Manipulation and coherence of a two-electron logical spin qubit using gallium arsenide double quantum dots

Spin qubits have been considered promising candidates for quantum computation due to their expected long coherence times. In a solid state environment, such as GaAs, electrons confined to quantum dots interact with an ensemble of about a million nuclei through hyperfine interaction. The polarization of the nuclear bath fluctuates around a zero mean which was considered to be a serious obstacle towards extending the qubit's coherence time T2 beyond approximately 1 microsecond. In addition, the inhomogeneous coherence time T*2 has been measured to be in the range of 10 ns, ultimately limiting the fidelity of the qubit.;Here we explore, using two electron logical spin qubits in GaAs, the possibility of achieving universal control of the qubit's state and of prolonging the qubits coherence times. Our main results include: (1) Dynamical polarization of the nuclei to reach local magnetic field gradients up to 200 mT. (2) The use of local field gradients to perform universal quantum control and quantum state tomography of the qubit. (3) The use of a quantum feedback mechanism intrinsic to our system in order to pump the nuclear bath and simultaneously narrow its distribution, thereby reducing the amplitude of its stochastic fluctuations that lead to inhomogeneous dephasing. (4) Using decoupling techniques well established in NMR we show that we can extend the homogeneous coherence time of the qubit by three orders of magnitude compared to previous reports. (5) Unraveling the contribution of spin-orbit coupling to the nuclear dynamic polarization using a pumping scheme that never exchanges electrons with the reservoir. The modulation of the polarization efficiency by the nuclear Larmor precession period is interpreted as arising from the coherent interplay of spin-orbit and hyperfine interaction.