Quantum delocalization, gauge and quantum optics: The light-matter interaction in relativistic quantum information

TL;DR

This paper analyzes the light-matter interaction in relativistic quantum systems, emphasizing gauge subtleties, the validity of effective models like UDW, and the importance of quantum delocalization of atomic centers of mass.

Contribution

It clarifies the regimes where effective models are valid and refines dipole interaction models for relativistically moving atoms, highlighting the role of gauge and multipole approximations.

Findings

The UDW model with quantum COM misses Röntgen terms.

Effective dipole models can be covariantly formulated for moving atoms.

Gauge and multipole effects are crucial in relativistic light-matter interactions.

Abstract

We revisit the interaction of a first-quantized atomic system (consisting of two charged quantum particles) with the quantum electromagnetic field, pointing out the subtleties related to the gauge nature of electromagnetism and the effect of multipole approximations. We connect the full minimal-coupling model with the typical effective models used in quantum optics and relativistic quantum information such as the Unruh-DeWitt (UDW) model and the dipole coupling approximation. We point out in what regimes different degrees of approximation are reasonable and in what cases effective models need to be refined to capture the features of the light-matter interaction. This is particularly important when considering the center of mass (COM) of the atom as a quantum system that can be delocalized over multiple trajectories. For example, we show that the simplest UDW approximation with a quantum COM fails to capture crucial R\"ontgen terms coupling COM and internal atomic degrees of freedom with each other and the field. Finally we show how effective dipole interaction models can be covariantly prescribed for relativistically moving atoms.