On 18 Jun 2017, at 21:17, Brent Meeker wrote:
On 6/18/2017 3:43 AM, Telmo Menezes wrote:
No, but it does mean that a quantum computer can have the
computational power of a lot of Turing machines acting in parallel,
and it is normal to ask "why?", and be unsatisfied with a theory
that
does not answer this question.
I have come across an interesting paper that discusses these
questions, and
comes to the conclusion that it is problematic to see quantum
computing as
accessing the computing power of other worlds.
Michael Cuffaro, http://arxiv.org/abs/1110.2514v2
Thanks for the paper, I finally had some time to read it. I also
write
in reply to Brent, I think we are all talking about the same thing.
First of all: I was too quick in accepting your definition of
world. I
think this is where our disagreement starts. You define a world
bottom-up, by saying that if two thing can interact, then they belong
to the same world. I would say that this is a good definition for
classical worlds, but it becomes useless when it is not longer clear
what "things" are these that can interact.
I prefer this definition: a world is the set of things that can be
observed. My definition forces one to invite theories of mind into
the
discussion. This is another thing I was too quick to accept: to talk
about "just physics". When we are discussing interpretations of QM,
it's no longer "just physics". Such a requirement makes the
discussion
non-sensical.
To be clear, let us consider the double-slit experiment.
Simplistically, under my definition there are three possible worlds:
the two ones where you know which sensor was activated and there is
no
interference, and the one where you do not know and there is
interference. The knowing/now knowing distinction is key here. The
world-as-what-can-be-observed can supervene on different subsets of
the totality, depending on which degrees of freedom are conserved. If
I can know which sensor was activated, then the world where I'm in is
not compatible with a superposition of states on the electron, and no
interference is observable. This is the fundamental breakdown of
"thingness" that QM brings to the table.
So back to quantum computation: what I think that QC demonstrates
(independently of it being realised by network models or cluster
states) is that the superposition of states really does mean that the
various states *exist*.
Superposition of states just means you choose a basis for the states
that did not have the right eigenstates. "Being in a superpostion"
is just a coordinate choice. It's like saying that going from
Bermuda to London is a superposition of north-traveling and east-
traveling.
? That resembles a confusion between pure state and mixed state. I am
sure you are aware of the difference so I find this answer very weird.
It brushes away all the interpretation problem we have of the facts
(not just the theory). Single photon (or molecule) interference would
not exist if pure state where not also superposition in the rotated
base.
Bruno
They are necessarily things, because they
provide a subtract for computation that does not exist otherwise. If
you wanted to deny this, you would have to be able to show me that
your interpretation of QC can be used to implement an equally
powerful
algorithm in a classical computer. But you cannot do that, cluster
state or not. This is what makes purely probabilistic interpretations
awkward, and I think this is the meat of Deutsch's intuition.
I think that the preferred basis problem operates at a lower level
than what I think is a useful definition of world. It's just a matter
of frame of reference. It's like refuting the idea that the Earth
follows and approximately elliptical orbit around the Sun by placing
the frame of reference in Venus. Both descriptions are valid, but the
first makes it easier to apprehend the actual underlying phenomenon.
An Ptolemy would have said it's a superposition of epicycles. That's
no reason to reify epicycles.
For me, the underlying phenomenon here is about what mind can
supervene on. Refusing to do this exercise seems absurd, because the
very point of interpreting quantum mechanics is to figure out what is
the reality that the equations describe, taking into account the
reality that we can observe. If we remove the problem of mind from
the
effort, there is no ground to stand on.
But if you take mind to supervene on the physics then observation
is just another physical interaction. What about it makes it
"underlying" or a something to define reality? As you know, I
resist the temptation to designate something as "fundamental".
I'd like to have an explanation of physics in terms of mind or
computation and vice versa.
But if you take mind to supervene on the physics then observation is
just another physical interaction. What about it makes it
"underlying" or a something to define reality? As you know, I
resist the temptation to designate something as "fundamental". I'd
like to have an explanation of physics in terms of mind and vice
versa.
If you want to take mind as basic then it seems that QBism is the
proper interpretation of QM. Most physicist dislike it because it
is personal - each person has their own idea of the wave function -
but that strikes me as a necessary aspect of a mind-based
interpretation.
It's just a popularity contest
between equally unfalsifiable hypothesis.
Interpretations of QM are unfalsifiable; that's what makes them
interpretations rather than theories.
The explanation for exponential speedup is:
"On this view, quantum computers are faster than classical
computers because
they perform fewer, not more, computations.
This is just circular reasoning, taking advantage of ambiguity on how
to count computations -- and begging the question when deciding how
to
count them.
It's no more circular than Deutsch's choice of how to count.
Brent
Otherwise, give me the algorithm so that I can test it
here, on my laptop.
Telmo.
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