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Re: How good is the chance of building a moderately sized quantum computer?

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On 2015-12-05, Mok-Kong Shen <mok-kong.shen@t-online.de> wrote:
> Am 05.12.2015 um 13:26 schrieb David H. Lipman:
>
>> If you want the content on Usenet then just post the content, not the
>> Zeta Board links.
>
> I am sorry for the mistake. Below is a copy of the content.
>
> M. K. Shen
> -------------------------------------------------------------------
>
> Being curious, as layman, of how quantum logic circuits could be verified,
> since simple measurements would have spoiled the qubits without providing
> useful informations, I asked and fianally got a hint to quantum tomography.
> Though lack of knowledge hindered me to capture the sophisticated 
> scientific
> arguments in an Wiki article, I read from it the following:
>
> "The number of experimental configurations (state preparations and
> measurements) required for full quantum process tomography grows
> exponentially with the number of constituent particles of a system.
> Consequently, in general, QPT is an impossible task for large-scale 
> systems."
>
> I think that this fairly clearly indicates that the chance of building a
> moderately sized quantum computer is extremely low from the very beginning,
> since verification of computer hardware is necessary in its design,
> manufacture and maintenance.
>
> [Addedum 1] In discussions elsewhere I got a reference
> http://web.physics.ucsb.edu/~martinisgroup/papers/Martinis2015.pdf
> Lacking knowledge, I couldn't understand much of what is written in it,
> not to say to attempt to counter-argue. However, I have the impression
> that the manner with which quantum tomography is mentioned at two places at
> the beginning of the paper is a little bit biased or anyway not very clear.
>
> One first reads there: "In level 1 ...... Quantum process tomography is
> often performed on one- and two-qubit gates, which is important as it
> proves that proper quantum logic has been achieved. ......."
>
> I suppose that readers of it would afterwards (highly likely) not have
> any doubts on a similar "importance" of QPT for more-than-two-qubit gates.
> (I personally think that QPT is extremely important for a really solid
> proof of correctness of any quantum gate or circuit, but this is of no
> relevance here.)
>
> Next one reads from the paper: "In level 2 ...... Process tompgraphy
> can be performed, but is typically abandoned because (i) the number of
> measurements scales rapidly with increasing number of qubits, (ii)
> information on error coherence is hard to use and (iii) it is difficult
> to separate out initialization and measurement errors. ......"
>
> In my view (a) this doesn't (clearly) tell the reader whether for cases
> of more than 2 qubits quantum process tomography remains important for
> the proof of "proper quantum logic" just as in the case of one or two
> qubits, (b) this (in my interpretation) is in fact merely a more detailed
> formulation of the sentence in Wiki on quantum tomography claiming that
> QPT is practically infeasible for cases of larger number of qubits and
> consequently, if the "importance" of (a) is true, then the stuffs of
> the higher levels treated in the paper would lose their significance.
>
> [Addendum 2] There is contained in [1] a IMHO remarkable citation from
> [2] (which refers to [3]) as follows:
>
> "Serge Haroche, while a leader at the frontier of experimental quantum
> computing, continues to deride the vision of practical quantum computers
> as an impossible dream that can come to fruition only in the wake of
> some as yet unglimpsed revolution in physics. As everyone at this meeting
> knows well, building a quantum computer will be an enourmous technical
> challenge, and perhaps the naysayers will be vindicated in the end.
> Surely their skepticism is reasonable. But to me, quantum computing is
> not an impossible dream; it is a possible dream. It is a dream that can
> be held without flouting the laws of physics as currently understood.
> It is a dream that can stimulate an enormously productive collaboration
> of experimenters and theorists seeking deep insights into the nature of
> decoherence. It is a dream that can be persued by responsible scientists
> determined to explore, without prejudice, the potential of a fascinating
> and powerful new idea. It is a dream that could change the world. So let
>   us dream."
>
> [1] D. P. DiVincenzo, "Origins of Quantum Information Science," in
> Lecture Notes of the 44th IFF Spring School "Quantum Information Processing"
> (ForsuchungsZentrum J??lich, 2013).
>
> [2] J. Preskill, "Quantum Computing: Pro and Con," Proc. Roy. Soc. Lond.
>      A454 (1998) 469-486.
>
> [3] S. Haroche, J-M. Raymond, "Quantum Computing: Dream or Nightmare?",
>      Physics Today 49(8),p.51 (1996).
>
> [Addendum 3] I suppose the following three citations could also be of
> some interest:
>
> (1) From F. H. W. Hossfeld, Komplexitaet und Berechenbarkeit: Ueber die
> Moeglichkeiten und Grenzen des Computers. Wiesbaden, 2000:
>
> "..... Auf die Frage P=NP? gibt es aber im Quantumcomputer-Modell vorerst
> wohl nur widerspruechliche Aussagen. Der Entwurf des Quantumcomputers
> und erst recht seine Verwirklichung ist somit eine Vision, die grosse
> Herausforderungen im naechsten Jahrhundert an die Interdisziplinaritaet
> stellen wird, denn der Quantumcomputer wird mehr noch als die
> "mathematische" Maschine des 20. Jahrhunderts zuvorerst eine 
> "physikalische"
> Maschine sein!"
>
> (2) From Preface p.xxiii in S. Aaronson, Quantum Computing since 
> Democritus,
>      2013:
>
> "Partly for that reason, if scalable quantum computing were proved to be
> IMpossible, that would excite me a thousand times more than if it were
> proved to be possible. For such a failure would imply something wrong or
> incomplete with our understanding of quantum mechanics itself: a revolution
> in physics! As a congenital pessimist, though, my GUESS is that Nature 
> won't
> be so kind to us, and that scalable quantum computing will turn out to be
> possible after all."
>
> "In summary, you could say that I'am in this field less because of what
> you could do with a quantum computer, than because of what the POSSIBILITY
> of quantum computers ALREADY does to our conception of the world. EITHER
> practical quantum computers can be built, and the limits of the knowledge
> are not what we thought they are; OR they can't be built, and the 
> principles
> of quantum mechanics themselves need revision; OR there's yet-undreamt
> method to simulate quantum mechanics efficiently using a conventional
> computer. All three of these possibilities sound like crackpu??t 
> speculations,
> but at least one of them is right! So whichever the outcome, what can 
> one say
> but - to reverse-plagiarize a certain TV commercial - 'that's 
> interesting!' "
>
> (3) From C. J. Meier, Eine kurze Geschichte des Quantumcomputers, p.171,
>      Hannover, 2015:
>
> "Nach wie vor allerdings mangelt es noch an Grundsaetzlichem: Bislang gibt
> es keinen Durchbruch zu einem skalierbaren Quantumrechner. Es gibt noch
> keine Analogie zur Entdeckung des Transitors in den 1940er Jahren bei der
> Entwicklung des klassischen Computers. Die Frage, ob so etwas wie einen
> Transistor fuer den Quantum Computer, also ein beliebig oft kopierbares
> Grundelement, gibt, ist noch offen."
>
> [Addendum 4] From P. Cockshott et al., Computation and its Limits, Oxford,
> 2012, p.172:
>
> "..... Current CMOS technology can achieve error rates of the order of
> 10^(-25). To achieve this error rate, the quantum gate would require around
> 100 joules for each switching operation. A single quantum gate operating
> in the GHz range would be using of the order of 100 MW of power (Kish, 
> 2004).
> Whilst quantum computing does hold promise as a means of reducing the
> complexity of algorithms, it seems unlikely that it will allow us to
> escape from the power-consumption limits posed by classical computing."
>
>
>

