Knight Field Enabled Nuclear Spin Polarization in Single Quantum Dots

TL;DR

This paper demonstrates nuclear spin polarization in single quantum dots without external magnetic fields, using optical excitation to induce an effective magnetic field that aligns nuclear spins, advancing quantum measurement techniques.

Contribution

It introduces a novel mechanism for nuclear spin polarization via Knight field effects induced by optical pumping, without the need for external magnetic fields.

Findings

Nuclear spin polarization achieved without external magnetic field.

Optical pumping induces an effective Knight field for spin alignment.

First step towards quantum measurement of the Overhauser field.

Abstract

We demonstrate dynamical nuclear spin polarization in the absence of an external magnetic field, by resonant circularly polarized optical excitation of a single electron or hole charged quantum dot. Optical pumping of the electron spin induces an effective inhomogeneous magnetic (Knight) field that determines the direction along which nuclear spins could polarize and enables nuclear-spin cooling by suppressing depolarization induced by nuclear dipole-dipole interactions. Our observations suggest a new mechanism for spin-polarization where spin exchange with an electron reservoir plays a crucial role. These experiments constitute a first step towards quantum measurement of the Overhauser field.