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If we want to simulate molecular systems that have quantum effects, with a quantum computer, is it a good fit? Say we want to create designer molecules for flo

If we want to simulate molecular systems that have quantum effects, with a quantum computer, is it a good fit? Say we want to create designer molecules for flow batteries. A few benzene rings and some "arms" with a few carbons and hydroxyl group at the end. The potential design space is absolutely humongous. Can a quantum computer simulate something like that relatively directly, and then see how that reacts in various situations, oxidizing and reducing? If you have a system with 20 atoms, how many qubits would you need, and so on? Right now researchers make a best guess, then synthesize the material. That can take months. Then they make experiments, and usually there are some surprises. Machine learning has been proposed to help but probably has limitations...

jlokier 4y ago
Little known fact: Simulating quantum physical systems was the original motivation for building quantum computers, because those simulations are infeasible without one, and someone figured out it would be, in principle, feasible to run those simulations using a computer based on quantum effects. There's a neat symmetry to it. We can't be certain quantum computers work at useful scales, until we've built one, because there's no feasible way to simulate the physics of a useful size quantum computer without using one.
- jacobr1 4y ago
  > there's no feasible way to simulate the physics of a useful size quantum computer without using one. What do the diverging growth curves look like? We probably could brute force simulate the exponential states within a small-n using conventional computing, just by paying for more parallelism and time. At some point that doesn't scale (more computers needed than atoms on earth, for longer than the heat-death of the universe or whatever) but surely there are interesting problems we can model before whatever inflection point exists? If so - what kind of scale is that?
ta988 4y ago
In theory yes, but we can't get one with enough qbits (you would need more qbits than your molecule because you need to encode positions and interactions of electrons at the very least), enough configurability (current q-computers are somewhat limited to which qbit can interact with which through which mean) and that is able to keep decoherence away (which limits how much it can scale). It seems that we are really hitting a wall every time we want to scale them to a useful size and keep the number of possible interactions between qbits high and noise/decoherence level low. So for now we don't have the technology (if ever), it is not just a problem of "just scaling by adding more qbits" we need a new approach.
- deleted 4y ago
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jlokier 4y ago

Little known fact: Simulating quantum physical systems was the original motivation for building quantum computers, because those simulations are infeasible without one, and someone figured out it would be, in principle, feasible to run those simulations using a computer based on quantum effects. There's a neat symmetry to it. We can't be certain quantum computers work at useful scales, until we've built one, because there's no feasible way to simulate the physics of a useful size quantum computer without using one.

jacobr1 4y ago
> there's no feasible way to simulate the physics of a useful size quantum computer without using one. What do the diverging growth curves look like? We probably could brute force simulate the exponential states within a small-n using conventional computing, just by paying for more parallelism and time. At some point that doesn't scale (more computers needed than atoms on earth, for longer than the heat-death of the universe or whatever) but surely there are interesting problems we can model before whatever inflection point exists? If so - what kind of scale is that?

ta988 4y ago

In theory yes, but we can't get one with enough qbits (you would need more qbits than your molecule because you need to encode positions and interactions of electrons at the very least), enough configurability (current q-computers are somewhat limited to which qbit can interact with which through which mean) and that is able to keep decoherence away (which limits how much it can scale). It seems that we are really hitting a wall every time we want to scale them to a useful size and keep the number of possible interactions between qbits high and noise/decoherence level low. So for now we don't have the technology (if ever), it is not just a problem of "just scaling by adding more qbits" we need a new approach.

deleted 4y ago
[deleted]