Lets see, you throw together a farago of quotes, some from very early in
the game, some from later, some from people who know what they are
talking about some from people who do not. Since you are (self
confessed)  from the far incompetent end of the spectrum regarding 
quantum computing, why not just sit back and see. 

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How good is the chance of building a moderately sized quantum computer? Mok-Kong Shen <mok-kong.shen@t-online.de> - 2015-12-04 20:33 +0100
  Re: How good is the chance of building a moderately sized quantum computer? "David H. Lipman" <DLipman~nospam~@Verizon.Net> - 2015-12-05 07:26 -0500
    Re: How good is the chance of building a moderately sized quantum computer? Mok-Kong Shen <mok-kong.shen@t-online.de> - 2015-12-05 21:58 +0100
      Re: How good is the chance of building a moderately sized quantum computer? William Unruh <unruh@invalid.ca> - 2015-12-05 22:42 +0000
        Re: How good is the chance of building a moderately sized quantum computer? Mok-Kong Shen <mok-kong.shen@t-online.de> - 2015-12-06 18:15 +0100
    Re: How good is the chance of building a moderately sized quantum computer? Mok-Kong Shen <mok-kong.shen@t-online.de> - 2015-12-06 18:28 +0100
      Re: How good is the chance of building a moderately sized quantum computer? William Unruh <unruh@invalid.ca> - 2015-12-06 17:36 +0000
        Re: How good is the chance of building a moderately sized quantum computer? Mok-Kong Shen <mok-kong.shen@t-online.de> - 2015-12-06 18:46 +0100
      Re: How good is the chance of building a moderately sized quantum computer? Rich <rich@example.invalid> - 2015-12-06 18:58 +0000
        Re: How good is the chance of building a moderately sized quantum computer? Mok-Kong Shen <mok-kong.shen@t-online.de> - 2015-12-07 23:19 +0100
          Re: How good is the chance of building a moderately sized quantum computer? Rich <rich@example.invalid> - 2015-12-07 22:51 +0000
            Re: How good is the chance of building a moderately sized quantum computer? Mok-Kong Shen <mok-kong.shen@t-online.de> - 2015-12-08 23:37 +0100
          Re: How good is the chance of building a moderately sized quantum computer? homer@lightlink.com - 2015-12-22 20:17 +0000
      Re: How good is the chance of building a moderately sized quantum computer? homer@lightlink.com - 2015-12-22 20:11 +0000
  Re: How good is the chance of building a moderately sized quantum computer? Mok-Kong Shen <mok-kong.shen@t-online.de> - 2015-12-20 15:35 +0100

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