Quantum computers inch closer?

Ed Gerck egerck at nma.com
Mon Sep 2 18:33:01 EDT 2002



David Wagner wrote:

> David Honig  wrote:
> >At 08:56 PM 8/30/02 -0700, 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, associated with a random
> >>key, and it is overwhelmingly likely that this key is one for which the
> >>recognizer returns 0.
> >
> >I thought the whole point of quantum-computer design is to build
> >systems where you *do* impose your arbitrary constraints on the system.
>
> Look again at those quantum texts.  AARG! is absolutely correct.
> Quantum doesn't work like the original poster seemed to wish it would;
> state vectors collapse into a random state, not into that one magic
> needle-in-a-haystack state you wish it could find.

The original poster was incorrect just in assuming that this would be an
effective method allowing Feistel ciphers to be broken.

The original poster is correct, however, in that a metric function can be defined
and used by a QC to calculate the distance between a random state and an
eigenstate with some desired properties, and thereby allow the QC to define
when that distance is zero -- which provides the needle-in-the-haystack solution,
even though each random state vector can be seen as a mixed state and will, with
higher probability, be representable by a linear combination of eigenvectors
with random coefficients, rather than by a single eigenvector.

Cheers,
Ed Gerck



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