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1. We aren't 2. Everything after the first sentence doesn't make sense. Any system that implements a restricted class of unitaries and features local measureme

by gaze 5y ago

1. We aren't 2. Everything after the first sentence doesn't make sense. Any system that implements a restricted class of unitaries and features local measurement as described by Born's rule can efficiently solve problems in BQP. We have no reason this should apply to 10 qubits and not 1000. The theory of quantum mechanics has been completely predictive in materials and has resulted in the most accurate predictions in nature (Anomalous magnetic dipole moment to a part in a trillion.) It doesn't matter if the theory is "Macroscopic" or whatever, it's predictive and within the constraints of its predictions you have known computational power up to problems in BQP. Whether BQP=P is an open question, but as is P=NP. Known problems are things like wiring, materials science/sources of noise -- eventually including things like cosmic rays. It should work and we have no reason to think it shouldn't due to fundamental limitations of the natural world. It is, however, one of the hardest scientific things humans have ever tried to do so far. To say it'll definitely work is hubristic.

lisper 5y ago
This is not quite true. There are spontaneous collapse models [1] that are consistent with the data, like the GRW model [2]. Personally I give long odds against these being true, but you can't rule them out on the basis of current data. And if they are true then they would present a fundamental limit on how big you can make a quantum computer before it fails. [1] https://plato.stanford.edu/entries/qm-collapse/#ContSponLocaModeCSL https://plato.stanford.edu/entries/qm-collapse/#ContSponLoca... [2] https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%93Weber_theory https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...
- gaze 5y ago
  There have been superpositions maintained on the scale of a meter using these fountain experiments. I think the chip traps, SC qubit chips, dot chips, etc… will be much smaller than this.
  - lisper 5y ago
    Not sure what you're referring to here. Maintaining a superposition is not hard. The light from distant galaxies has been in superposition across literally billions of (light) years before we detect it. What is harder to maintain over large distances are entanglements, but even those have been maintained across hundreds of miles. When I talked about a "fundamental limit on how big you can make a quantum computer" I wasn't talking about it's physical size, I was talking about the number of mutually entangled qbits it contains, and the amount of time it is able to maintain those entanglements in a coherent state.
m101 5y ago
As you can see I don't really know much about this stuff. There's apparently noise in the system - how do we know that the reality is different to the model, and what we are putting down to noise is actually reality being different to the model? Kind like Einstein's correction of Newton.

by gaze 5y ago

lisper 5y ago

This is not quite true. There are spontaneous collapse models [1] that are consistent with the data, like the GRW model [2]. Personally I give long odds against these being true, but you can't rule them out on the basis of current data. And if they are true then they would present a fundamental limit on how big you can make a quantum computer before it fails. [1] https://plato.stanford.edu/entries/qm-collapse/#ContSponLocaModeCSL https://plato.stanford.edu/entries/qm-collapse/#ContSponLoca... [2] https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%93Weber_theory https://en.wikipedia.org/wiki/Ghirardi%E2%80%93Rimini%E2%80%...

gaze 5y ago
There have been superpositions maintained on the scale of a meter using these fountain experiments. I think the chip traps, SC qubit chips, dot chips, etc… will be much smaller than this.
- lisper 5y ago
  Not sure what you're referring to here. Maintaining a superposition is not hard. The light from distant galaxies has been in superposition across literally billions of (light) years before we detect it. What is harder to maintain over large distances are entanglements, but even those have been maintained across hundreds of miles. When I talked about a "fundamental limit on how big you can make a quantum computer" I wasn't talking about it's physical size, I was talking about the number of mutually entangled qbits it contains, and the amount of time it is able to maintain those entanglements in a coherent state.

m101 5y ago

As you can see I don't really know much about this stuff. There's apparently noise in the system - how do we know that the reality is different to the model, and what we are putting down to noise is actually reality being different to the model? Kind like Einstein's correction of Newton.