Spontaneous Rotational Symmetry Breaking in a Kramers Two-Level System

TL;DR

This paper models a Kramers two-level system respecting time-reversal symmetry, revealing spontaneous rotational symmetry breaking due to quantum vacuum fluctuations, with implications for Casimir-Polder forces and nonreciprocal polarizability.

Contribution

It introduces a formalism for Kramers two-level systems showing spontaneous symmetry breaking caused by quantum vacuum effects, especially in the context of Casimir-Polder interactions.

Findings

Spontaneous rotational symmetry breaking occurs in the ground state.

Nonreciprocal electric polarizability arises due to spin-orbit interaction.

Stable orientations can deviate from symmetric configurations in specific conditions.

Abstract

Here, I develop a model for a two-level system that respects the time-reversal symmetry of the atom Hamiltonian and the Kramers theorem. The two-level system is formed by two Kramers pairs of excited and ground states. It is shown that due to the spin-orbit interaction it is in general impossible to find a basis of atomic states for which the crossed transition dipole moment vanishes. The parametric electric polarizability of the Kramers two-level system for a definite ground-state is generically nonreciprocal. I apply the developed formalism to study Casimir-Polder forces and torques when the two-level system is placed nearby either a reciprocal or a nonreciprocal substrate. In particular, I investigate the stable equilibrium orientation of the two-level system when both the atom and the reciprocal substrate have symmetry of revolution about some axis. Surprisingly, it is found that when chiral-type dipole transitions are dominant the stable ground state is not the one in which the symmetry axes of the atom and substrate are aligned. The reason is that the rotational symmetry may be spontaneously broken by the quantum vacuum fluctuations, so that the ground state has less symmetry than the system itself.