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The big theoretical use case of quantum computing is simulating quantum systems, with applications in chemistry, biology, medicine/pharmacology, material scienc

The big theoretical use case of quantum computing is simulating quantum systems, with applications in chemistry, biology, medicine/pharmacology, material science and quantum physics. In theory, doing things like calculating the ground state of a complex molecule would be viable on a quantum computer while a classical computer would require enormous amounts of energy, memory and time. Popular science outlets love to talk about "quantum computing breaking encryption" and "instant search with Grover's algorithm", etc., but from what I've seen, barely anyone in modern quantum computing "industry" is really interested in those. Companies like Volkswagen have been interested in QC for its potential in improving EV batteries; agriculture, medicine and pharma are interested in things like protein folding and other simulations with complex molecules and chemical processes, which aren't just viable to a good degree of accuracy on any classical computer nowadays. Another application, which is a bit closer to the popular view of quantum algorithms, is logistics, where potential speed ups in NP-complete problem solving may be useful. (Oh, and there is a so-called post-quantum cryptography with quite fascinating real-world tests)

credit_guy 3y ago
> The big theoretical use case of quantum computing is simulating quantum systems And in practice, how close are we to being able to solve n-body quantum problems on actual quantum computing hardware? Can we do that with D-Wave type of computers, or we have to wait for perfectly entangled qubits (which will never happen)?
- freetonik 3y ago
  We don't need perfectly entangled qubits, but we do need a large number of physical qubits in order to achieve quantum error correction (and in practice get a smaller amount of perfect logical qubits). We are not very close to that level today, but the progress in the last decade was quite radical.
  - credit_guy 3y ago
    This is really cool, thank you for providing the details. I did a bit of digging, and I think the crucial result that I did not know 25 years ago is the "threshold theorem" [1], which to me sounds like an immensely important result. However, it appears you need quite a bit of noisy cubits, wikipedia mentions 1000 to 10000 for a single logical qubit, but it appears that that's for a level of noise well below what's currently achievable. In any case, quantum n-body problems with tens of entangled electrons are doable with classicla algorithms, so to make an impact you'd need many millions of noisy quantum cubits, maybe even billions. After (admittedly fairly shallow) googling, I found out that IBM plans to have a quantum computer with 100000 noisy cubits ... sometime soon. On the other hand it appears that solid advances happened on the theory side, according to this editorial [2] in the Journal of Chemical Theory and Computation. Still, I can't shake the feeling that it's all just another case of nuclear fusion. When it will happen it will change the world, but that's going to be in 30 years, 30 years from now. [1] https://en.wikipedia.org/wiki/Threshold_theorem https://en.wikipedia.org/wiki/Threshold_theorem [2] https://pubs.acs.org/doi/epdf/10.1021/acs.jctc.3c01043 https://pubs.acs.org/doi/epdf/10.1021/acs.jctc.3c01043
    - freetonik 3y ago
      Yup, the threshold theorem is the holy grail. Going back to the number of qubits: IBM and some other research institutes have been working on an idea of connecting multiple quantum computers in a grid. This would potentially help alleviate the problem of "how to fit 1000s of qubits, with all the wiring, into a single fridge", as well as "how to fabricate high-quality single chip reliably". In theory, we could fabricate and wire-up medium sized-chips, setup full quantum computers and connect them with some sort of quantum memory and quantum communication channels. Fabrication is tough; the company I work at does it all, from fabrication of quantum chips to final on-premise installation, and fabrication is as difficult as one can imagine.

credit_guy 3y ago

> The big theoretical use case of quantum computing is simulating quantum systems And in practice, how close are we to being able to solve n-body quantum problems on actual quantum computing hardware? Can we do that with D-Wave type of computers, or we have to wait for perfectly entangled qubits (which will never happen)?

freetonik 3y ago
We don't need perfectly entangled qubits, but we do need a large number of physical qubits in order to achieve quantum error correction (and in practice get a smaller amount of perfect logical qubits). We are not very close to that level today, but the progress in the last decade was quite radical.
- credit_guy 3y ago
  This is really cool, thank you for providing the details. I did a bit of digging, and I think the crucial result that I did not know 25 years ago is the "threshold theorem" [1], which to me sounds like an immensely important result. However, it appears you need quite a bit of noisy cubits, wikipedia mentions 1000 to 10000 for a single logical qubit, but it appears that that's for a level of noise well below what's currently achievable. In any case, quantum n-body problems with tens of entangled electrons are doable with classicla algorithms, so to make an impact you'd need many millions of noisy quantum cubits, maybe even billions. After (admittedly fairly shallow) googling, I found out that IBM plans to have a quantum computer with 100000 noisy cubits ... sometime soon. On the other hand it appears that solid advances happened on the theory side, according to this editorial [2] in the Journal of Chemical Theory and Computation. Still, I can't shake the feeling that it's all just another case of nuclear fusion. When it will happen it will change the world, but that's going to be in 30 years, 30 years from now. [1] https://en.wikipedia.org/wiki/Threshold_theorem https://en.wikipedia.org/wiki/Threshold_theorem [2] https://pubs.acs.org/doi/epdf/10.1021/acs.jctc.3c01043 https://pubs.acs.org/doi/epdf/10.1021/acs.jctc.3c01043
  - freetonik 3y ago
    Yup, the threshold theorem is the holy grail. Going back to the number of qubits: IBM and some other research institutes have been working on an idea of connecting multiple quantum computers in a grid. This would potentially help alleviate the problem of "how to fit 1000s of qubits, with all the wiring, into a single fridge", as well as "how to fabricate high-quality single chip reliably". In theory, we could fabricate and wire-up medium sized-chips, setup full quantum computers and connect them with some sort of quantum memory and quantum communication channels. Fabrication is tough; the company I work at does it all, from fabrication of quantum chips to final on-premise installation, and fabrication is as difficult as one can imagine.