On 07-08-2019 00:45, Bruce Kellett wrote:
On Wed, Aug 7, 2019 at 5:03 AM smitra <[email protected]> wrote:
On 06-08-2019 20:00, 'Brent Meeker' via Everything List wrote:
On 8/6/2019 6:38 AM, Bruno Marchal wrote:
If the QC does its task effectively, the output basis qbits
will
be put into definite states,
Relatively to the observer, but in the global state, the
observer
will inherit the superposition state, by linearity of the tensor
products and of the evolution.
In something like Shor's algorithm there is only one final state
with
non-vanishing probability. Yet this is the kind of algorithm
that
Deutsch cites as proving there must be many worlds.
Deutsch's does mention this, but his main argument is that you
could in
principle implement entire observers within a QC, simply because
the
universe is ultimately quantum mechanical and observers are
internal to
the universe. Note that locality implies that observations cannot
be
fundamentally based on permanent records due to environmental
decoherence.
I note that Deutsch does not believe in the non-locaility implied by
violation of the Bell inequalities. Non-locality is now an accepted
part of the understanding of quantum mechanics and of decoherence.
That's a different notion of non-locality (which Deutsch doesn't
accept). What's not in dispute is that there are only local
interactions.
If you do an experiment in the lab and make an observation
then there are still only a finite number of physical degrees
involved
in the entanglement with the observed quantum system. Whatever it
physically means to have observed a definite outcome would remain
unaffected when performed within a sufficiently large QC simulation
that
doesn't undergo decoherence itself (although it will end up
simulating a
decohering system).
The trouble with that argument is that in any simulation, you get to
set the rules of physics that obtain. There is then no guarantee that
the results of your simulation have any relation to physics in the
real (unassimilated) world. For decoherence to work, all that is
required is a sufficient number of environmental degrees of freedom
for multiple copies of the result to be recorded by the "environment
as witness", in Zurek's words. Quantum Darwinism then ensures that the
result is permanent and irreversible.
If you measure the z-component of a spin polarized in the x-direction,
then however astronomically large the number of environmental degrees of
freedom there are that get entangled with the spin, it's still a finite
number. One minute after the measurement all the degrees of freedom that
can be entangled are within one light-minute of the experimental set-up.
So, the recording of the result in the environment are going to be a
superposition of the two possible recordings.
Saibal
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