quantum computer factors number

[Ben Laurie]

Steve Bellovin wrote:
> 
> A quantum computer has been built that has actually factored a number: 15.
> It's not a very interesting number from a cryptographic perspective,
> but it is real.  http://www.nature.com/nature/links/011220/011220-2.html

Its worth noting that not only is the number not very interesting (15),
but various properties of it have been used to make the quantum computer
simpler. The quantum computer would not be capable of performing any
other calculation. In particular, addition has been substituted for
multiplication in one part of the calculation, and the other
multiplication has been changed to a completely different operation (bit
swapping). Once simplified thus, several operations were omitted because
they were known to not actually influence the outcome or because enough
of their inputs were known to make them guaranteed to be null
operations.

These changes are described as optimisations - but since in any real
case they would involve performing most of the calculation on a
classical computer (AFAICS), its difficult to see how this experiment
demonstrates anything other than a remarkable ability to control and
measure the quantum state of 7 atoms in a molecule. Which is impressive
in itself, but it seems hardly fair to describe it is factorisation.

Probably the coolest thing about this experiment is that they have
produced what appears to be a very accurate model of the decoherence
effects, which should allow quantum computers to be modelled with some
certainty in the future.

Cheers,

Ben.

--
http://www.apache-ssl.org/ben.html       http://www.thebunker.net/

"There is no limit to what a man can do or how far he can go if he
doesn't mind who gets the credit." - Robert Woodruff



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