The Quantum Nature of Information

The information revolution is based on what a physicist would call a classical view of information.  Quantum effects were long regarded as a mere nuisance for information processing, preventing information in microscopic objects from being observed or copied accurately, but are now known to make possible feats like quantum cryptography and possible dramatic speedups of some computations. More importantly, the quantum approach has led to a more coherent and powerful way of thinking about information.  We review this approach, especially the uniquely private form of correlation known as entanglement, which plays a role in many ways complementary to classical information.  Quantum principles help explain the origin of randomness, why the future is more uncertain than the past, and why some information is evanescent, while other information becomes ever more durable by being redundantly replicated throughout the environment.  The most private information, such the path of an unobserved particle in the famous two-slit experiment, evades this replication and exists only temporarily: after the experiment is over, no record remains anywhere in the universe of what "happened".  Less private information includes classical secrets, facts known only to a few, and information like the lost literature of antiquity that once was public but has been forgotten over time. Finally there 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 a proliferation of such information, eroding personal privacy along with the ability of tyrants to conceal their misdeeds.  Although one might hope that whenever information is amplified to macroscopic size, it becomes ineradicable, we argue that most once-classical, macroscopic information---for example the pattern of drops in last week's rainfall---is impermanent, soon becoming nearly as ambiguous, from a terrestrial perspective, as the path of an unobserved particle. Depending on the diligence and forgetfulness of their enemies, the fate of mysteriously disappeared persons like Jimmy Hoffa, thought to have been murdered in 1977, may by now have acquired this ambiguous status. Finally we discuss the physical 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. Thermodynamic disequilibrium, long regarded as key to the origin and maintenance of the world's complexity, also underlies its superficially classical appearance, allowing quantum laws to have remained  undiscovered until the 20th century.