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A Reality Check on Quantum Computers

wsj.com

11–20 of 36 posts

Re: A Reality Check on Quantum Computers

#11
post #7

What does this article really ‘reality check’? The editor’s understanding of quantum computing?

The pop-sci headlines hinting that quantum supremacy shenanigans means that we are close to some new paradigm of general purpose quantum computers. I suppose.

Re: A Reality Check on Quantum Computers

#13
post #7

What does this article really ‘reality check’? The editor’s understanding of quantum computing?

I think the intent is to reality check some of the marketing coming out of Google/IBM/Intel/Rigetti that seem to be trying to imply that quantum computers actually exist today (as opposed to laboratory science experiments which are what actually exist now) and are just a few years away from commercialization after which some form of Moore's Law will take hold. I've seen a number of Pop Sci articles that say things like "today's quantum computers fill an entire room like 1960s mainframes" or "today's leading edge quantum computers contain 53 qubits, experts say we'll need n-qubits to solve x problem which is intractable on a classical computer". All of this is intended to imply we just need to figure out how to shrink these existing quantum computers down while expanding the number of qubits available and we're about to have a quantum repeat of the PC Revolution. Nothing could be further from the truth.

Re: A Reality Check on Quantum Computers

#14
I'm disappointed that this article doesn't answer my only real question about quantum computers: What does a reasonable expectation look like? What is the range of potential performance gains?

I understand that this is the frontier and no one has any definite answers, but I hear everything from "trivial" improvements to "insta-crack AES-256" (thousands of trillion trillion trillion trillions times faster).

Does anyone have a well informed upper/lower bound estimate?

Re: A Reality Check on Quantum Computers

#15

I'm disappointed that this article doesn't answer my only real question about quantum computers: What does a reasonable expectation look like? What is the range of potential performance gains? I understand that this is the frontier and no one has any definite answers, but I hear everything from "trivial" improvements to "insta-crack AES-256" (thousands of trillion trillion trillion trillions times faster). Does anyon…

Read Quantum Computing Since Democritus. Its the best you can do as a lay person.

Re: A Reality Check on Quantum Computers

#16
Wikipedia blurb about the author:

Frank Anthony Wilczek (/ˈwɪltʃɛk/;[2] born May 15, 1951) is an American theoretical physicist, mathematician and a Nobel laureate. He is currently the Herman Feshbach Professor of Physics at the Massachusetts Institute of Technology (MIT), Founding Director of T. D. Lee Institute and Chief Scientist Wilczek Quantum Center, Shanghai Jiao Tong University (SJTU), Distinguished Origins Professor at Arizona State University (ASU) and full Professor at Stockholm University.

Re: A Reality Check on Quantum Computers

#17

I'm disappointed that this article doesn't answer my only real question about quantum computers: What does a reasonable expectation look like? What is the range of potential performance gains? I understand that this is the frontier and no one has any definite answers, but I hear everything from "trivial" improvements to "insta-crack AES-256" (thousands of trillion trillion trillion trillions times faster). Does anyon…

There's a lot of research at the "P = BQP?", "NP I can tell you that Shor's algorithm for factoring takes O((log n)^2 (log log n)(log log log n)) (we can round that up to O((log n)^4) if you want) to a the best known classical algorithm of

O(exp(1.9 * (log n)^(1/3) (log log n)^(2/3)))

But unless I tell you how fast each gate is, that tells you nothing about the constant time factors.

Also, all these quantum algorithms are constructed out of logical gates, but each logical gate might be built out of 10 physical gates, using some error correcting code.

Since you really need to know the physical characteristics of the particular architecture to know how much error correction you need, it's hard to even say how many gates something will use and how fast they are until we pick a particular architecture.

All we can really say is that given a big enough number, a quantum computer can factor it faster than a classic computer.

Re: A Reality Check on Quantum Computers

#19
post #17

I'm disappointed that this article doesn't answer my only real question about quantum computers: What does a reasonable expectation look like? What is the range of potential performance gains? I understand that this is the frontier and no one has any definite answers, but I hear everything from "trivial" improvements to "insta-crack AES-256" (thousands of trillion trillion trillion trillions times faster). Does anyon…

There's a lot of research at the "P = BQP?", "NP I can tell you that Shor's algorithm for factoring takes O((log n)^2 (log log n)(log log log n)) (we can round that up to O((log n)^4) if you want) to a the best known classical algorithm of O(exp(1.9 * (log n)^(1/3) (log log n)^(2/3))) But unless I tell you how fast each gate is, that tells you nothing about the constant time factors. Also, all these quantum algorithm…

But could quantum computing accelerate AI workloads such as graph traversal/rewriting?

Iff quantum computing is only useful at breaking encryption, I don't see the point in funding quantum computing research.

Re: A Reality Check on Quantum Computers

#20

I'm disappointed that this article doesn't answer my only real question about quantum computers: What does a reasonable expectation look like? What is the range of potential performance gains? I understand that this is the frontier and no one has any definite answers, but I hear everything from "trivial" improvements to "insta-crack AES-256" (thousands of trillion trillion trillion trillions times faster). Does anyon…

Quantum computers won't insta-crack AES. They only give a square-root speedup for symmetric crypto (so you can double the key length for equivalent security) with Grover's algorithm. It's public-key crypto algorithms based on prime factorization or the descrete log problem which will be broken by Shor's. However, running Shor's algorithm on production key sizes requires a huge quantum computer (with millions of qbits) and Scott Aaronson says he would be "astounded" if this was accomplished within the next decade.

Improvements are believed to be exponential when simulating physical quantum systems.

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