4 ms·

QM predicts that QCs will work. If QCs don't work, QM has a big problem. Wave collapse interpretation is an orthogonal topic.

lukan 3y ago
Not being able to build something, is not the same as proofing it is impossible. (Unless one has a very high ego.) It might as well require some unknown tech, or material or rather a real understanding of the domain.
- juitpykyk 3y ago
  Sure, but parent was saying what if we manage to build QC as we envision them and then they don't work.
sesm 3y ago
QM doesn’t say anything about how long a system can stay in quantum entangled state, so it doesn’t say what is the limit to computations that can be performed on a quantum computer. Some computations were successfully performed, as QM predicts, but now it seems that researchers are hitting a limit.
- juitpykyk 3y ago
  QM says QC as we envision them will work. QM says a quantum entangled state will remain like that until disturbed/measured. If there is some limit as you suggest, that means QM is at least partially broken.
  - sesm 3y ago
    That’s right, but it doesn’t specify what exactly is disturbance/measurement. We know from observation that interaction with a macroscopic system causes wave function collapse seemingly instantly. But we don’t know much about in-between behavior, like what happens when interacting with another microscopic system and does wave function of a microscopic quantum system collapse on it’s own given enough time.
jlokier 3y ago
> QM predicts that QCs will work. Although that is true, QC isn't ruled in or out by other current observations, despite the extreme accuracy with which quantum physics theories match a very wide range of observations. If QCs can't work, QM will need to be amended but very subtly. It's even possible that useful QC can't work for such a subtle reason that there's no way to be sure. According to current physics and computability theories, it is thought you need a useful QC to calculate the behaviour of any physical system that has the properties required by a useful QC. Where "useful" means large enough to be out of reach of classical computers, and "calculate" means to sufficient accuracy to test if calculated behaviours match observation. This means, the universe might deviate from the appealing math of quantum physics just enough to prevent a highly-entangled QC from ever working, at the same time as agreeing with everything we can calculate from what we have observed so far. Examples of things we can measure but not calculate accurately: Chemical behaviours, material properties and phase transitions (at least some of them) depend on highly-entangled quantum effects, even higher than any QC is aiming for. You might think, if we know the basic physics, and we can accurately measure temperature, energy, reaction rates and other properties, those measurements surely gives us a good idea if quantum physics theory matches the universe. But where there is high entanglement, we can't do the calculations accurately enough to say for sure. They are limited by computational complexity on conventional computers. We have good approximations that are very useful, but they aren't precise enough to rule QC feasibility one way or the other.
- juitpykyk 3y ago
  QM is quite simple. But then we also have QFT. The difficult things you were talking about fall more on the QFT side of things. In my view QM predicts QC will work just like math predicts Turing machines will work. But the practical realization of an advanced Turing machine can indeed be a very complicated thing - in fact we need quantum physics to understand how modern transistors work. So if practical useful QC don't work, wouldn't that be more of a QFT engineering problem? I fully agree that we might not be able to build a working QC, just like we are struggling with fusion power for over 50 years now despite theory predicting it should work, and in theory it being a simpler problem than QCs. Maybe I'm being pedantic with the distinction between QM/QFT...

lukan 3y ago

Not being able to build something, is not the same as proofing it is impossible. (Unless one has a very high ego.) It might as well require some unknown tech, or material or rather a real understanding of the domain.

juitpykyk 3y ago
Sure, but parent was saying what if we manage to build QC as we envision them and then they don't work.

sesm 3y ago

QM doesn’t say anything about how long a system can stay in quantum entangled state, so it doesn’t say what is the limit to computations that can be performed on a quantum computer. Some computations were successfully performed, as QM predicts, but now it seems that researchers are hitting a limit.

juitpykyk 3y ago
QM says QC as we envision them will work. QM says a quantum entangled state will remain like that until disturbed/measured. If there is some limit as you suggest, that means QM is at least partially broken.
- sesm 3y ago
  That’s right, but it doesn’t specify what exactly is disturbance/measurement. We know from observation that interaction with a macroscopic system causes wave function collapse seemingly instantly. But we don’t know much about in-between behavior, like what happens when interacting with another microscopic system and does wave function of a microscopic quantum system collapse on it’s own given enough time.

jlokier 3y ago

> QM predicts that QCs will work. Although that is true, QC isn't ruled in or out by other current observations, despite the extreme accuracy with which quantum physics theories match a very wide range of observations. If QCs can't work, QM will need to be amended but very subtly. It's even possible that useful QC can't work for such a subtle reason that there's no way to be sure. According to current physics and computability theories, it is thought you need a useful QC to calculate the behaviour of any physical system that has the properties required by a useful QC. Where "useful" means large enough to be out of reach of classical computers, and "calculate" means to sufficient accuracy to test if calculated behaviours match observation. This means, the universe might deviate from the appealing math of quantum physics just enough to prevent a highly-entangled QC from ever working, at the same time as agreeing with everything we can calculate from what we have observed so far. Examples of things we can measure but not calculate accurately: Chemical behaviours, material properties and phase transitions (at least some of them) depend on highly-entangled quantum effects, even higher than any QC is aiming for. You might think, if we know the basic physics, and we can accurately measure temperature, energy, reaction rates and other properties, those measurements surely gives us a good idea if quantum physics theory matches the universe. But where there is high entanglement, we can't do the calculations accurately enough to say for sure. They are limited by computational complexity on conventional computers. We have good approximations that are very useful, but they aren't precise enough to rule QC feasibility one way or the other.

juitpykyk 3y ago
QM is quite simple. But then we also have QFT. The difficult things you were talking about fall more on the QFT side of things. In my view QM predicts QC will work just like math predicts Turing machines will work. But the practical realization of an advanced Turing machine can indeed be a very complicated thing - in fact we need quantum physics to understand how modern transistors work. So if practical useful QC don't work, wouldn't that be more of a QFT engineering problem? I fully agree that we might not be able to build a working QC, just like we are struggling with fusion power for over 50 years now despite theory predicting it should work, and in theory it being a simpler problem than QCs. Maybe I'm being pedantic with the distinction between QM/QFT...