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There is a lot of circular reasoning involved in quantum information and thermodynamics. It's all totally, perfectly self consistent, but it does not derive fr

There is a lot of circular reasoning involved in quantum information and thermodynamics. It's all totally, perfectly self consistent, but it does not derive from first principles like set theory or mathematical logic do. Physics is an experimental science; they are not required to state their axioms. Oftentimes they do (QFT for example), but the most glaring case where they don't is anything involving information. The whole postulate that information is physical is something that was stumbled upon, and then turned out to explain a whole bunch of other weird things like heat and entropy, and some of those explanations in turn implied that information is physical. I suspect that our current efforts to build cryptographically-relevant quantum computers are a lot like the efforts to build perpetual motion machines in the 1700s. Our current understanding of things isn't wrong, but there is some undiscovered general principle that we keep butting up against, so we'll keep trying to build these things until we figure out why nature keeps blocking us. That discovery -- rather than a computationally-useful device -- will be the most important result of all the quantum computing research going on right now.

adgjlsfhk1 5y ago
that seems plausible, but currently very unsupported by evidence. in the past decade, quantum computers have gotten way more powerful, and progress doesn't seem to be stalling out.
wizzwizz4 5y ago
> until we figure out why nature keeps blocking us A quantum computer only behaves like our mathematical ideal quantum computer if it's sufficiently isolated from the rest of the universe: otherwise you don't get the “indistinguishable states” thing and the amplitudes don't sum, and the computer stops computing and starts being a regular ol' physics experiment. It's an engineering problem, as far as I understand. Get things cold enough, get things isolated enough, so they stay entangled with each other and not with the rest of the universe.
- pishpash 5y ago
  Is it? In engineering problems you have to consider practical feasibility. Would it turn out that for the expenditure of energy to get things isolated we may as well spend it on classical computation?
  - wizzwizz4 5y ago
    On Earth? Maybe. In deep space? Probably not.
    - evanb 5y ago
      The cosmic microwave background makes even deep intergalactic space warmer (3 or 4 K) than the inside of Earth’s best fridges (the best as low as nanoK). 3 or 4 K is easy to achieve using liquid helium cooling, which is presumably why eg. Google’s Sycamore chip operators in that range. But achieving those low temperatures is a regular occurrence in labs all over the world—-you call AirGas or somebody and they deliver a dewar of liquid He.
      - wizzwizz4 5y ago
        And, close to a star, you have several neutrinos flying through your experiment. You have vibrations from earthquakes. You have electromagnetic coupling. etc, etc. The temperatures are easy enough; you just have to compensate for thermal noise. The isolation isn't; without isolation, your signal is wrong.
        pishpash 5y ago
        But unless you're doing a computation without I/O (i.e. no reading back the results, no providing inputs), you need coupling into the rest of the universe, and typically into a low-entropy part of the universe like the Earth, where you have entities that care about the computation. So, I am not convinced having a deeply isolated part of the universe is the answer; in fact, the isolation vs. signal quality tradeoff makes it sound more and more like a fundamental limitation of practical concern.
- octoberfranklin 5y ago
  > It's an engineering problem, as far as I understand. That is far from being clear. People in the 1700s were totally convinced that building a perpetual motion machine was just "an engineering problem".

adgjlsfhk1 5y ago

that seems plausible, but currently very unsupported by evidence. in the past decade, quantum computers have gotten way more powerful, and progress doesn't seem to be stalling out.

wizzwizz4 5y ago

> until we figure out why nature keeps blocking us A quantum computer only behaves like our mathematical ideal quantum computer if it's sufficiently isolated from the rest of the universe: otherwise you don't get the “indistinguishable states” thing and the amplitudes don't sum, and the computer stops computing and starts being a regular ol' physics experiment. It's an engineering problem, as far as I understand. Get things cold enough, get things isolated enough, so they stay entangled with each other and not with the rest of the universe.

pishpash 5y ago
Is it? In engineering problems you have to consider practical feasibility. Would it turn out that for the expenditure of energy to get things isolated we may as well spend it on classical computation?
- wizzwizz4 5y ago
  On Earth? Maybe. In deep space? Probably not.
  - evanb 5y ago
    The cosmic microwave background makes even deep intergalactic space warmer (3 or 4 K) than the inside of Earth’s best fridges (the best as low as nanoK). 3 or 4 K is easy to achieve using liquid helium cooling, which is presumably why eg. Google’s Sycamore chip operators in that range. But achieving those low temperatures is a regular occurrence in labs all over the world—-you call AirGas or somebody and they deliver a dewar of liquid He.
    - wizzwizz4 5y ago
      And, close to a star, you have several neutrinos flying through your experiment. You have vibrations from earthquakes. You have electromagnetic coupling. etc, etc. The temperatures are easy enough; you just have to compensate for thermal noise. The isolation isn't; without isolation, your signal is wrong.
      - pishpash 5y ago
        But unless you're doing a computation without I/O (i.e. no reading back the results, no providing inputs), you need coupling into the rest of the universe, and typically into a low-entropy part of the universe like the Earth, where you have entities that care about the computation. So, I am not convinced having a deeply isolated part of the universe is the answer; in fact, the isolation vs. signal quality tradeoff makes it sound more and more like a fundamental limitation of practical concern.
octoberfranklin 5y ago
> It's an engineering problem, as far as I understand. That is far from being clear. People in the 1700s were totally convinced that building a perpetual motion machine was just "an engineering problem".