[tagrun]

They're using a "clever trick" to approximately evaluate the overall gate from this paper 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]

> 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]

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]

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]

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).

[YeGoblynQueenne]

The theory of epicycles very accurately explained observed phenomena, and though the conditions for science at that time were very different, its popularity and accuracy very much comparable to those of quantum theory.

In the Hipparchian and Ptolemaic systems of astronomy, the epicycle (from Ancient Greek: ἐπίκυκλος, literally upon the circle, meaning circle moving on another circle[1]) was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets. In particular it explained the apparent retrograde motion of the five planets known at the time. Secondarily, it also explained changes in the apparent distances of the planets from the Earth.

(...)

Epicycles worked very well and were highly accurate, because, as Fourier analysis later showed, any smooth curve can be approximated to arbitrary accuracy with a sufficient number of epicycles. However, they fell out of favour with the discovery that planetary motions were largely elliptical from a heliocentric frame of reference, which led to the discovery that gravity obeying a simple inverse square law could better explain all planetary motions.

https://en.wikipedia.org/wiki/Deferent_and_epicycle

So the epicycles worked very well to explain and predict observations, but for reasons irrelevant to what really caused the motion of the planets. It's still possible that quantum mechanics will fall in the same way.

Note I don't have a horse in this race. I have no opinion on whether quantum mechanics is right or wrong.

[trhway]

>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.

sorry, i think you're doing a slew of hands here. Quantum computing relies not just on QM, it relies on Copenhagen interpretation of it - superposition being a physical reality, not just statistical description. That interpretation is tested by the Bell experiments and granted where have been a bunch of them which do look like confirming the Copenhagen.

>Even classical computers rely on it.

all that confirms QM, not the Copenhagen interpretation. Wrt. Google supremacy demonstration - it would work the same in statistical aggregate interpretation too thus actually not showing anything quantum computing.

[tagrun]

No. An interpretation is just that. No matter what interpretation you use, the experimental measurement results are the same.

If they don't give the same results, it won't be interpretations: you'd have two competing theories and one of them will be wrong since it can be ruled out experimentally.

This is also why majority of physicists don't care much about such philosophical aspects. You can argue that they should, are there are a few people working on foundations of quantum mechanics, but most physicists (including me) see it as semantics and choose to spend their time on practical physics. At least that's what my field (condensed matter physics) is about, which also encompasses the realization of these quantum computers. You can't change the conductivity of a material, or the measured charge state of a transmon qubit by using a different interpretation.

[trhway]

>No matter what interpretation you use, the experimental measurement results are the same

Sorry, no. Bell experiments do produce different measurements for different interpretations thus ruling one of them true. As it stands now they seems to confirm Copenhagen for pretty much everyone.

[tagrun]

Then I don't know which crackpot "interpretation" (that doesn't even agree with the experiments, unlike MWI etc) you are referring to, but you can rest assured that nothing in these experiments or condensed matter physics in general depend on it.

[mlyle]

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]

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]

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]

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.

[mlyle]

Right-- so if we need 10-1000 qubits per high-quality error corrected boolean-ish qubit and 20,000 error-corrected qubits to really tackle interesting real world problems, we need 3-5 orders of magnitude improvement.

The fact that we have exponential improvements to parts of the process is nice. But exponential improvement doesn't continue forever. Extrapolating from the current status 5 orders of magnitude out seems reckless.