Quantum information, the ambiguity of the past, and the complexity of the present
by
Prof.Charles Bennett,(IBM, Yorktown Heights)
→
Europe/Rome
Aula Conversi (Dip. di Fisica - Edificio G. Marconi)
Aula Conversi
Dip. di Fisica - Edificio G. Marconi
Description
Quantum information theory provides a coherent picture of the origin
of randomness and the emergence and decay of correlations, even in
macroscopic systems exhibiting few traditional quantum hallmarks. It
helps explain why the future is more uncertain than the past, and how
correlations can become macroscopic and classical by being redundantly
replicated throughout a system's environment. The most private
information, exemplified by which path a particle takes through an
interferometer, is not replicated and exists only transiently: after
the experiment is over, no record remains anywhere in the universe of
what "happened". At the other extreme is information that has been
replicated and propagated so widely as to be infeasible to conceal and
unlikely to be forgotten. Modern information technology has caused an
explosion of such information, eroding privacy while making it harder
for tyrants to rewrite the history of their misdeeds; and it is
tempting to believe that all macroscopic information is permanent,
making such cover-ups impossible in principle. But we argue, by
comparing entropy flows into and out of the Earth with estimates of
the planet's storage capacity, that most macroscopic classical
information---for example the pattern of drops in last week's
rainfall---is impermanent, eventually becoming nearly as ambiguous,
from a terrestrial perspective, as the which-path information of an
interferometer. Finally we discuss prerequisites for a system to
accumulate and maintain in its present state, as our world does, a
complex and redundant record of at least some features of its past.
Not all dynamics and initial conditions lead to this behavior, and in
those that do, the behavior itself tends to be temporary, with the
system losing its memory as it relaxes to thermal equilibrium.
