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Quantum Computer Factors 15

From: andrew cooke <andrew@...>

Date: Thu, 24 Sep 2009 08:03:29 -0400

This is impressive - a "real number" has just been factorised by a
quantum computer -
http://www.schneier.com/blog/archives/2009/09/quantum_compute.html

If that doesn't make much sense, here's a very quick background:

Some numbers are prime - they cannot be written as the product of any
other numbers (except themselves and 1).  For example, 5 and 3 are
prime, but 15 (=5x3) is not.

A lot of public key cryptography (that's the kind of thing that's used
a lot on the internet to show that you are who you say you are - for
example, it's used in SSL certificates (https) so that web sites can
show that they are "for real") use numbers that are the product of two
primes (like 15).

This cryptography is reliable (so it's effectively impossible for
another website to pretend to be amazon.com when using SSL) because
for large numbers, it's hard to find out what the two primes used in
the product are.

However, in theory quantum computing can solve this problem - find the
two numbers - quickly.

Until now, people have been assuming that although this was possible
in theory, it would be hard - perhaps impossible - in practice.  The
news above is a surprising advance that suggests this is going to
happen much more quickly than was expected.

Andrew

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