Re: Quantum computing

Date: Sat, 6 Jan 1996 14:51:49 +1
From: Arkadiusz Jadczyk <ajad@physik.uni-bielefeld.de>
To: quantum-d@teleport.com
Subject: Re: Quantum computing

Mitch asked about quantum computing - what other
interpretations say about the subject. Here is my
response, although I am well aware of the fact that
what I say may not satisfy him...

I think that input and output of any quantum computer
must be classical. Clear "yes-no". Classical bits.
In practice: quantum computer must have a classical
computer as an interface. We ask classical questions,
we get classical answers. So we need a theory
that describes the working of such a hybrid machine.
We need equations. Or better: we need algorithm. 
We now that Schroedinger equation is insufficient to
explain (or simulate) individual events. One has to
go beyond. There may be several possibilities. Many
world interpretation may be one of such. But I fail
to see that it provides an algorithm that is needed.
I know of one such algorithm - it has its strong, but 
also some weak points. This is PDP (piecewise 
deterministic process) algorithm (Phys. Lett.
A 203(1995) 260-266 and Ann. Physik 4(1995)583-599).
Using this algorithm we can simulate any quantum
computer on a classical one (I do it on my PC). 
What we learn is the following: quantum computer
(we may call it quantum analog machine) does
effortlessly two things: 

1) it manages to solve Schroedinger equation for 
   many degrees of freedom in almost no time at all.
2) it manages to do the integration over the region
   of interface with the classical computer at each
   time step
3) it has an intrinsic random number generator

Otherwise there is nothing spectacular (if the above
is not enough). 

In fact, I did not try to simulate the elementary
building blocks of a quantum computer with the PDP
algorithm. But it should be rather straightforward 
and it can be a nice subject for a PhD thesis.

To summarize: I believe that the PDP algorithm that
is at the basis of the "event enhanced quantum theory"
described in the papers quoted above is the simplest
way of describing and explaining action and properties
of quantum computers. It allows us also to simulate
them on classical digital machines.

Let me also add this: after reading and rereading
David Deutsch's papers I failed to understand their
content. It is certainly my fault - not his, but
the result was that I was forced to produce some
other way of understanding quantum theory - and PDP
is the result. But now, having PDP at hand, it is
possible to see the many world interpretation in
somewhat new light. Because PDP can be also interpreted
as the algorithm for world splitting! And this is
precisely what was missing: WHEN worlds split? 

aj
  
+-----------------------------------------------------------------------+

 Arkadiusz Jadczyk                tel (0049)521-106-6199
 BiBoS                            fax (0049)521-106-2961
 University of Bielefeld
 Universitaetstr. 25              e-mail: ajad@physik.uni-bielefeld.de
 D 33615 Bielefeld                            ajad@ift.uni.wroc.pl
 Germany                          
 
         www: http://www.ift.uni.wroc.pl/~ajad/datapage.htm

+-----------------------------------------------------------------------+