Re: poly: quantum catalysis

Another interesting QM paper came out recently, announced by David Deutsch.
Deutsch is one of the pioneers of quantum computing and an advocate of the
unconventional no-collapse "many worlds" approach to QM. He writes:

   [A]nyone interested in the technical details of how quantum theory
   can be completely local despite Bell's theorem should read the
   following paper, which we have just submitted to Proceedings of the
   Royal Society:

   ----------------------------------

   Title: Information Flow in Entangled Quantum Systems
   Authors: David Deutsch, Patrick Hayden

   All information in quantum systems is, notwithstanding Bell's
   theorem, localised. Measuring or otherwise interacting with a
   quantum system S has no effect on distant systems from which S is
   dynamically isolated, even if they are entangled with S. Using the
   Heisenberg picture to analyse quantum information processing makes
   this locality explicit, and reveals that under some circumstances
   (in particular, in Einstein-Podolski-Rosen experiments and in quantum
   teleportation) quantum information is transmitted through 'classical'
   (i.e. decoherent) information channels.

   ----------------------------------

   You can download it in pdf format from
   http://xxx.lanl.gov/pdf/quant-ph/9906007

I was not able to follow the paper very well, because he uses the
Heisenberg formalism for QM (based on matrices), while in school
I learned the Schrodinger approach. Although these two methods are
mathematically equivalent they lead to very different interpretations of
how information flows in the EPR and quantum teleportation experiments
that Deutsch considers.

Deutsch argues that quantum information is essentially always localized.
Although there are situations where information is latent and cannot be
accessed locally, he draws an analogy to encryption using a one time pad.
A message encrypted in this way cannot, by itself, be deciphered; the
information is there but it must be combined with information elsewhere
to be revealed. A similar effect occurs in quantum teleportation: the
classical signal which must be sent from source to destination is shown
to carry latent quantum information which allows reconstruction of the
source state, but is not accessible in the classical signal itself.

This is in contrast to conventional interpretations of the phenomenon
which have the quantum information propagating instantaneously, or perhaps
going back into the past and then into the future, or behaving in other
bizarre ways. Deutsch shows that in the Heisenberg formulation the
information travels along a very prosaic path, carried by the particles
which communicate the classical measurement results.

Another interesting aspect of the paper is that Deutsch performs
his analysis using a quantum computing model. Rather than photons
or electrons flying around, he uses qubits and operates on them with
quantum gates. Since quantum computers can simulate any quantum system,
his results are general.

Deutsch summarizes,

   Given that quantum theory is entirely local when expressed in the
   Heisenberg picture, but appears nonlocal in the Schrodinger picture,
   and given that the two pictures are mathematically equivalent, are we
   therefore still free to believe that quantum theory (and the physical
   reality it describes) is nonlocal?

   We are not - just as we should not be free to describe a theory
   as "complex" if it had both a simple version and a mathematically
   equivalent complex version. The point is that a "local" theory is
   defined as one for which there exists a formulation satisfying the
   locality conditions that we stated at the end of Section 1 (and a local
   reality is defined as one that is fully described by such a theory).

And since the Heisenberg interpretation provides such an interpretation,
quantum mechanics should properly be viewed as local.

It will be interesting to see whether this attempt to demystify QM is
accepted by other workers in the field.

Hal
Received on Wed Jun 2 16:57:06 1999