From: <hal@finney.org>

Date: Wed Jun 02 1999 - 16:53:37 PDT

Date: Wed Jun 02 1999 - 16:53:37 PDT

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

*
This archive was generated by hypermail 2.1.8
: Tue Mar 07 2006 - 14:45:30 PST
*