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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

#31

Post by neolefty »

sebzim4500 wrote: post #14
neolefty wrote: post #4

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. My question too. I've had a vague feeling about thi…

>It must get exponentially harder per qbit to eliminate thermal noise by cooling down closer to absolute zero Why? Cooling a large object is not exponentially harder than cooling a small object.

Not the size, but the temperature. If you have to cool to a microkelvin for a certain number of qbits to retain coherence, how low do you need to go to add one more qbit, and how much energy will that require?

My thermodynamic instinct says that the cooling effort required rises with the resolving power — which is exponential with the number of qbits. But it's just instinct, not grounded very well in science or engineering.

Re: What You Shouldn't Know About Quantum Computers

#32

Post by bee_rider »

threecheese wrote: post #28
bee_rider wrote: post #21

Earlier quoted context omitted.

When a prediction has a probability attached to it, it should be possible to check the math. In some sense it isn’t really a prediction about the future, so much as a statement about current information (which is not complete). Or, possibly, they are using “90% confidence” colloquially as “pretty sure.” If so, that should probably be made more clear. He’s using the fact that the researchers agree as an argument from…

It’s a quote, and you are gonna laugh when you find out who said it.

Attributed to Yogi Berra, I just don’t think it fits the situation here very well.

Re: What You Shouldn't Know About Quantum Computers

#33

Post by JanisErdmanis »

andrewla wrote: post #3

The article suffers from skipping between layers of abstraction in an attempt to make a point, and in the end in doing so fails to make the initial point. In the first "Myth" section, that "nobody understands this quantum stuff", the example is used of the transistor. It's true that we have no way of making a classical model of a transistor, and our understanding of how transistors work relies on quantum mechanics. B…

> But we did not invent transistors from quantum physics -- we created transistors long before we had an explanation of how they worked, and we have continued to improve and iterate by making advances in material sciences and experimentation, not by applying first principles. Things like blue LEDs were invented by tinkering rather than by solving Lagrangians.

In a similar spirit ships were built, steam engines made and applications of superconductivity imagined long before the objects were understood from first principles. Creation often proceeded theory nevertheless it is still useful for optimizing designs at later stages.

Re: What You Shouldn't Know About Quantum Computers

#34

Post by nyrikki »

renonce wrote: post #25

> For example, would quantum computers work by trying all possible answers in parallel? Sorry, no, that's too good to be true: Quantum computers work by choreographing a pattern of interference, where the contributions to the amplitude of each wrong answer cancel each other out, while the contributions to the right answer's amplitude reinforce each other. Only for special problems, as it turns out, do we know how to…

Neither NP or NP∩coNP, are contained in BQP, for those that want a complexity theory version of the above.

BQP: Bounded-Error Quantum Polynomial-Time, bounded by a max error of 1:3

BQP is the complexity class thought to contain problems with practical solutions for quantum computers.

IIRC the main limit being the transition amplitudes are subject to the Church–Turing thesis and must be computable functions.

Hopefully useful buzzwords for those who want to dig deeper.

Re: What You Shouldn't Know About Quantum Computers

#35

Post by tom-from-july »

bee_rider wrote: post #22

Is this really an appropriate use of arxiv? I thought it was for physics preprints. This might be a neat work, but it appears to be a 143 page blog post in a PDF (unless I missed the references section?)

The text also available as a printed book, so I guess a pdf of it fits any reasonable definition of preprint.

Re: What You Shouldn't Know About Quantum Computers

#36

Post by jessriedel »

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…

Co-author of the paper here.

The “largest integer factored” metric is terrible for several reasons, which is well recognized by researchers and is why people rarely publish those sorts of claims any more. Without net-positive error correction, it’s basically a function of how low your error rate is and how many bits that allows you to compute on with low chance of error, but as soon as the error rate crosses the fault-tolerance threshold then suddenly your storage is limited only by how many qubits you can build. So you can’t use this metric (if people were even publishing it) to predict anything after fault tolerance is achieved.

On more meaningful metrics, there has been steady progress.

Re: What You Shouldn't Know About Quantum Computers

#38

Post by jessriedel »

bee_rider wrote: post #22

Is this really an appropriate use of arxiv? I thought it was for physics preprints. This might be a neat work, but it appears to be a 143 page blog post in a PDF (unless I missed the references section?)

This is in the arXiv section called “physics and society” which is specifically intended for high-quality popular physics book, among other things.

Re: What You Shouldn't Know About Quantum Computers

#39

Post by pbhjpbhj »

Fwiw, there's an event I was just looking at from UCL on the "Future of Quantum Computing" (online webinar), https://www.eventbrite.co.uk/e/the-future-of-quantum-computi...

It's a panel headed by Prof Al-Khalili. I suspect it will be a beginner level presentation.

>Professor Jim Al-Khalili CBE FRS is a theoretical physicist at the University of Surrey where he holds a Distinguished Chair in physics and leads the Quantum Foundations and Technologies Research Group in the School of Mathematics and Physics. As well as his academic work he is a well-known popular science author and broadcaster on BBC radio and television.

No affiliation, just may be of interest.

Re: What You Shouldn't Know About Quantum Computers

#40

Post by wadadadad »

jessriedel wrote: post #36
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…

Co-author of the paper here. The “largest integer factored” metric is terrible for several reasons, which is well recognized by researchers and is why people rarely publish those sorts of claims any more. Without net-positive error correction, it’s basically a function of how low your error rate is and how many bits that allows you to compute on with low chance of error, but as soon as the error rate crosses the faul…

Which metrics would be more meaningful?
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