We demonstrate adiabatic rapid passage on a subpicosecond time scale in a single semiconductor quantum dot, enabling the exploration of a regime of strong (and rapidly varying) Rabi energies for optical control of excitons. An observed dependence of the exciton inversion efficiency on the sign of the pulse chirp demonstrates the dominance of phonon-mediated dephasing, which is suppressed for positive chirp at low temperature. Our findings will support the realization of dynamical decoupling strategies and suggest that multiphonon emission and/or non-Markovian effects should be taken into account.

Structure, magnetization, and low-temperature spin dynamic behavior of zincblende Mn-rich Mn(Ga)As nanoclusters embedded in GaAsWe have investigated the carrier and magnetization dynamics in a GaMnAs structure with perpendicular uniaxial anisotropy using time-resolved pump probe techniques. Experiments were performed over two orders of magnitude variation in pump fluence, revealing an ultrafast demagnetization response that saturates at fluence values larger than 1 mJ/ cm 2 . Dichroic bleaching contributions exhibit no dependence on the circular polarization state of the pump beam, indicating no signature of electron spin dynamics, in contrast to experiments at similar pump pulse fluence in other III-Mn-V semiconductors. We observe no evidence of a transient hole spin depolarization despite the strong demagnetization effects in our experiments, suggesting that more studies are needed to elucidate the influence of hot holes on the nonlinear optical response of diluted magnetic semiconductors. Differential reflectivity experiments indicate an electron trapping time of 1 ps, followed by carrier recombination on a time scale of several nanoseconds. The demagnetization observed is incomplete, reaching only 80% of the equilibrium magnetization at saturation. We attribute this to the optical saturation of the band edge absorption in GaMnAs.

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Femtosecond optical control of the magnetization and coercive field is demonstrated in GaMnAs using time-resolved magneto-optical Kerr effect techniques. These experiments reveal a near-complete, subpicosecond collapse of the hysteresis loop, consistent with femtosecond demagnetization. On longer time scales ͑ϳ300 ps͒ an increase in coercivity is observed, attributed to hole-mediated enhancement of the domain wall energy.

In optimal quantum control (OQC), a target quantum state of matter is achieved by tailoring the phase and amplitude of the control Hamiltonian through femtosecond pulse-shaping techniques and powerful adaptive feedback algorithms. Motivated by recent applications of OQC in quantum information science as an approach to optimizing quantum gates in atomic and molecular systems, here we report the experimental implementation of OQC in a solid-state system consisting of distinguishable semiconductor quantum dots. We demonstrate simultaneous high-fidelity π and 2π single qubit gates in two different quantum dots using a single engineered infrared femtosecond pulse. These experiments enhance the scalability of semiconductor-based quantum hardware and lay the foundation for applications of pulse shaping to optimize quantum gates in other solid-state systems.