Re: Anyone seen the 'quantum cryptanalysis' thread on sci.crypt?

>Date: Wed, 28 Sep 1994 11:21:45 -0700
>From: doug@OpenMind.com (Doug Cutrell)


>I won't argue whether Shor's work will be implemented or not within any
>given time period, but I thought that one of the most important properties
>of it is that once (and if) achieved, the resources required to factor
>increasingly large moduli lengths go up only polynomially, not
>exponentially.

The real roadblock might be that there are limits to how many bits there
can be in a register.  NIST's recent Quantum Computation conference
included discussion of the expected lifetime of a computation (what
fraction of a second the computation would have to complete in before the
internal state space goes incoherent).  The more bits are bound together,
the shorter the lifetime of those bits, according to one result.  However,
the more bits you have the longer the computation has to be.  This suggests
that any given Quantum Computer technology point will lead to a maximum
state size (likely in a small number of bits) for a given application.


Trouble is, I'm not a quantum physicist, so my understanding of this is
superficial.  Any real physicists out there?

 - Carl

> 
> The real roadblock might be that there are limits to how many bits there
> can be in a register.  NIST's recent Quantum Computation conference
> included discussion of the expected lifetime of a computation (what
> fraction of a second the computation would have to complete in before the
> internal state space goes incoherent).  The more bits are bound together,
> the shorter the lifetime of those bits, according to one result.  However,
> the more bits you have the longer the computation has to be.  This suggests
> that any given Quantum Computer technology point will lead to a maximum
> state size (likely in a small number of bits) for a given application.
> 
Something that might be relevant here is that relationship between energy and
lifetime for virtual particle generation. When a virtual particle is generated
it can have a random amount of energy. However, the larger the energy level
is the shorter its lifetime is. It is related to Plank's Constant in a 
relationship that I don't have on hand, but should be in most quantum texts.