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What You Shouldn't Know About Quantum Computers

arxiv.org

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Re: What You Shouldn't Know About Quantum Computers

#91

Post by sebzim4500 »

naasking wrote: post #69
sebzim4500 wrote: post #18

Earlier quoted context omitted.

>The final section was of particular interest to me; Gil Kalai's work on quantum error correction is very interesting to me and I am in the camp that believes that quantum computing is not possible in any useful sense; in particular a quantum computer will not be capable of being significantly more powerful than a classical computer, in the quantum supremacy sense. Maybe his arguments have improved over the decades.…

> (i) take it as a point of religious faith that noise will be magically correlated in order to break error correcting codes This possibility doesn't strike me as any more magical than QM's conjugate variables or its unusual correlations, which also seemed magical to classical physics. Arguably these + contextuality would make noise correlated with the system's configuration in some way, we just don't have a thorough…

But the reason people believe QM is because of overwhelming empirical evidence, not because it seems right.

You can't just go "QM has many surprising aspects" to "this other theory also has surprising aspects, it's probably true".

If we had been having this conversation before some of the more bizarre quantum effects had been observed it would have been a fair comparison, but we are way past that point.

Re: What You Shouldn't Know About Quantum Computers

#92

Post by sebzim4500 »

nyrikki wrote: post #58
drdeca wrote: post #29

Earlier quoted context omitted.

Huh? What are you saying is an exponential function? x^3 is not an exponential function, in the sense relevant here.

You aren't thinking about the exponential decay of emissivity near absolute zero. It isn't linear like we get to assume to make the math easier. Thus why IBMs largest refrigerator can only dissipate tiny amounts when cold. > enabling close to ~10 mW at 100 mK cooling power, and over 24 W of cooling power at 4 K temperatures. Finally, the weight of the entire system — 6.7 metric tons They aren't building single huge q…

It seems to me your argument hinges on the ratio between the surface area and the volume dropping to zero as the object gets bigger.

This is only true if the object enlarges in every direction equally. If it spreads out along a flat plane then the ratio is essentially constant, for example.

Re: What You Shouldn't Know About Quantum Computers

#93

Post by naasking »

sebzim4500 wrote: post #91
naasking wrote: post #69

Earlier quoted context omitted.

> (i) take it as a point of religious faith that noise will be magically correlated in order to break error correcting codes This possibility doesn't strike me as any more magical than QM's conjugate variables or its unusual correlations, which also seemed magical to classical physics. Arguably these + contextuality would make noise correlated with the system's configuration in some way, we just don't have a thorough…

But the reason people believe QM is because of overwhelming empirical evidence, not because it seems right. You can't just go "QM has many surprising aspects" to "this other theory also has surprising aspects, it's probably true". If we had been having this conversation before some of the more bizarre quantum effects had been observed it would have been a fair comparison, but we are way past that point.

> You can't just go "QM has many surprising aspects" to "this other theory also has surprising aspects, it's probably true".

I didn't say it's probably true because of this argument, you're saying it's probably false because it seems magical, and that's the implication I'm disputing with that analogy.

I do think it's plausible that noise could be correlated for the reasons I specified, but not because of the "magical" analogy.

Re: What You Shouldn't Know About Quantum Computers

#94

Post by jessriedel »

JohnKemeny wrote: post #90
jessriedel wrote: post #88

Earlier quoted context omitted.

Not really accurate. There are tons of tenured profs who are well-positioned to reveal reasons why QCs are fundamentally infeasible (and those kind of stories play well in the media). You can read about Gil Kalai's arguments here: https://www.quantamagazine.org/the-argument-against-quantum-... In any case, I'm happy to bet on this.

What would be a statement that you would put, say, $100k on, that would be determined within 2040. And which odds would you need on that?

The existence of a handful of logical qubits. Like I can initialize them, swap them, entangle them, measure them, etc., all performed with essentially perfect fidelity (say, 1e-10 error rate). I’d immediately bet $100k on that by 2040 at 50-50 odds, and probably 80-20 if I had a day or two to review the literature and confirm I wasn’t being adversely selected against.

If you wanted a useful machine, say factoring RSA-2048, you would need to push the date to more like 2050 or 2060.

Most of the uncertainty comes from the economy, engineering cost, public interest in QC, AI doom, etc. If we could somehow make a platonic bet on “is a large QC constructible if dedicated $10T/yr for 200 years”, then I am over 95-5.

Re: What You Shouldn't Know About Quantum Computers

#95

Post by jessriedel »

zeroonetwothree wrote: post #16
sampo wrote: post #7

> Researchers like Jaime Sevilla and Jess Riedel support this timeline, publishing a report in late 2020 that claimed a 90% confidence of RSA-2048 being factored before 2060. I am skeptical. 36 years is a long time, but in the past 10 years there hasn't been much progress: year 2001: factorization of 15 (IBM) year 2012: factorization of 21 (University of Bristol) year 2019: factorization of 35 attempt, failed (IBM) h…

It’s hard to make predictions, especially about the future.

We are quite clear in the abstract and the paper that the predictions are conditional on the continuation of the smooth progress seen to date. Contrary to the GP's interpretation, these predictions are holding up well.

Re: What You Shouldn't Know About Quantum Computers

#96

Post by vlovich123 »

jessriedel wrote: post #94
JohnKemeny wrote: post #90

Earlier quoted context omitted.

What would be a statement that you would put, say, $100k on, that would be determined within 2040. And which odds would you need on that?

