Quantum computers inch closer?

[John S. Denker]

"AARG!Anonymous" wrote:
> 
> The problem is that you can't forcibly collapse the state vector into your
> wished-for eigenstate, the one where the plaintext recognizer returns a 1.
> Instead, it will collapse into a random state,

Sorry, that's a severe mis-characterization.

> David Honig  wrote:
>
> >I thought the whole point of quantum-computer design is to build
> >systems where you *do* impose your arbitrary constraints on the system.

David Wagner wrote:
> 
> Look again at those quantum texts.  

That's good advice.

> Quantum doesn't work like the original poster seemed to wish it would;
> state vectors collapse into a random state, 

Random is not the right word.

> not into that one magic
> needle-in-a-haystack state you wish it could find.

C'mon folks, let's cut down on extreme statements like 
the-whole-point-is-this or the-whole-point-is-that
and using words like "magic" to describe finding the
right answer.

1) Computer design has many points that must be
taken into consideration.  Quantum computer design
is in some ways more powerful but in other ways more
constrained than classical computer design.

2) One of the points is that yes, the computer should
compute what you want it to compute.  OTOH it takes
more than wishing to bring such a computer into 
existence.

3) A sufficiently well designed quantum computer can, 
in principle, find some needles in some haystacks, 
precisely because the structure of the machine, acting 
according to the laws of quantum mechanics, does in fact 
"collapse" the wave-function into a representation of 
the wished-for answer.  (PS most of what has been 
written about "collapse" of wave-functions is baloney, 
but we need not pursue that tangent just now.)

=================

A general remark about parallel computing:  For every
parallel algorithm (running on P processors) there 
exists a corresponding uniprocessor algorithm:  just 
set P=1 and turn the crank.

The converse does not hold.  The existence of a uni-
processor algorithm may or may not be a guide to the 
creation of a parallel algorithm.  As Brooks famously 
said, creating a baby requires nine months, no matter 
how many mothers are assigned to the task.

The same applies even more strongly to quantum computing:
It would be nice if you could take a classical circuit,
automatically convert it to "the" corresponding quantum
circuit, with the property that when presented with a
superposition of questions it would produce "the" 
corresponding superposition of answers.  But that cannot 
be.  For starters, there will be some phase relationships 
between the various components of the superposition of 
answers, and the classical circuit provides no guidance 
as to what the phase relationships should be.

So let's not guess about what quantum algorithms exist.
It is possible to construct such algorithms, but it 
requires highly specialized skills.

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