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The big one is simulation of quantum systems, I don’t get how that’s not enough by itself? Classical computers will never be able to simulate quantum systems ef

The big one is simulation of quantum systems, I don’t get how that’s not enough by itself? Classical computers will never be able to simulate quantum systems efficiently so we need quantum computers for that. In general achieving arbitrary precision control of quantum states is something that will open avenues in many areas like sensing as well, so just from that angle alone a working quantum computer would open a new branch of technological progress. But yeah it’s not guaranteed or even likely that quantum computers will be very useful for many computer science problems, though I’m also optimistic about that given the progress made in the last 30 years. Physics / science isn’t guaranteed to be easy or progress at a steady pace.

eru 2y ago
> In general achieving arbitrary precision control of quantum states is something that will open avenues in many areas like sensing as well, [...] And developing new materials.
jncfhnb 2y ago
What does it mean to “simulate” a quantum system and why is that useful
- throwaway63467 2y ago
  Simulate roughly means designing a quantum computer that mimicks the dynamics of a real quantum system and then run a simulation of that to learn how it would behave under certain conditions e.g. to optimize its parameters. Similar to how we simulate planes or complex circuits today. Quantum systems are everywhere e.g. lasers, cold atoms, transistors or other complex semiconductors, superconductors, enzymes, … I’d say if we don’t develop the ability to understand these systems through simulation we will never progress beyond a certain technological boundary, and quantum computers are a necessary technology for that, just like classical computers were instrumental for the past scientific revolution.
  - jncfhnb 2y ago
    That sounds… wrong. A simulation of a plane is a model, implemented by humans, of how physics works. A simulation of a quantum system is still a human implementation of mechanics. A quantum computer per my understanding should not be able to add anything to that. You’re not able to just say well this thingy has quantum mechanics and this is a quantum computer so it’s better able to do that. Quantum computers are about speeding up calculations by structuring specific problems such that wrong answers are not calculated. Not emulating quantum mechanics and then pulling the answer from the ether. So… what am I missing here? How could quantum computer aided search spaces specifically aid simulating quantum systems that are not quantum computer aided search spaces?
    - cwillu 2y ago
      “Wrong answers are not calculated” is a popular simplification of this, but it's misleading you here. Quantum computers are a means of systematically creating and modifying complicated sums of exponentially large FFT's, and then efficiently sampling from the resulting distribution. Note that you typically still need to sample many times to get a meaningful answer, which is where the “wrong answers are not calculated” ultimately comes from: if you can arrange for most or all of the factors corresponding to “wrong” answers in the sampled distribution to cancel out (such as the term for the number 4 when trying to factor 15), then when you sample several times from that distribution, very few or perhaps none of those samples will have been drawn from the “wrong” part of the distribution, and so you waste less time testing bad samples. A quantum computer is potentially useful for simulating quantum systems because the _models_ for those systems are ridiculously complex in _exactly_ this way. It won't help if the model is wrong, but our problem is currently that we can't really run the calculations our current models imply beyond slightly-larger-than-toy examples.
      - jncfhnb 2y ago
        > so you waste less time testing bad samples. How is this not “wrong answers are not calculated”? You gave a lot more detail on the mechanics of how these probability amplitudes are canceling each other out but the answer seems the same? I don’t follow how this maps to helping simulate the quantum systems. Quantum computers are good at finding solutions to problems efficiently. But the quantum systems we are describing are not solution seeking systems. They’re going to be just interacting components with entanglement whatever’s going on. How would the avoidance of bad samples aid the simulation of a system like that?

eru 2y ago

> In general achieving arbitrary precision control of quantum states is something that will open avenues in many areas like sensing as well, [...] And developing new materials.

jncfhnb 2y ago

What does it mean to “simulate” a quantum system and why is that useful

throwaway63467 2y ago
Simulate roughly means designing a quantum computer that mimicks the dynamics of a real quantum system and then run a simulation of that to learn how it would behave under certain conditions e.g. to optimize its parameters. Similar to how we simulate planes or complex circuits today. Quantum systems are everywhere e.g. lasers, cold atoms, transistors or other complex semiconductors, superconductors, enzymes, … I’d say if we don’t develop the ability to understand these systems through simulation we will never progress beyond a certain technological boundary, and quantum computers are a necessary technology for that, just like classical computers were instrumental for the past scientific revolution.
- jncfhnb 2y ago
  That sounds… wrong. A simulation of a plane is a model, implemented by humans, of how physics works. A simulation of a quantum system is still a human implementation of mechanics. A quantum computer per my understanding should not be able to add anything to that. You’re not able to just say well this thingy has quantum mechanics and this is a quantum computer so it’s better able to do that. Quantum computers are about speeding up calculations by structuring specific problems such that wrong answers are not calculated. Not emulating quantum mechanics and then pulling the answer from the ether. So… what am I missing here? How could quantum computer aided search spaces specifically aid simulating quantum systems that are not quantum computer aided search spaces?
  - cwillu 2y ago
    “Wrong answers are not calculated” is a popular simplification of this, but it's misleading you here. Quantum computers are a means of systematically creating and modifying complicated sums of exponentially large FFT's, and then efficiently sampling from the resulting distribution. Note that you typically still need to sample many times to get a meaningful answer, which is where the “wrong answers are not calculated” ultimately comes from: if you can arrange for most or all of the factors corresponding to “wrong” answers in the sampled distribution to cancel out (such as the term for the number 4 when trying to factor 15), then when you sample several times from that distribution, very few or perhaps none of those samples will have been drawn from the “wrong” part of the distribution, and so you waste less time testing bad samples. A quantum computer is potentially useful for simulating quantum systems because the _models_ for those systems are ridiculously complex in _exactly_ this way. It won't help if the model is wrong, but our problem is currently that we can't really run the calculations our current models imply beyond slightly-larger-than-toy examples.
    - jncfhnb 2y ago
      > so you waste less time testing bad samples. How is this not “wrong answers are not calculated”? You gave a lot more detail on the mechanics of how these probability amplitudes are canceling each other out but the answer seems the same? I don’t follow how this maps to helping simulate the quantum systems. Quantum computers are good at finding solutions to problems efficiently. But the quantum systems we are describing are not solution seeking systems. They’re going to be just interacting components with entanglement whatever’s going on. How would the avoidance of bad samples aid the simulation of a system like that?