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. 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).
Saibal
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