5 ms·

That's getting a bit ahead of where they're at. To the best of my knowledge, the fundamental assumption behind the QC paradigm, the Copenhagen Interpretation,

That's getting a bit ahead of where they're at. To the best of my knowledge, the fundamental assumption behind the QC paradigm, the Copenhagen Interpretation, has never really been tested.[1] It is possible that a device like this will constitute the definitive test. The question will be whether such a system can actually perform a calculation across a superposition of states. In the systems I am aware of where such a calculation has been claimed, what actually happened was that a large number of atoms were manipulated simultaneously, and the "superposition" was potentially across the specific states in those individual atoms. My own opinion is that the QC scenario is a reductio ad absurdum for the Copenhagen Interpretation, and that when a true QC calculation is set up, it will fail. This is actually an optimistic view, because the Copenhagen Interpretation is a fundamentally pessimistic assumption: that because we don't know what's behind a certain observed random distribution, we can't. See the following link for more details. (But IAMAP, and I'm not really keeping up with this area.) [1] For more on how the Copenhagen Interpretation evolved, and just how little empirical basis there is for it, see http://books.google.com/books?id=tTN4HuUNXjgC&lpg=PP1&pg=PA328#v=onepage&q&f=false http://books.google.com/books?id=tTN4HuUNXjgC&lpg=PP1&#3...

gjm11 16y ago
Quantum computation does not in any way depend on the Copenhagen interpretation.
- Estragon 16y ago
  The QC scenario relies on the superposition of states which collapse probabilistically when the system is observed. It is precisely this superposition which enables the massive parallelism the QC scenario promises. This superposition and probabilistic collapse is a central component of the Copenhagen Interpretation, as I understand it.
  - Jach 16y ago
    Superposition isn't part of the Copenhagen interpretation, nor is decoherence. Copenhagen is the addition to standard QM (which QC is based on) and states that the other possibilities we don't observe, also stop existing, making there only be "one true world". But it's not predicted by standard QM which predicts MWI: http://arxiv.org/pdf/0905.1283v1 http://arxiv.org/pdf/0905.1283v1 http://www.hpl.hp.com/breweb/quiprocone/Protected/Lecture_2.htm http://www.hpl.hp.com/breweb/quiprocone/Protected/Lecture_2....
    - Estragon 16y ago
      This might be a matter of semantics. When I say "superposition," I mean the view that all we could possibly know about a system is represented in its wave function. This implies an ontological superposition of states until the wave function is collapsed, as is famously embodied in Schroedinger's cat. It is this ontological superposition of states which the QC scenario depends on for massive parallelism: If Schroedinger's cat was the input to your QC function, you would get to run the function with an alive-cat and a dead-cat input at the same time. Of course, you only get to see the output for the state the cat's in when the wave function is collapsed, and a key aspect of search algorithms like Shor's QC factorization scheme is to make sure that all the other states cancel out. (Shor's algorithm basically tries all possible divisors of the target number at the same time, through a superposition scheme.) But if the probabilistic component of the QM wave function instead merely represents our degree of ignorance about the entire system, rather than some such magical superposition, then the QC system is actually only in one of the many states on which the computation is supposed to be run simultaneously, and the promised massive parallelism is just a fantasy. (Yes, I think a hidden-variables explanation is more likely than the Copenhagen interpretation. Yes, I know about the EPR experiment.)
      - gjm11 16y ago
        It sounds to me as if what you don't believe in is not the Copenhagen interpretation but quantum mechanics. The things on which quantum computation depends, and that you are being skeptical about, are common to all interpretations of QM. (Which is just as well, because all interpretations make the same predictions about our observations -- aside perhaps from some funny "anthropic" things -- and whether quantum computation works or not is, at least once the necessary engineering has been done, eminently observable.)
        Estragon 16y ago
        > The things on which quantum computation depends, and > that you are being skeptical about, are common to all > interpretations of QM. I disagree: http://en.wikipedia.org/wiki/Hidden_variable_theory http://en.wikipedia.org/wiki/Hidden_variable_theory (The "Motivation" section is particularly relevant to this discussion.) This is not the standard QM, but to the best of my knowledge it is consistent with the results of all the standard QM experiments.
Estragon 16y ago
> But IAMAP, Gah, that should be IANAP. (Too late to edit it.)

gjm11 16y ago

Quantum computation does not in any way depend on the Copenhagen interpretation.

Estragon 16y ago
The QC scenario relies on the superposition of states which collapse probabilistically when the system is observed. It is precisely this superposition which enables the massive parallelism the QC scenario promises. This superposition and probabilistic collapse is a central component of the Copenhagen Interpretation, as I understand it.
- Jach 16y ago
  Superposition isn't part of the Copenhagen interpretation, nor is decoherence. Copenhagen is the addition to standard QM (which QC is based on) and states that the other possibilities we don't observe, also stop existing, making there only be "one true world". But it's not predicted by standard QM which predicts MWI: http://arxiv.org/pdf/0905.1283v1 http://arxiv.org/pdf/0905.1283v1 http://www.hpl.hp.com/breweb/quiprocone/Protected/Lecture_2.htm http://www.hpl.hp.com/breweb/quiprocone/Protected/Lecture_2....
  - Estragon 16y ago
    This might be a matter of semantics. When I say "superposition," I mean the view that all we could possibly know about a system is represented in its wave function. This implies an ontological superposition of states until the wave function is collapsed, as is famously embodied in Schroedinger's cat. It is this ontological superposition of states which the QC scenario depends on for massive parallelism: If Schroedinger's cat was the input to your QC function, you would get to run the function with an alive-cat and a dead-cat input at the same time. Of course, you only get to see the output for the state the cat's in when the wave function is collapsed, and a key aspect of search algorithms like Shor's QC factorization scheme is to make sure that all the other states cancel out. (Shor's algorithm basically tries all possible divisors of the target number at the same time, through a superposition scheme.) But if the probabilistic component of the QM wave function instead merely represents our degree of ignorance about the entire system, rather than some such magical superposition, then the QC system is actually only in one of the many states on which the computation is supposed to be run simultaneously, and the promised massive parallelism is just a fantasy. (Yes, I think a hidden-variables explanation is more likely than the Copenhagen interpretation. Yes, I know about the EPR experiment.)
    - gjm11 16y ago
      It sounds to me as if what you don't believe in is not the Copenhagen interpretation but quantum mechanics. The things on which quantum computation depends, and that you are being skeptical about, are common to all interpretations of QM. (Which is just as well, because all interpretations make the same predictions about our observations -- aside perhaps from some funny "anthropic" things -- and whether quantum computation works or not is, at least once the necessary engineering has been done, eminently observable.)
      - Estragon 16y ago
        > The things on which quantum computation depends, and > that you are being skeptical about, are common to all > interpretations of QM. I disagree: http://en.wikipedia.org/wiki/Hidden_variable_theory http://en.wikipedia.org/wiki/Hidden_variable_theory (The "Motivation" section is particularly relevant to this discussion.) This is not the standard QM, but to the best of my knowledge it is consistent with the results of all the standard QM experiments.

Estragon 16y ago

> But IAMAP, Gah, that should be IANAP. (Too late to edit it.)