| This thesis presents research on the initialization, control, and readout of electron spin states in gate defined GaAs quantum dots. The first three experiments were performed with Singlet-Triplet spin qubits in double quantum dots, while the remaining two experiments were performed with an Exchange-Only spin qubit in a triple quantum dot.;The first experiment examines the relationship between a dynamic nuclear polarization (DNP) process and the increase in measured singlet return probability in a Singlet-Triplet qubit. We find that the DNP process creates an Overhauser field difference, ΔBz, between dots, which enhances the relaxation of triplet states during the measurement. We propose a model which explains this relaxation mechanism, and find it in good agreement with the data.;The second and third experiments investigate the effects of dynamical decoupling sequences on the Singlet-Triplet qubit. We demonstrate the preservation of singlet-triplet superpositions by interlacing qubit rotations with Carr-Purcell (CP) sequences. We test the preservation of a singlet state with different dynamical decoupling schemes, finding a maximum coherence time of T2 ~ 80 μs for the CP sequence. The coherence time is studied for a number of π-pulses in the CP sequence, yielding an exponential dependence on even numbers of π-pulses. This allowed us to estimate the functional form of the noise spectrum influencing the qubit.;The fourth experiment studied the initialization, complete electrical control, and readout of an Exchange-Only spin qubit in a triple quantum dot. We demonstrate over 75 qubit rotations at 47 GHz and create a method of measurement and state tomography, enabling a quantification of the leakage from the qubit subspace.;In the final experiment, we bias the triple quantum dot in a regime that is protected from charge noise and leakage from the qubit subspace. In this regime we demonstrate a resonant two-axis control using microwaves, which offer superior control capabilities over the pulsed interactions used in the rest of this thesis. A 64 π-pulse dynamical decoupling sequence yields a coherence time of ~ 19 μs and a noise power spectrum that is heavily dominated by low frequency noise. |
