Optical Orientation of Nuclear Spins in an Individual Quantum Dot

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Electron spins trapped in solid-state systems can exhibit strong hyperfine interactions with a nuclear spin reservoir, which is normally fluctuating and randomly oriented. As this represents a fundamental decoherence mechanism for the electron spin, several theoretical scenarios to suppress this effect in semiconductor quantum dots (QDs) have been proposed. However, implementing these proposals requires a deeper understanding of the properties of the QD nuclear spin ensemble and of the possibilities to manipulate the nuclear spins. The present work describes an experimental assessment of these topics. Optical preparation and detection of the spin and energy of QD electrons was used to manipulate and measure the average nuclear spin polarization (NSP) in an individual QD. Studying NSP as a function of external paramters such as magnetic fields, QD charge and of time revealed the nonlinear character of electron-nuclear spin coupling and led to the identification of the dominant spin-relaxation mechanisms for QD nuclei. Ultimately, further understanding of the subtle interactions that govern the QD nuclear spin system could enable us to improve the spin coherence time of QD electrons.