We investigate the coherent exciton spin dynamics in a single InAlAs/AlGaAs quantum dot using time-resolved quantum state tomography. Under two-longitudinal-optical (LO)-phonon quasiresonant excitation of neutral exciton, we observe pronounced quantum beats in the circular and diagonal polarization components, reflecting the fine-structure splitting ($\mathrm{\Delta }\approx 19.6\mu \text{eV}$). 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 ($1.1\pm 0.2\mathrm{ns}$) that exceeds the exciton lifetime ($\sim 767$ ps). Our analysis indicates that the buildup of the exciton spin polarization exhibits no resolvable delay relative to the exciton population dynamics within the experimental time resolution. In addition, no discernible Overhauser shift was observed within the present signal-to-noise limit, suggesting a negligible influence of the local nuclear environment under the present experimental conditions. These results provide a quantitative benchmark for the three-dimensional spin-state reconstruction using differential polarization signals, demonstrating the feasibility of using quasiresonant excitation for stable spin initialization in semiconductor nanostructures.

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• Luminescence
• Quantum coherence & coherence measures
• Quantum correlations in quantum information
• Quantum interference effects
• Spin dynamics
• Quantum dots
• Semiconductors
• Quantum master equation
• Time-resolved photoluminescence