Alignment of nuclear spins in single quantum dots using unpolarized light

TL;DR

This paper demonstrates a method to align nuclear spins in single quantum dots using unpolarized light, revealing a polarization mechanism independent of light polarization and wavelength.

Contribution

It introduces a novel optical pumping technique that aligns nuclear spins via virtual spin-flip transitions, independent of light polarization.

Findings

Nuclear spin polarization occurs via virtual spin-flip transitions.

The polarization sign depends on exciton energy asymmetry.

Nuclear spin alignment is achieved with unpolarized light.

Abstract

We report optical pumping of neutral quantum dots leading to nuclear spin alignment with direction insensitive to polarization and wavelength of light. Measurements of photoluminescence of both "dark" and "bright" excitons in single dots reveal that nuclear spin pumping occurs via a virtual spin-flip transition between these states accompanied by photon emission. The sign of the nuclear spin polarization is determined by asymmetry in the exciton energy spectrum, rather than by the sign of the exciton spin polarization.