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Courtesy of centre for quantum computation.

Quantum information

Is information local?

All of the basic laws of physics that are known to approximate nature to some
reasonable degree can be expressed as fields evolve in space and time according to
hyperbolic partial differential equations.
This is true for all quantum fields in the "Standard Model" of particle physics and the classical
gravitational field in General Relativity.

Normally an evolution that can be fully described with partial differential equations
means that nature is "local", i.e. to describe what happens or is known at a
certain place depends only on what happens in its immediate neighbourhood.
Still, quantum mechanics seems to imply that this is not true in certain
situations. This leaves a tension between the basic field equations of physics and
the idea of non-local quantum information, that has been
impossible to understand for me.

Recently David Deutsch ,  a pioneer of quantum computing, argued in an interview  that
within Everett´s  relative-state interpretation  of quantum mechanics we can:
"...blow the 'quantum non-locality' misconception clean out of the water."
In my mind, if true,  this would be a most convincing argument in favour of the relative-state
(or "many-worlds" MWI)  interpretation.
Contrary to this view many MWI-specialists, like e.g. Hans-Dieter Zeh, are adamant that even in the MWI
non-locality is unavoidable. I had an e-mail discussion  about these opposing views in the spring
of 2001 with Henry Stapp. I also asked David Deutsch for his standpoint on this issue see
here  for his answer. In view of the motto of this site, it comes as no surprise that David Deutsch
likes Karl Popper.

Can the many-worlds interpretation be tested?

Based on an experiment done by Y.Kim and Y.Shih [quant-ph/9905039] Rainer Plaga  recently
proposed an practical test of the relative state interpretation:
"An extension of ``Popper´s experiment``  can test interpretations of  quantum mechanics
[quant-ph/0010030]  "(Found.Phys.Lett. 13, 461(2000)).
Since this paper was published, there have been controversial discussions if Kim and  Shih
gave a correct account of the meaning of their experimental results. I do not think anymore
that they did, see  here  why. See also quant-ph/0005063  by Tony Short. This withdraws the basis from my paper.
The search for a practical test - or a proof of its impossibility - of the relative-state
interpretation is this still on.

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