X-Message-Number: 6110
Date: Sun, 21 Apr 1996 13:16:14 -0700
From: John K Clark <>
Subject: SCI.CRYONICS  Quantum Computers


In the April 12 1996 issue of Science there is an article on
Quantum  Computers. It makes clear that a practical Quantum Computer 
has not been proven to be possible, nevertheless the article had a 
very optimistic tone, an optimism I did not see just one year ago. 
If such a machine could be built the ramifications are mind boggling.

When a conventional 64 bit single processor computer performs an
operation, it does it on ONE 64 bit number at a time. When a 64
bit (actually a 64 qubit) single processor QUANTUM computer
performs an operation it does it on ALL 64 bit numbers at the
same time, all 2^64 of them, more than a billion billion, 
and any increase in the number of qubits the computer can handle
will increase it's already astronomical power exponentially. 

It gets even wilder, because the quantum mechanical state of the
matter in the machine's memory determines the output, Seth Lloyd 
of MIT thinks you could run the machine in  reverse and the result 
would be a quantum mechanical micromanipulator.

Despite this enormous increase in performance and a possible
short cut to Nanotechnology, most weren't very interested because 
it didn't seem like a  Quantum Computer could ever be built. 
The slightest error or interaction with the outside environment would 
render the machine inoperative, conventional error correcting codes 
don't work for in the quantum domain and most said that correcting codes 
for quantum mechanical information was impossible. 
They were wrong.

Late last year Peter Shor of ATT showed how to encode a piece of
quantum  information in a 9 qubit system so that the information
is retained even if there is an error in one of the 9 qubits. 
A few months later researchers at IBM refined Shor's technique so that 
only 5 qubits was needed, and found  ways to correct for multiple errors. 

We still don't know for sure if a Quantum Computer is possible
because these error correcting codes work for storing and
transmitting quantum information not for the actual calculation,
but most now think the problem is solvable. In fact both Shor
and Lloyd have privately circulated ideas on how to do this but
have not yet published. If this problem could be solved then we
just need someone to put all the pieces together and make a machine.

I find all this very exciting, it must have been like this in
the late 1930's when reports trickled in about nuclear fission
and the idea occurred to people that a bizarre device like a
nuclear bomb might actually be able to exist in the real world.

                                             John K Clark     

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