is when. We already have had exponential growth in single qubit coherence times in the past decade, we have very good entangling gates, and there isn't any fundamental reason why the number of qubits can't be increased. It's not like there is an invisible great barrier ahead of us, and nothing in the physics of these devices say we can't. By the way, they aren't using quantum error correction methods right now, basically because it's not worth it: you need a lot of physical qubits to encode a high quality logical qubits.