
The Argument Against Quantum Computers and the Quantum Laws of Nature [pdf] - luu
https://gilkalai.files.wordpress.com/2020/04/laws-blog.pdf
======
Gil-Kalai
Hi everybody, thanks for the nice discussion. Here is
[https://youtu.be/_Yb7uIGBynU](https://youtu.be/_Yb7uIGBynU) a very nice panel
discussion about the Google supremacy claims moderated by Sandy Irani with
Scott Aaronson, Dorit Aharonov, Boaz Barak, Sergio Boixo, Adam Bouland, Umesh
Vazirani, and me (Gil Kalai). (This is related to Sections 5 and 6 in my
paper.)

~~~
camjohnson26
Thanks for keeping the paper so accessible. More experts need to write in a
way that’s readable to non experts to get them interested in the topic. It
takes confidence in your ideas to be willing to keep them simple.

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nabla9
More accessible lecture from Kalai: Noise stability, noise sensitivity and the
quantum computer puzzle – Gil Kalai – ICM2018
[https://www.youtube.com/watch?v=oR-
ufBz13Eg](https://www.youtube.com/watch?v=oR-ufBz13Eg)

How Quantum Computers Fail:Quantum Codes, Correlations in Physical Systems,
and Noise Accumulation
[http://www.ma.huji.ac.il/~kalai/Qitamar.pdf](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.

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anewvillager
Scott Aarson didn't comment on this paper specifically but gave a FAQ[0] on
his blog (he believes Google actually attained quantum supremacy). See also
this[1] when the announcement was done.

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.

[0]:
[https://www.scottaaronson.com/blog/?p=4317](https://www.scottaaronson.com/blog/?p=4317)

[1]:
[https://www.scottaaronson.com/blog/?p=4372](https://www.scottaaronson.com/blog/?p=4372)

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estevaoam
Check this video from minutephysics about Shor's algorithm:
[https://youtu.be/lvTqbM5Dq4Q](https://youtu.be/lvTqbM5Dq4Q)

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jacobush
Upvoting in the hope that someone will explain ... anything about this. The
comments so far seem to imply a level of understanding I don't have.

~~~
vaidhy
Just to be clear, everyone agrees on the advantages of QC and no one disputes
that Google has built a quantum computer. Let me try to paraphrase the
arguments.

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.

~~~
crazygringo
Thank you! That's extremely helpful.

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gnramires
My intuition points to Kalai being right. If that's the case, I believe the
greatest contribution of Quantum Computing will be advancing our understanding
of QM and Thermodynamics. Are we trying to build a modern version of Maxwell's
Daemon?

edit: I really don't want to discredit the QC effort. Prove or disprove, there
is a lot to learn.

~~~
reikonomusha
As a practitioner in the field of quantum computation, my advice is to be very
careful with intuition.

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.

~~~
ShorsHammer
> among some of the most complex pieces of engineering

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.

~~~
roenxi
Sometimes stuff that is 20 years from now happens in 20 years. They're up past
50 qbits and it looks like it might be another exponential growth process with
doubling time ~4 years [0].

[0] [https://s3.amazonaws.com/cbi-research-portal-
uploads/2019/01...](https://s3.amazonaws.com/cbi-research-portal-
uploads/2019/01/14180930/quantum-computing-qubit-timeline-1024x768.png)

~~~
fsh
Counting qubits is not a useful metric. Google actually had to go back down
from 72 to 53 in order to get their quantum supremacy experiment working. Also
none of these machines are error-corrected, so they cannot run useful quantum
algorithms such as Shor's or Grover's algorithm. It is still an open question
whether large error-corrected quantum computers are at all possible.

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seibelj
Even if true, the thought experiment of quantum computers and Shor’s Algorithm
are very interesting to think about. It was one of those “wow this is mind-
blowing” moments of my computer education when I understood its implications
[https://en.m.wikipedia.org/wiki/Shor%27s_algorithm](https://en.m.wikipedia.org/wiki/Shor%27s_algorithm)

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yetkin
This article does not refer to this one but uses the same title? What is the
relation between them? [https://spectrum.ieee.org/computing/hardware/the-case-
agains...](https://spectrum.ieee.org/computing/hardware/the-case-against-
quantum-computing)

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sgt101
To me his argument seems to be that he doesn't think that the errors in the
system are independent and therefore the fidelity is lower than claimed. This
begs the question - what is the implication of the errors in the system being
systematic?

