Comment by abdullahkhalids | Hacker News Reader

Comment by abdullahkhalids | Hacker News Reader

abdullahkhalids

> The most profound issue, however, concerns the meaning of quantum supremacy. After all, it doesn’t take qubits to solve important quantitative problems faster than any classical computer. Any carbon atom can “calculate” the solution of a very important practical problem—how does carbon behave?—simply by doing its thing.

The author [1] is trivializes the notion of computational machine. Calculating using a classical/quantum computer vs doing the experiment, differ in at least two important respects.

(1) Computers are fully programmable while experimental setups are usually specific to calculating a choice few parameters. The universality of various computational models implies exactly this: a well designed physical machine can simulate any other physical system, assuming you have the correct model of the system. A carbon-spectra calculating experimental setup does not have this universal programmability.

(2) A computer often yields the correct answer with a lot fewer resources, again assuming your model of the simulated system is correct. This is because, a well designed computation can ignore unneeded part of the model, which the experimental system must include. In fact, computers are precisely useful where we can get such an advantage, and useless where we can't. As the author points out this is not possible for some carbon atoms, and it also is not possible for medical drugs. But it is possible for a wide variety of other systems - say rockets to the moon.

[1] a Nobel prize winner, so take my words with a grain of salt.

I think he is well aware of this. The current quantum device Google build is however not much different than a sufficiently complex other physics experiment and so far they have not been able to demonstrate that it is capable of fault-tolerant error-corrected computation (which they basically won't be able to do in the foreseeable future).

Everyone agrees that Sycamore isn't fault tolerant, but I don't think many people agree that it is "not much different than a sufficiently complex other physics experiment". Here's a test: do you think there is any device that could be achieve quantum supremacy without being fault tolerant? If not, then your complaint isn't specific to Sycamore at all.

There are a lot of interesting applications that doesn't require a "computational machine".. While the carbon atom example seems excessively specific, there are other setups like D-Waves quantum computer that works in a sort of in-between way, with programmable links between "atoms" so you can setup a custom Hamiltonian that should be minimized by the computer.

That way you get sort of the best by both worlds, you get a (limited) programmability and you get easier observations than you would have access to just by for example designing a molecule encapsulating the problem you want to solve, as you would have to observe it "doing its stuff" somehow.

Just because it can't crack crypto doesn't mean a large-scale version of a setup like this can't be valuable. The field of quantum chemistry needs to use huge supercomputer clusters to simulate stuff like this.

The first aspect that you mention is actually not a disagreement but the whole point of his message - the recently popularized demonstrations of "quantum supremacy" did not allow for universal programmability, but instead were more comparable to an experimental setup for a particular problem of how do atoms behave in that setup.

Having a proper quantum computer with enough fully programmable bits to demonstrate quantum supremacy would be valuable, but (as the author states) the meaning of quantum supremacy is tricky, and we currently have achieved "quantum supremacy" only if we use a very limited, "cheating" definition of quantum supremacy.

> The first aspect that you mention is actually not a disagreement but the whole point of his message - the recently popularized demonstrations of "quantum supremacy" did not allow for universal programmability, but instead were more comparable to an experimental setup for a particular problem of how do atoms behave in that setup.

Where does Wilczek claim that? The only sentence I can see where he arguably comments on the programmability of Google's device is "the computation that Sycamore performed is very specialized". But this is before he brings up the carbon example, and he never says the programmability of Sycamore is comparable to the programmability of carbon.

Indeed, Sycamore is programable. It's obviously not capable of arbitrary-length computations, because the noise level is too high to be fault tolerant, but it has a certain accessible state space in which it can be arbitrarily configured. Here's Scott Aaronson if you don't believe me (emphasis mine):

> [Question:] Even so, there are countless examples of materials and chemical reactions that are hard to classically simulate, as well as special-purpose quantum simulators (like those of Lukin’s group at Harvard). Why don’t these already count as quantum computational supremacy?

> [Answer:] Under some people’s definitions of “quantum computational supremacy,” they do! The key difference with Google’s effort is that they have a fully programmable device—one that you can program with an arbitrary sequence of nearest-neighbor 2-qubit gates, just by sending the appropriate signals from your classical computer.

> In other words, it’s no longer open to the QC skeptics to sneer that, sure, there are quantum systems that are hard to simulate classically, but that’s just because nature is hard to simulate, and you don’t get to arbitrarily redefine whatever random chemical you find in the wild to be a “computer for simulating itself.” Under any sane definition, the superconducting devices that Google, IBM, and others are now building are indeed “computers.”

https://www.scottaaronson.com/blog/?p=4317

Therefore, I think abdullahkhalids's interpretation is right: Wilczek's carbon analogy is inapplicable.

Devil's advocate: if you make a computerized system for generating a lot of similar atomic arrangements with random variations, then can't you call the process of placing the atoms "programming"?

The more I think about the distinction people are drawing, the fuzzier it gets in my mind.

Yes, as long as you can achieve a complete gate set (over some N-qubit logical subspace) by arranging the atoms, and as long as the computation and read-out is reliable enough to get results, I think that's literally what it means to build a computer. (To formalize this, more needs to be said in terms of what the gate set is and how reliable it needs to be, but I think that's the basic idea.)

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