Exact Decoherence and Orthogonal Pointer States Brought by One Degree of Freedom: von Neumann Equation Approach and Examples

TL;DR

This paper rigorously confirms a quantum measurement model that achieves exact decoherence and orthogonal pointer states using a von Neumann equation approach, with specific examples involving spin-1/2 particles and harmonic oscillators.

Contribution

It re-examines and rigorously validates a previous measurement model by solving the von Neumann equation, providing concrete examples of exact decoherence in quantum systems.

Findings

Exact decoherence and orthogonal pointer states are confirmed under specific initial conditions.

The von Neumann equation approach reproduces previous results with added rigor.

Illustrations include measurements of spin-1/2 particles and harmonic oscillators.

Abstract

In a quantum measurement setting, it is known that environment-induced decoherence theory describes the emergence of effectively classical features of the quantum system-measuring apparatus composite system when the apparatus is allowed to interact with the environment. In [E.A. Galapon {\it EPL} {\bf 113} 60007 (2016)], a measurement model is found to have the feature of inducing exact decoherence at a finite time via one internal degree of freedom of the apparatus provided that the apparatus is decomposed into a pointer and an inaccessible probe, with the pointer and the probe being in momentum-limited initial states. However, an issue can be raised against the model: while the factorization method of the time evolution operator used there is formally correct, it is not completely rigorous due to some unstated conditions on the validity of the factorization in the Hilbert space of the model. Furthermore, no examples were presented there in implementing the measurement scheme in specific quantum systems. The goal of this paper is to re-examine the model and confirm its features independently by solving the von Neumann equation for the joint state of the composite system as a function of time. This approach reproduces the joint state obtained in the original work, leading to the same conditions for exact decoherence and orthogonal pointer states when the required initial conditions on the probe and pointer are imposed. We illustrate the exact decoherence process in the measurement of observables of a spin-1/2 particle and a quantum harmonic oscillator by using the model.