Velocity-like maximum polarization: irreversibility and quantum measurements

TL;DR

This paper explores the limits of polarization in scattering processes, revealing a velocity-like addition rule that prevents polarization from exceeding unity, and discusses its implications for quantum measurement and time irreversibility.

Contribution

It introduces a velocity-like addition rule for polarization, linking quantum measurement to time irreversibility and analyzing specific cases for spin 1/2 and 1.

Findings

Polarization in scattering processes cannot exceed 1.

Photon linear polarization increases monotonically in Thomson scattering.

The polarization behavior relates to quantum measurement and time irreversibility.

Abstract

The polarization emerging in the subsequent scattering processes can never exceed $1$ which corresponds to the fully polarized pure state. This property is shown to be provided by the addition rule similar to that for relativistic velocities never exceeding the speed of light. The cases of spin $1/2$ and $1$ are considered. The photon linear polarization in Thomson scattering is monotonically increasing. This directness is shown to be a consequence of spin measurement procedure and may be the particular example of ithe anticipated relation between quantum measurement and time irreversibility. The emergent polarization may be considered as a case of opposing time's arrows corresponding to microscopic (spin) and macroscopic (momentum) degrees of freedom, respectively.