Mechanical rotation via optical pumping of paramagnetic impurities

TL;DR

This paper explores how optical pumping of paramagnetic impurities can induce mechanical rotation in hybrid quantum systems, revealing angular momentum transfer mechanisms that could be observed with advanced torsional or nanoparticle systems.

Contribution

It introduces a theoretical framework for understanding optical pumping-induced rotation in systems with different spin centers, highlighting angular momentum transfer via cross-relaxation.

Findings

Optical illumination induces rigid rotation of the host crystal.

Angular momentum transfer is robust against phonon scattering.

The effect is observable with current torsional and nanoparticle technologies.

Abstract

Hybrid quantum systems exhibiting coupled optical, spin, and mechanical degrees of freedom can serve as a platform for sensing, or as a bus to mediate interactions between qubits with disparate energy scales. These systems are also creating opportunities to test foundational ideas in quantum mechanics, including direct observations of the quantum regime in macroscopic objects. Here, we make use of angular momentum conservation to study the dynamics of a pair of paramagnetic centers featuring different spin numbers in the presence of a properly tuned external magnetic field. We examine the interplay between optical excitation, spin evolution, and mechanical motion, and theoretically show that in the presence of continuous optical illumination, inter-spin cross-relaxation must induce rigid rotation of the host crystal. The system dynamics is robust to scattering of spin-polarized phonons, a result we build on to show this form of angular momentum transfer should be observable using state-of-the-art torsional oscillators or trapped nanoparticles.