The existence of a handful of logical qubits. Like I can initialize them, swap them, entangle them, measure them, etc., all performed with essentially perfect fidelity (say, 1e-10 error rate). I’d immediately bet $100k on that by 2040 at 50-50 odds, and probably 80-20 if I had a day or two to review the literature and confirm I wasn’t being adversely selected against. If you wanted a useful machine, say factoring RSA…

With that kind of statement, it seems fundamentally immature to be deploying PQC on the Internet today when we're 20+ years out a useful machine. Prognostication this far into the future should not be the basis of making changes today - we're not good at it.

Re: What You Shouldn't Know About Quantum Computers

#97

Post by vlovich123 »

jessriedel wrote: post #89
vlovich123 wrote: post #42

Earlier quoted context omitted.

I thought the error rate has stayed pretty exponential in terms of the number of physical qubits needed to express a logical qubit and it’s not actually known if we’re any closer on that metric vs other more easily achieved metrics. Additionally, I was under the impression that not all QCs being built are able of executing Shor’s algorithm which added additional challenges that aren’t solved. My final impression is t…

> I thought the error rate has stayed pretty exponential in terms of the number of physical qubits needed to express a logical qubit There is finite a threshold error rate (roughly 0.1-1%) at which you can produce a single logical qubit with an unbounded number of physical qubits (infinite overhead). For error rates below the threshold, the overhead becomes much less. People expect overheads in the thousands. See Fig…

> I can't really parse the claim, but I think your impression is wrong. Supremacy has always been a fuzzy bound, since it's defined in terms of the best known classical algorithms. But the supremacy results have gotten more unambiguous over time.

By that I mean that integer factorization is still slower than classical machines even though the numbers that can be factored have gotten larger. Similarly, with the exception of very specific toy problems specifically constructed to demonstrate quantum supremacy, we haven't achieved supremacy on any interesting and useful problems (not sure if DWave's quantum annealing machine really has any useful applications but presumably it must, but also not clear that it's a meaningful step on the path to a QC).

Re: What You Shouldn't Know About Quantum Computers

#98

Post by jessriedel »

vlovich123 wrote: post #96
jessriedel wrote: post #94

Earlier quoted context omitted.

The existence of a handful of logical qubits. Like I can initialize them, swap them, entangle them, measure them, etc., all performed with essentially perfect fidelity (say, 1e-10 error rate). I’d immediately bet $100k on that by 2040 at 50-50 odds, and probably 80-20 if I had a day or two to review the literature and confirm I wasn’t being adversely selected against. If you wanted a useful machine, say factoring RSA…

With that kind of statement, it seems fundamentally immature to be deploying PQC on the Internet today when we're 20+ years out a useful machine. Prognostication this far into the future should not be the basis of making changes today - we're not good at it.

Disagree. I still put a few percentage points of probability on it happening much faster, maybe by 2030 or 2035. The whole internet and a good chunk of the global economy is predicated on secure encryption. A small chance of that being disrupted is worth substantial investment. Deploying PQC protocols for testing is cheap.

Not to mention the fact that we still don’t know if the current candidate PQC protocols are actually secure. Security is mostly a game of back-and-forth over years, so it could take a while.

Re: What You Shouldn't Know About Quantum Computers

#99

Post by jessriedel »

vlovich123 wrote: post #97
jessriedel wrote: post #89

Earlier quoted context omitted.

> I thought the error rate has stayed pretty exponential in terms of the number of physical qubits needed to express a logical qubit There is finite a threshold error rate (roughly 0.1-1%) at which you can produce a single logical qubit with an unbounded number of physical qubits (infinite overhead). For error rates below the threshold, the overhead becomes much less. People expect overheads in the thousands. See Fig…

> I can't really parse the claim, but I think your impression is wrong. Supremacy has always been a fuzzy bound, since it's defined in terms of the best known classical algorithms. But the supremacy results have gotten more unambiguous over time. By that I mean that integer factorization is still slower than classical machines even though the numbers that can be factored have gotten larger. Similarly, with the except…

Those two sentences are correct, but don't really support your original points as I understand them.

Re: What You Shouldn't Know About Quantum Computers

#100

Post by NohatCoder »

jessriedel wrote: post #88
NohatCoder wrote: post #86

Earlier quoted context omitted.

The strong expert consensus has been reached between a lot of people who would be out of a job if their consensus was the opposite.

Not really accurate. There are tons of tenured profs who are well-positioned to reveal reasons why QCs are fundamentally infeasible (and those kind of stories play well in the media). You can read about Gil Kalai's arguments here: https://www.quantamagazine.org/the-argument-against-quantum-... In any case, I'm happy to bet on this.

Yes, there are people who don't agree with your consensus, that is part of my point, the consensus is not all that universal. And the other part of my point is that if you only count people who have a full time job developing quantum computers as experts then you get a massive bias in opinion, as the sceptics are more likely to pick different jobs.

As for my own belief, I don't know how it will pan out, but I'm inherently sceptical of people who are too certain that it is one way or another, the hard evidence simply does not exist yet.

I'm personally most inclined to believe that the universe is computationally classical, and while quantum physics necessitates that there must be quite a lot more information and processing happening than a Newtonian model requires, it is still a finite, thus putting a finite limit on what speed-up we can gain from a quantum computer. Most likely this pans out as the error correction limit being impossible to reach, but there is also the possibility that more complicated quantum states result in higher per-gate error rates.

I'm open to other conjectures, but if you want to dispel mine I would like to see some hard evidence.

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