ca314159 (ca314159@bestweb.net)
Fri, 19 Nov 1999 18:22:51 -0800
I.Vecchi wrote:
>
> Stephen Paul King wrote:
> >
> > Hi All,
> >
> > Robert Fung is making some great points!
> >
> > Later,
> >
> > Stephen
> >
> > ... entanglement is an additional problem when you consider
> > not just the state-space of a single particle, but the
> > state space of two particles that interacted and so their
> > PD's and PDF's have some memory of that event as if they
> > were two bell's (or impulse response functions[1]) that
> > once clanged together and when separated, they maintained
> > a "memory" of that event in their separate sets of PDs and PDFs.
> > Those separate memorys are what allow the two particles
> > to be non-locally correlated, or "entangled".
> >
> > Those memories however tend fade away (decohere) after a while.
> > But they should be maintainable, by a _local_ resonant
> > communications between the entangled particles.
> >
> > Of what use that may be to quantum cryptography &c.,
> > I am not concerned with, as I think there are more significant
> > implications than that.
> >
>
> Decoherence is indeed a slippery concept, often used in an improper way.
> The above statement about "fading memories" is in my opinion confusing.
> The point is that decoherence does not destroy long-range quantum
> superpositions. Decoherence just limits the ability of an observer
> subject to the second principle of thermodynamics to keep track of such
> superpositions.
>
Superpositions are not necessarily entanglements.
A superposition is what happens at a beam splitter.
An entanglement is what happens in a non-linear crystal.
Zeilinger et al, don't use beamsplitters to make entangled
particle pairs. Superpositions and entanglements are different.
Wavefunction collapse can be alot like tapping a computer
programmer on the shoulder while s/he is in deep superposition
of many associated concepts; destoying their concentration.
Decoherence is like you say, a loss of the ability to
make meaningful distinguishments, or the loss of the ability
to maintain a record of a past event (entanglement) and
this diffusion or dispersion does not destroy all record
of the past event but makes it increasingly unrecoverable
in a theromdynamic sense, which itself can probably be modelled
as a damping in an impulse-response (Green's functions) sense,
like a pair of bells having clanged against each other
and that ringing of each bell, fading like a memory of
the past as the energy of the event disperses within the bell.
Single-moded spatio-temporal solitons (light-bullets)
are recognized for their ability to resist such dispersion
and are proposed for this reason as candidates for qubits
which resist decoherence. Other possibilities for
decoherence resistent qubits might be BE condensates,
superconductors, and superfluids, and extremely stabilized
laser-light (non-interaction experiments); all of which
effectively seem to homogenize their component units to the
point where they act in extreme unison (coherence).
Pure coherence, unadulterated, is apparently not possible
in any finitely bounded system. But we none-the-less have
computers, which function almost completely as if this
idealism of both continuity and closure were attainable in a
finite system.
The Zen master will tap the meditating student on the shoulder
to remind him that closures are indeed just as much a part of
life as coherence. Decisions have to be made.
An electric battery represents distinguishment. Its potential
energy is meaningless without continuity (a circuit).
But shorting out the terminals of the battery and explosively
releasing all its energy is also meaningless.
We build an electric circuit with resistance (decoherence) and
yet it can still have meaning, but that meaning is (as Stephan
minds me) is generally derived from outside the energy
economy of that circuit.
The battery's energy is eventually depleted and it fails
to serve its purpose to make some form of distinguishment.
In order for its purpose to survive, the energy must
be replenished from the outside, but not with excessive
greed (we live off the land and should respect the Time
it takes to feed us and itself).
--
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