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Magnetization is induced in an ensemble of quantum dots, each charged with a single electron, when it is illuminated with a short circularly polarized light pulse that is resonant with the fundamental energy gap of the quantum dots. In this investigation, a quantum-mechanical model for the light-induced magnetization is presented. The phase of the magnetization precession as a function of the strength of the magnetic field in a Voigt geometry is in excellent agreement with experimental data measured on (In,Ga)As singly charged quantum dot ensembles. It is demonstrated that the precession of the hole in the trion plays a vital role because it determines the amplitude and phase of the magnetization precession. The model could also be easily extended to describe positively charged quantum dots. We also suggest that our theory, combined with measurements of the phase as a function of magnetic field, can be used as a technique to measure the resonant trion lifetime as a function of QD emission energy.Due to the long lifetime of the confined electron spin state, quantum dots (QDs) charged with a single electron are promising candidates for spin-based functional devices (see, for example, Ref.[1]). Of all methods, the manipulation and readout of the confined electron spin state using light is the fastest possible, and therefore has attracted the wide attention of researchers. Understanding the physics behind light control of magnetism is essential to advance this field and device applications based on it. A standard experiment that can be performed is to illuminate an ensemble of charged QDs with a short light pulse resonant with the fundamental energy gap of the QDs and measure, using the pump-probe technique, the magnetization of the ensemble as a function of time [2,3]. In this case a circularly polarized π pulse induces a magnetization that precesses around a magnetic field [3,4] (see Fig. 1 for an example). The long-term magnetization that remains after the recombination of photoexcited particles is associated with the electrons that are permanently resident in the QD. The long-term oscillation of the z component of the magnetization is described by [2,5,6] where M z0 is the amplitude of the oscillation, T * 2 the ensemble dephasing time [6], = g e eB 2m 0 the Larmor frequency, g e the electron g factor, and φ the phase of the oscillation. The minus sign in (1) has been added for convenience of the phase definition. In this Rapid Communication we associate the magnitude of parameters M 0 and φ to the mechanism by which light creates magnetization in a QD ensemble. We present a quantum-mechanical theory for the magnetization process and compare its predictions with our pump-probe experimental data obtained on (In,Ga)As QDs. Our analysis shows that the coherence of the spin of the photoexcited holes plays a vital role in light-induced magnetization and spin coherence generation of QD ensembles.When a magnetic field is applied along the y direction, there will be a thermal distribution in the ensemble of both spin ori...

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