Quantum computing

Robert J. Hansen rjh at sixdemonbag.org
Fri Apr 20 02:25:10 CEST 2007

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> Which I also remarked in the original post. However, when (if?)
> commercial interests grab a hold of quantum computing, huge leaps in
> cost of production perhaps could be achieved, making memory-rich  
> quantum
> computers abundant -- at least, from my chair, there's no  
> obstruction to
> this future. (?)

Eh.  I'm still unconvinced.  It wasn't until last year that the final  
physics hurdle to large-scale QC was addressed (large systems have a  
strong tendency to near spontaneously decohere, turning your quantum  
computer into an expensive paperweight).  We still have no idea how  
to apply this physics knowledge, however.

Just knowing that something is possible doesn't mean the ability to  
do it is around the corner.  We can teleport atoms in laboratories at  
the speed of light and we know how to do it for macro-scale items,  
but the engineering difficulties are so large that I doubt we'll see  
it in our lifetimes.

While I agree that commercial development _may_ lead to developments  
in QC, I think it's equally likely that the engineering difficulties  
will be insurmountable.  Which means that, from where I sit, we  
should just shrug and say "we really can't say with any confidence  
what the future will or will not hold".

> found -- this pragmatism causes me to ponder the scenario in which
> something like Rice' theorem could be established for quantum  
> computers'
> ability (or traditional computers' inability):

What do you mean?  Rice's theorem applies to QC.

Computational theory is computational theory.  We've already got very  
robust mathematics to describe the computational properties of QC.   
We know that BQP is a superset of P, that it does not encompass NP- 
COMPLETE, that it has some overlap with NP, etc., etc.

It's true that in mathematics there could always be a proof delivered  
tomorrow by some hungry graduate student which will utterly shatter  
our knowledge of math as we know it.  But this is true for all of  
mathematics.  It's not as if this risk is special to QC.  You should  
be just as concerned about the prospect of P=NP.

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