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> Before this milestone, there were still reasons to believe that quantum computers could not, in principle, beat classical computers The laws of Quantum Mecha

by sampo 5y ago

> Before this milestone, there were still reasons to believe that quantum computers could not, in principle, beat classical computers The laws of Quantum Mechanics clearly allow quantum computers. So those beliefs must have included something that goes beyond known physics i.e. the assumption of some supernatural effect?

reikonomusha 5y ago
The laws of Newtonian physics also “allow” frictionless, spherical cows. The laws of relativity and nuclear physics “allow” a tablespoon of salt to provide 453 GWh of energy.
jlokier 5y ago
Nothing supernatural. It may help to understand that a prime motivation for building quantum computers is that they provide the only known way to perform quantum mechanics calculations of general physical systems at significant scale. In other words, although you can simulate a quantum computer in principle, and calculate the effects of the laws of quantum mechanics in principle, in practice classical (non-quantum) computers hit a computional wall doing either of those things above a certain level of complexity which occurs in the natural world. That makes it impossible to work backwards from physical observations of sufficiently complex systems to confirm that the underlying process is following the laws of quantum mechanics as we currently formulate them. Basically, we need a quantum computer to do the kinds of calculations needed to relate physics observations of complex quantum systems to their quantum mechanics underpinnings. We just can't do the math otherwise. Quantum mechanics itself has been verified to extraordinary precision over a wide range of phenomena. We know the physics and mathematics works and is remarkably accurate, for the things we can test. That includes many macroscopic observations; it's not a size problem. We have all sorts of clever ways of calculating different kinds of physics observations from underlying quantum mechanics. But the calculations are all limited in the quantum information complexity they can handle, in practice. In particular, nothing with the high level of coherent but complex quantum state required for a substantial size quantum computer is calculatable without a quantum computer, as far as we know. So, remarkably, quantum mechanics being verified over a wide range of phenomena isn't enough to tell us if quantum computation at significant scale is possible, because we can't do the calculations that show if the observed universe is following mathematical quantum mechanics closely enough to support one. Ironically, to measure if the universe supports quantum computation, we have to build a quantum computer just to be able to do the calculations that show us if we can build a quantum computer.
simiones 5y ago
jlokier has explained this beautifully and in more detail than I would have known how. I will just add one thing: basically Quantum Supremacy is yet another test of QM. As long as quantum supremacy could not be demonstrated, there remained a possibility that QM was wrong in this prediction - just like until the Higgs boson was detected, there remained a possibility that the Standard Model was wrong/incomplete. If you're curious of some proposed possible barriers to quantum supremacy from the time before this experiment was completed, Gil Kalai is a major advocate of the impossibility of achieving QS in practice. This paper [0] details some of this arguments. In very short, his argument is that it is fundamentally impossible to control quantum states to a sufficient degree of precision to realize an actual quantum computer, that errors will always compound too much (he doesn't believe the Google paper will stand the test of time). I am not a believer in his view, but as far as I understand it is (or was, before the Google experiments) not completely absurd; and it is definitely not supernatural. [0] https://arxiv.org/abs/1908.02499 https://arxiv.org/abs/1908.02499

by sampo 5y ago

reikonomusha 5y ago

The laws of Newtonian physics also “allow” frictionless, spherical cows. The laws of relativity and nuclear physics “allow” a tablespoon of salt to provide 453 GWh of energy.

jlokier 5y ago

Nothing supernatural. It may help to understand that a prime motivation for building quantum computers is that they provide the only known way to perform quantum mechanics calculations of general physical systems at significant scale. In other words, although you can simulate a quantum computer in principle, and calculate the effects of the laws of quantum mechanics in principle, in practice classical (non-quantum) computers hit a computional wall doing either of those things above a certain level of complexity which occurs in the natural world. That makes it impossible to work backwards from physical observations of sufficiently complex systems to confirm that the underlying process is following the laws of quantum mechanics as we currently formulate them. Basically, we need a quantum computer to do the kinds of calculations needed to relate physics observations of complex quantum systems to their quantum mechanics underpinnings. We just can't do the math otherwise. Quantum mechanics itself has been verified to extraordinary precision over a wide range of phenomena. We know the physics and mathematics works and is remarkably accurate, for the things we can test. That includes many macroscopic observations; it's not a size problem. We have all sorts of clever ways of calculating different kinds of physics observations from underlying quantum mechanics. But the calculations are all limited in the quantum information complexity they can handle, in practice. In particular, nothing with the high level of coherent but complex quantum state required for a substantial size quantum computer is calculatable without a quantum computer, as far as we know. So, remarkably, quantum mechanics being verified over a wide range of phenomena isn't enough to tell us if quantum computation at significant scale is possible, because we can't do the calculations that show if the observed universe is following mathematical quantum mechanics closely enough to support one. Ironically, to measure if the universe supports quantum computation, we have to build a quantum computer just to be able to do the calculations that show us if we can build a quantum computer.

simiones 5y ago

jlokier has explained this beautifully and in more detail than I would have known how. I will just add one thing: basically Quantum Supremacy is yet another test of QM. As long as quantum supremacy could not be demonstrated, there remained a possibility that QM was wrong in this prediction - just like until the Higgs boson was detected, there remained a possibility that the Standard Model was wrong/incomplete. If you're curious of some proposed possible barriers to quantum supremacy from the time before this experiment was completed, Gil Kalai is a major advocate of the impossibility of achieving QS in practice. This paper [0] details some of this arguments. In very short, his argument is that it is fundamentally impossible to control quantum states to a sufficient degree of precision to realize an actual quantum computer, that errors will always compound too much (he doesn't believe the Google paper will stand the test of time). I am not a believer in his view, but as far as I understand it is (or was, before the Google experiments) not completely absurd; and it is definitely not supernatural. [0] https://arxiv.org/abs/1908.02499 https://arxiv.org/abs/1908.02499