文档介绍：Information Flow in Entangled
Quantum Systems
David Deutsch and Patrick Hayden
Centre for putation
The Clarendon Laboratory
University of Oxford, Oxford OX1 3PU, UK
June 1999
All information in quantum systems is, notwithstanding BellÕs 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
ÔclassicalÕ (. decoherent) information channels.
1. Quantum information
It is widely believed (see . t and Shor (1998)) that in general plete
description of posite quantum system is not deducible plete
descriptions of its subsystems unless the ÔdescriptionÕ of each subsystem S depends
on what is going on in other subsystems from which S is dynamically isolated. If
this were so, then in quantum systems information would be a nonlocal quantity Ð
that is to say, the information in posite system would not be deducible from
the information located in all its subsystems and, in particular, changes in the
distribution of information in a spatially extended quantum system could not be
understood wholly in terms of information flow, . in terms of subsystems carrying
information from one location to another. In this paper we shall show that this belief
David Deutsch and Patrick Hayden Information Flow in Entangled Quantum Systems
is false. It has given rise to a wide range of misconceptions, some of which we shall
also address here, but our main concern will be with the analysis of information flow
in quantum information-processing systems.
Any quantum Ôtwo-stateÕ system such as the spin of an electron or the polarisation of
a photon can