## Dynamics of quantum measurement and the measurement problem.

**Speaker:**Theo M. Nieuwenhuizen, Institute for Theoretical Physics University of Amsterdam

**When:**July 17, 2018 (Tue), 12:00PM to 01:30PM (add to my calendar)

**Location:**SCI 328

*This event is part of the Condensed Matter Theory Seminar Series. *

Quantum mechanics allows ideal measurements to be treated by postulates, which has led to various postulate-based interpretations of varying merit.

In a laboratory a measurement is performed with a physical apparatus. Much can be learned from solving the Curie-Weiss model for quantum measurement, where the z-component of a spin 1/2 is measured with a Curie-Weiss magnet.

In this exactly solvable model the which-basis question and the ready-state of the apparatus find obvious meanings. Three dynamical mechanisms have been identified: 1) truncation of the density matrix (disappearance of off-diagonal "Schrodinger cat" terms) 2) registration, where the macroscopic pointer benefits from a first order phase transition 3) subensemble relaxation inside the magnet, after decoupling it from the spin

While 1) and 2) are concordant with the postulates, 3) is related to the quantum measurement problem: how can one describe individual events in the ensemble approach? While this is trivial classically (a coin lies face up or down), it is considered to be unsolvable within quantum mechanics. Our approach nevertheless allows to formulate minimal postulates to connect to the reality in laboratories.

The Born rule is connected to indications of the macroscopic magnet, while the state of the microscopic spin is inferred from it. Most "quantum probabilities" should be left without interpretation. The frequency interpretation emerges from the addition rule for density matrices.