Elucidating the quantum measurement problem

Note: Pizza served at 11:45 AM
Speaker: Theo Nieuwenhuizen, Institute for Theoretical Physics, UvA, Amsterdam, The Netherlands

When: April 29, 2011 (Fri), 12:00PM to 01:00PM (add to my calendar)
Location: SCI 352
Hosted by: Paul Krapivsky

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract
Textbooks describe ideal quantum measurements by two postulates: the collapse of the wave packet and Born’s rule for the probabilities of outcomes. The quantum evolution of a system then has two components: unitary (Hamiltonian) in between measurements and non-unitary when a measurement is performed. This split was considered as unsatisfactory by many people, including Einstein, Bohr, de Broglie, von Neumann and Wigner.
The quantum measurement problem, that is, understanding why a unique outcome is obtained in each individual run of an experiment, is tackled by solving a Hamiltonian model for a quantum measurement within standard quantum statistical mechanics. The model describes the measurement of the z-component of a spin through interaction with a magnetic memory. The latter apparatus is modeled by a Curie-Weiss magnet having N >> 1 spins weakly coupled to a phonon bath.
The Hamiltonian evolution exhibits several time scales. The reduction, a rapid decay of the off-diagonal blocks of the system-apparatus density matrix, arises from the many degrees of freedom of the pointer (the magnetization). The registration occurs due to a phase transition from the initial metastable state to one of the final stable states, triggered by the tested system. It yields a stationary state in which the apparatus and the system are correlated. Under proper conditions the process satisfies all features of ideal measurements, including collapse and Born’s rule. The irreversibility of the measurement is ensured by the large size of the apparatus. Nothing else than the usual quantum statistical mechanics and Schrodinger equation is needed, and the results support a specific version of the statistical interpretation. The solution of the quantum measurement problem requires a combination of the reduction and the registration, the properties of which arise from the irreversible dynamics.
This talk summarizes work done in last decade in collaboration with Armen Allahverdyan of the Yerevan Physics Institute and Roger Balian of the IPhT, CEA Saclay.