Coherent exciton spin dynamics and three-dimensional quantum state tomography in a single InAlAs quantum dot

TL;DR

This study demonstrates detailed three-dimensional quantum state tomography of exciton spins in a single InAlAs quantum dot, revealing long coherence times and the influence of exchange interactions under quasi-resonant excitation.

Contribution

It introduces a comprehensive method for quantum state tomography of exciton spins, highlighting the dynamics and coherence properties in a single quantum dot with minimal nuclear influence.

Findings

Observed quantum beats indicating fine-structure splitting of ~19.6 μeV.

Long-lived spin coherence of approximately 1.1 ns was measured.

Spin-formation time is shorter than the instrument response, enabling precise spin dynamics analysis.

Abstract

We investigate the coherent exciton spin dynamics in a single InAlAs/AlGaAs quantum dot using time-resolved quantum state tomography. Under two-LO-phonon quasi-resonant excitation of neutral exciton, we observe pronounced quantum beats in the circular and diagonal polarization components, reflecting the fine-structure splitting ($\Delta \approx 19.6 \ \mu\text{eV}$). By employing a global fitting procedure across three orthogonal polarization bases, we demonstrate that the spin evolution is consistently described by a unified Hamiltonian dominated by the anisotropic exchange interaction. While the initial degree of circular polarization is limited to $\approx 0.28$ due to fast relaxation processes during carrier cooling, the subsequent dynamics reveal a long-lived spin coherence ($1.1 \pm 0.2$ ns) that exceeds the exciton lifetime ($\sim 767$ ps). Our analysis reveals that the spin-formation time is significantly shorter than the instrument response function, and the absence of a discernible Overhauser shift confirms a negligible influence from the local nuclear environment under the present conditions. These results provide a quantitative benchmark for the three-dimensional reconstruction of spin trajectories using differential polarization signals, demonstrating the feasibility of using quasi-resonant excitation for stable spin initialization in semiconductor nanostructures.