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Quantum Algorithms for Lattice Problems

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Re: Quantum Algorithms for Lattice Problems

#91
post #38
post #19

I work on homomorphic encryption, and there are some rumors circulating that, if this checks out, it will break some of the leading FHE schemes like BFV, where the moduli used are quite large (in the hundreds of bits or even over a thousand bits).

… only if scalable quantum computers exist.

I wouldn't bet against the existence of a modern Bletchley Park analogue.

Re: Quantum Algorithms for Lattice Problems

#92
post #80

Earlier quoted context omitted.

The record for integer factoring on quantum computers was on the order of factoring fifteen into three times five the last time I checked. Can we do three digits now?

Significantly larger numbers than 15 have been factored [1] but not using Shor's algorithm. Shor's algorithm is particularly sensitive to noise/errors in your quantum computer and isn't going to be useful unless we get a properly error corrected machine working. The algorithms used in [1] are considerably less fancy (with worse asymptomatic performance) but are more resilient to noise. [1] https://arxiv.org/abs/2012.…

Interesting - why is Shor's sensitive to noise? Is that the Rphase gates?

Re: Quantum Algorithms for Lattice Problems

#93
post #85

Earlier quoted context omitted.

You should check again. Numbers like 1099551473989 have been factored successfully by now. The arxiv link in the sibling post is a good start.

biggest number factored by a quantum computer isn't the right question. the right question is biggest number factored using a polynomial time algorithm. the answer to that as far as I know of still 15 (although I would be interested in papers that show more progress)

Application of Shor's algorithm is currently limited by available error correction. Long-lived qubits would eliminate that need and drastically increase capabilities.

Re: Quantum Algorithms for Lattice Problems

#94
post #85

Earlier quoted context omitted.

You should check again. Numbers like 1099551473989 have been factored successfully by now. The arxiv link in the sibling post is a good start.

biggest number factored by a quantum computer isn't the right question. the right question is biggest number factored using a polynomial time algorithm. the answer to that as far as I know of still 15 (although I would be interested in papers that show more progress)

This is one of the things I really resent about QC as a field - there's so much chaff where one paper will say "we can do x" and the reality is that x does not mean what everyone thought that they meant. Number of Qbits is another thing - also what gates are implemented in the devices; how long they can run for etc etc etc.

Re: Quantum Algorithms for Lattice Problems

#95
post #92

Earlier quoted context omitted.

Significantly larger numbers than 15 have been factored [1] but not using Shor's algorithm. Shor's algorithm is particularly sensitive to noise/errors in your quantum computer and isn't going to be useful unless we get a properly error corrected machine working. The algorithms used in [1] are considerably less fancy (with worse asymptomatic performance) but are more resilient to noise. [1] https://arxiv.org/abs/2012.…

Interesting - why is Shor's sensitive to noise? Is that the Rphase gates?

Yeah, for Shor's algorithm to factor an integer of order 2^k you need controlled phase gates with phases roughly order 2^{-k} (very roughly, with some caveats, but lets just say you need some small ones) these very small phase gates are susceptible to even very small errors.

This is a gross oversimplification. For the true version see here

https://arxiv.org/abs/2306.10072

Re: Quantum Algorithms for Lattice Problems

#96

Earlier quoted context omitted.

Significantly larger numbers than 15 have been factored [1] but not using Shor's algorithm. Shor's algorithm is particularly sensitive to noise/errors in your quantum computer and isn't going to be useful unless we get a properly error corrected machine working. The algorithms used in [1] are considerably less fancy (with worse asymptomatic performance) but are more resilient to noise. [1] https://arxiv.org/abs/2012.…

that paper is factoring with an algorithm that almost certainly isn't polynomial time. That paper is only slightly better than the quantum factoring algorithm of making a quantum computer perform trial division.

I agree

Re: Quantum Algorithms for Lattice Problems

#97

Earlier quoted context omitted.

Significantly larger numbers than 15 have been factored [1] but not using Shor's algorithm. Shor's algorithm is particularly sensitive to noise/errors in your quantum computer and isn't going to be useful unless we get a properly error corrected machine working. The algorithms used in [1] are considerably less fancy (with worse asymptomatic performance) but are more resilient to noise. [1] https://arxiv.org/abs/2012.…

And to extend off this comment, there are methods being worked on for building qubits that are intrinsically noise-free and don’t need the exponential number of error correcting operations. When those are available, you’ll see a step function increase in capabilities.

>When those are available

Pretty big if

Re: Quantum Algorithms for Lattice Problems

#98
post #80

Earlier quoted context omitted.

We are within 20 years of scalable quantum computers already.

The record for integer factoring on quantum computers was on the order of factoring fifteen into three times five the last time I checked. Can we do three digits now?

I'm not sure that's the right question. It's more, is there a chance at all of anyone figuring it out, and given the enormous scale of the security risk that poses, we should start proactively mitigating those threats. If fusion energy goes from perpetually 10 years away to suddenly here, that's pretty much just a white swan. If quantum computers happen, that's a global security risk before it's a civilizational upgrade.

Re: Quantum Algorithms for Lattice Problems

#99

Earlier quoted context omitted.

Significantly larger numbers than 15 have been factored [1] but not using Shor's algorithm. Shor's algorithm is particularly sensitive to noise/errors in your quantum computer and isn't going to be useful unless we get a properly error corrected machine working. The algorithms used in [1] are considerably less fancy (with worse asymptomatic performance) but are more resilient to noise. [1] https://arxiv.org/abs/2012.…

And to extend off this comment, there are methods being worked on for building qubits that are intrinsically noise-free and don’t need the exponential number of error correcting operations. When those are available, you’ll see a step function increase in capabilities.

For a circuit of size C, the size of a fault tolerant circuit to compute the same thing is O(C polylog C)

https://arxiv.org/abs/quant-ph/9906129

Re: Quantum Algorithms for Lattice Problems

#100
post #97

Earlier quoted context omitted.

And to extend off this comment, there are methods being worked on for building qubits that are intrinsically noise-free and don’t need the exponential number of error correcting operations. When those are available, you’ll see a step function increase in capabilities.

>When those are available Pretty big if

We’re working on it.
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