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They are classical computers in the sense they are are no stronger than classical computers - they provide a different physical implementation, but computationa

They are classical computers in the sense they are are no stronger than classical computers - they provide a different physical implementation, but computationally can't achieve anything that a classical computer can't. Everything those "quantum computers" do can be simulated with a classical computer. The entire motivation behind the development of QC is supremacy, and the existence of those "quantum computers" alone doesn't provide any proof that supremacy is possible - so it is definitely not just a question of scale. Google already has a 72 qubit processor (Bristlecone), but whether they or anybody will be able to demonstrate supremacy in the current, next, or some future generation is entirely conjectural. The main limit is error correction, and there are theoretical arguments (Gil Kalai) for why achieving supremacy might be impossible.

sp332 8y ago
Well I don't want to get into a full-on argument in the comments here, but I 100% disagree with your definition of a quantum computer. If a computer uses quantum logic gates then it's a quantum computer.
- pavelrub 8y ago
  It’s not particularly important how you define the term “quantum computer”, nor have I presented any definition of a "quantum computer" that you can disagree with. The point is that, as I wrote before, regardless of your definition - currently there is no discernible advantage of quantum computers over classical computers in the only metric that matters: computational power. This is what people refer to when they talk about the impossibility of QC. Nobody has demonstrated that physically realizable quantum computers can solve BQP problems in polynomial time, or provide any superpolynomial speedups over classical computers, and there is certainly no proof that this is simply a matter of scale.
  - sp332 8y ago
    Grover's algorithm runs faster than any classical algorithm can https://en.wikipedia.org/wiki/Grover%27s_algorithm https://en.wikipedia.org/wiki/Grover%27s_algorithm and it was first run on quantum hardware in 1998 according to the timeline I posted above. Literally the only difference between the state of the art and quantum supremacy is scale.
    - pavelrub 8y ago
      Now you are simply posting incorrect or misleading information. There is no existing implementation of Grover's algorithm on quantum hardware that demonstrated any speedup over classical computers. In fact the largest implementation of Grover's algorithm to date, on ibmqx5, has an unusable error rate. > Literally the only difference between the state of the art and quantum supremacy is scale. This is simply incorrect. At the moment it's unclear that quantum supremacy can be achieved at any scale whatsoever, and it's certainly possible that it's unachievable even in principle.

sp332 8y ago

Well I don't want to get into a full-on argument in the comments here, but I 100% disagree with your definition of a quantum computer. If a computer uses quantum logic gates then it's a quantum computer.

pavelrub 8y ago
It’s not particularly important how you define the term “quantum computer”, nor have I presented any definition of a "quantum computer" that you can disagree with. The point is that, as I wrote before, regardless of your definition - currently there is no discernible advantage of quantum computers over classical computers in the only metric that matters: computational power. This is what people refer to when they talk about the impossibility of QC. Nobody has demonstrated that physically realizable quantum computers can solve BQP problems in polynomial time, or provide any superpolynomial speedups over classical computers, and there is certainly no proof that this is simply a matter of scale.
- sp332 8y ago
  Grover's algorithm runs faster than any classical algorithm can https://en.wikipedia.org/wiki/Grover%27s_algorithm https://en.wikipedia.org/wiki/Grover%27s_algorithm and it was first run on quantum hardware in 1998 according to the timeline I posted above. Literally the only difference between the state of the art and quantum supremacy is scale.
  - pavelrub 8y ago
    Now you are simply posting incorrect or misleading information. There is no existing implementation of Grover's algorithm on quantum hardware that demonstrated any speedup over classical computers. In fact the largest implementation of Grover's algorithm to date, on ibmqx5, has an unusable error rate. > Literally the only difference between the state of the art and quantum supremacy is scale. This is simply incorrect. At the moment it's unclear that quantum supremacy can be achieved at any scale whatsoever, and it's certainly possible that it's unachievable even in principle.