2.1.1 Structure of Quantum Theory

The relation of quantum mechanics to physical reality is highly abstract and indirect. It does not give any intuitive image of the physical system but only provides a set of recipes to produce numbers that are predictions for the outcomes of experimental procedures. The theory has emerged from the treatment of experimental realities; the historical development is reviewed in many places.

In this book we do not follow the historical path to learn the theory; instead, we want to present it as an instrument to use in physical situations. As far as is possible, we avoid any too-specific interpretations; rather, we offer the results as concisely as we are able to. We present a sort of minimal interpretation of the apparatus provided by quantum theory. We do this partly because it avoids getting sidetracked by confusing arguments about interpretation and partly because the rapidly evolving field of quantum information casts a novel light on the standard procedures, and thus new interpretations may lurk just beyond the horizon. In this situation, any unnecessary attachment to views of the past may be a hindrance.

In many texts, the structure of quantum theory is presented as a formal axiomatic structure. Here we regard it, rather, as a toolbox providing instructions as to how to proceed in various situations. Hence, we do not formulate the structure as axioms but rather as a set of rules agreed upon by the ...

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