How Quantum Computers Fail:Quantum Codes, Correlations in Physical Systems, and Noise Accumulation http://www.ma.huji.ac.il/~kalai/Qitamar.pdf In the paper Gil Kalai presents 4 conjectures:
Conjecture 1: The process for creating a quantum error correcting code will necessarily lead to a mixture of the desired code word with undesired code words.
Conjecture 2: A noisy quantum computer is subject to noise in which information leaks for two substantially entangled qubits have a substantial positive correlation.
Conjecture 3: In any quantum computer at a highly entangled state there will be a strong effect of error-synchronization
Conjecture 4: Noisy quantum processes are subject to detrimental noise.
Moreover, the naturalness argument is pretty strange. Of course complexity arguments are useless if big constants dominate the run time, but, why is it justified to assume the constants are small enough? Is it to make our life simpler? Real life is seldom that simple.
1. QC needs qbits. 1 logical qbit needs many (tens to hundreds) of physical qbits.
2. We need Quantum Error Correction(QEC) to read cleanly from qbits. Current algorithms for QEC need about a 1000 logical qbits.
3. Current state of technology has only 50 - 200 qbits has very low signal to noise and the noise is inherent in the system (you cannot engineer it away)
4. The current systems, specifically Google’s, produce a sequence of numbers. It is possible to have a low complexity algorithm on classical computer which can produce similar sequences (i.e. pass the test for QC produced sequence)
4 is possible because reading from the qbits is noisy and such a noisy data can at best replicate a classical computer. So, the current state of the art, Noisy Intermediate State Quantum computers are not useful for any computation and we need to get to 1000+ qbits to achieve the supremacy. Of course, not everyone agrees that the noise cannot be engineered away or even if the noise is such a big deal.
edit: I really don't want to discredit the QC effort. Prove or disprove, there is a lot to learn.
Quantum computers are among some of the most complex pieces of engineering. They generally require a relatively sophisticated understanding of mathematics and physics, much of which is very non-intuitive (e.g., high-dimensional Lie groups, random probability distributions of probability distributions, low-temperature thermodynamics, ...). Piecing it all together requires careful threading. I think Gil’s notes don’t represent the pinnacle of rigorous care, but it is a useful “punch list” of broad considerations worth talking about.
I am not questioning your or anybody’s intuition directly or personally, but it’s easy—especially for the tech-savvy world of software engineers that frequent HN—to jump to conclusions based off of our intuitions about the mechanics and failure of (classical) systems and make (baseless) conclusions about contemporary quantum ones.
I think somebody like Scott Aaronson—who has made an academic career in computer science and quantum computing—would gladly accept reality were it not to be what he hopes it is, and would be entirely willing to stand on a platform and broadcast the truth. Aaronson is in the business of understanding fundamental truths about computation, not (as far as anybody knows) being a shill for big business.
 Disclaimer: I’m not deeply familiar business-wise with all the companies listed, so take my “existential tie” with a grain of salt.
 I’m not claiming that any of these companies would conduct business dishonestly in the event that they were “wrong”.
The Intelligence Trap is an excellent book that among other things explains how being very clever can make it harder to get at the truth. Formidable intellects can patch any holes that appear in their world view by constructing convincing and credible arguements.
"Science advances on funeral at time" paraphrased from Max Planck - the individuals he was thinking of were truth seeking super brains as well.
Do you think it will end up in the "20 years from now" category like many other technologies? I'm on the fence regarding how fast we can actually advance, QC is tough.
I've seen the same things said about fusion energy. Fusion is fairly simple, you can do it in your garage right now if you were so inclined, however getting net positive output is truly pushing the limits of what we know and can build. Quantum Computers have far more going on in comparison.
I’m “bullish” on the whole thing. In my opinion, it would be hugely unwise for anybody in the field right now to call it quits without seeing the technology through further. There are too many “interesting” developments happening.
I think that in about a year we will know the true nature of the Google supremacy claims as experts from many disciplines both stew and weigh in on the matter. If supremacy (continues to) hold up, that would be evidence against the “20 years away” sentiment.
I agree with what you said, QC has a lot more going on / going for it than fusion does. They both still reside in roughly the same “technological class”, but QC at least seems to be improving regularly with a combination of steady engineering as well as scientific breakthroughs.
I think you are right in your assessment that there is understanding to be advanced.