Physical interpretation of the Wigner rotations and its implications for relativistic quantum information

TL;DR

This paper offers a new physical interpretation of Wigner rotations affecting the spin of relativistic particles, challenging previous assumptions and suggesting that standard spin operators are inadequate for certain measurements, with an experimental test proposed.

Contribution

It introduces a novel physical interpretation of Wigner rotations that alters the understanding of spin measurements in relativistic quantum information.

Findings

Reduced density matrix for spin cannot be defined.

Pauli-Lubanski operators are unsuitable for certain measurements.

Proposes an experimental test of the new formulation.

Abstract

We present a new treatment for the spin of a massive relativistic particle in the context of quantum information based on a physical interpretation of the Wigner rotations, obtaining different results in relation to the previous works. We are lead to the conclusions that it is not possible to define a reduced density matrix for the particle spin and that the Pauli-Lubanski (or similar) spin operators are not suitable to describe measurements where spin couples to an electromagnetic field in the measuring apparatus. These conclusions contradict the assumptions made by most of the previous papers on the subject. We also propose an experimental test of our formulation.