Quantum measurements and equilibration: modelling the emergence of objectivity via entropy maximisation

Textbook quantum physics features two types of dynamics: reversible unitary dynamics and irreversible measurements.

The latter stands in conflict with the laws of thermodynamics and has evoked much debate. With the help of modern quantum statistical mechanics, we take the first step in formalising the hypothesis that quantum measurements are instead driven by the natural tendency of closed systems to maximise entropy, a notion that we call the Measurement-Equilibration Hypothesis. In this paradigm, we investigate how objective measurement outcomes can emerge within an purely unitary framework, and find that: (i) the interactions used in standard measurement models fail to spontaneously feature emergent objectivity and (ii) while ideal projective measurements are impossible, we can (for a given form of interaction) approximate them exponentially well as we collect more physical systems together into an "observer system". We thus lay the groundwork for self-contained models of quantum measurement, and conclude by proposing some improvements to our simple scheme.