Information contents and architectural requirements of observer "ready" states

Abstract

A functional analysis of the task of observing multiple macroscopic quantum systems over an extended period of time and then reporting the accumulated results is used to investigate the information that must be encoded in the "ready" state |O^r> of any finite, macroscopic observer O capable of performing this task. Decoherence considerations show that this task can be considered as involving local observations under classical conditions (LOCC), allowing the use of classical automata theory to define a minimal observer. It is shown that such a minimal observer must implement a functional architecture equivalent to a classical Turing machine and must encode in |O^r> a classical specification of the complete set of reportable apparatus states. The observation task is then re-characterized employing an explicit model of such a minimal observer, and it is shown that both the assumption that external systems have well-defined boundaries against the environment and the assumption of decoherence are unnecessary for the characterization of measurements made by a minimal observer. It is shown that the observables available to a minimal observer are positive operator-valued measures (POVMs) and that the measurement results reported by a minimal observer comply with the Born rule. The differences in underlying physical assumptions between this "systems-free" treatment of observation and that traditionally employed in analyses of quantum measurement and quantum communication are discussed.