Leonard Susskind discusses that the principal power of quantum computers will be simulating quantum systems: instead of solving difficult equations classically, one can build a controllable quantum model, run it more slowly or modify it, and make quantum measurements to learn about the target system. He contrasts this practical role with special-case quantum algorithms like factoring, which seem exceptional and unrelated to physical quantum phenomena. Susskind highlights many application domains—quantum chemistry, solid-state physics, materials science, quantum field theory, and quantum gravity—and stresses that large quantum computers share physics in common with large quantum black holes, a connection useful in research. On the question of whether brains exploit quantum mechanics, he reports that most neuroscientists he knows treat brains as essentially classical, though macroscopic quantum materials (topological insulators, superconductors) demonstrate that some large systems do exhibit strongly quantum behavior.
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Foreign. As far as I can tell, the great power of quantum computers will actually be to simulate quantum systems. If you're interested in a certain quantum system and it's too hard to simulate classically, you simply build a version of the same system. You build a version of it, you build a model of it that's actually functioning as the system. You run it, and then you do the same thing you would do to the quantum system. You make measurements on it, quantum measurements on it. The advantage is you can run it much slower. You could say, why bother? Why not just use the real system? Why not just do experiments on the real system? Well, real systems are kind of limited. You can't change them. You can't manipulate them. You can't slow them down so that you can poke into them. You can't modify them in arbitrary kinds of ways to see what would happen if I, if I change the system a little bit. So I think that quantum computers will be extremely valuable in, in understanding quantum systems.
At the lowest level, the fundamental laws.
They'Re actually satisfying the same laws as the systems that they're simulating. That's right. Okay. So on the one hand, you have things like factoring. Okay, Factoring is the great thing of quantum computers. Factoring large numbers, that doesn't seem that much to do with quantum mechanics. Right. It seems to be almost a fluke that, that a quantum computer can solve the factoring problem in a short time. So. And those problems seem to be extremely special, rare, and it's not clear to me that there's going to be a lot of them. On the other hand, there are a lot of quantum systems chemistry. There's solid state physics, there's material science, there's quantum gravity, there's all kinds of quantum field theory. And some of these are actually turning out to be applied sciences as well as very fundamental sciences. So we probably will run out of the ability to solve equations for these things. You know, solve equations by the standard methods of pencil and paper, solve the equations by the method of classical computers. And so what we'll do is we'll build versions of these systems, run them and run them under controlled circumstances, or we can change them, manipulate them, make measurements on them, and find out all the things we want to know.
So in finding out the things we want to know about very small systems right now, the. Is there something we can also find out about the macro level, about something about the function and forgive me, of our brain biological systems, the, the stuff that's about 1 meter in size versus much, much smaller.
Well, what all the excitement is about among the people that I interact with is understanding black holes.
Black holes.
Black holes are big things. They are many, many degrees of freedom. There is another kind of quantum system that is big. It's a large quantum computer. And one of the things we've learned is that the physics of large quantum computers is in some ways similar to the physics of large quantum black holes. And we're using that relationship now. You asked. You didn't ask about quantum computers as systems. You didn't ask about black holes. You asked about brains.
Yeah, about stuff that's in the middle of the two.
It's different.
So black holes are. There's something fundamental about black holes that feels to be very different than brains.
Yes. And they also function in a very quantum mechanical way. Right. Okay. It is, first of all, unclear to me, but of course it's unclear to me. I'm not. I'm not a neuroscientist. I have. I don't even have very many friends who are neuroscientists. I would like to have more friends who are neuroscientists. I just don't run into them very often. Among the few neuroscientists I've ever talked about about this, they are pretty convinced that the brain functions classically. That it is not intrinsically a quantum mechanical system or doesn't make use of the special features, entanglement, complex, coherent superposition. Are they right? I don't know. I sort of hope they're wrong. Just because I like the romantic idea that the brain is a quantum system. But I think probably not. The other thing, big systems can be composed of lots of little systems, materials. The materials that we work with and so forth can be large systems, large piece of material, but they're big, and they're made out of quantum systems. Now, one of the things that's been happening over the last good number of years is we're discovering materials and quantum systems which function much more quantum mechanically than we imagine. Topological insulators, this kind of thing. That kind of thing. Those are macroscopic systems, but they just. Superconductors. Superconductors have a lot of quantum mechanics in them. You can have a large chunk of superconductor. So it's a big piece of material. On the other hand, its functioning and its properties depend very, very strongly on quantum mechanics. And to analyze them, you need the tools of quantum mechanics.
