Filip K. Malinowski

A thesis submitted June 2017 for the degree of Doctor of Philosophy and defended august, 2017.

The PhD School of Science, Center for Quantum Devices, Faculty of Science, Niels Bohr Institute, University of Copenhagen

Academic supervisors:
Charles M. Marcus
Ferdinand Kuemmeth

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Noise suppression and long-range exchange coupling for gallium arsenide spin qubits

This thesis presents the results of the experimental study performed on spin qubits realized in gate-defined gallium arsenide quantum dots, with the focus on noise suppression and long-distance coupling.

First, we show that the susceptibility to charge noise can be reduced by reducing the gradient of the qubit splitting with respect to gate voltages. We show that for singlet-triplet and resonant exchange qubit this can be achieved by operating a quantum dot array in a highly symmetric configuration. The symmetrization approach results in a factor-of-six improvement of the double dot singlet-triplet exchange oscillations quality factor while the dephasing times for the threeelectron resonant exchange qubit are marginally longer.

Second, we present the study of the Overhauser field noise arising due to interaction with the nuclear spin bath. We show that the Overhauser field noise conforms to classical spin diffusion model in range from 1 mHz to 1 kHz. Meanwhile the megahertz-scale noise spectrum is focused in three narrow bands related to relative Larmor precession of the three nuclear species. Application of the dynamical decoupling sequence designed to notch-filter the narrowband noise enables us to put the highest, up to date, lower bound on the electron spin coherence time in gallium arsenide: 870 ms.

Later, we study the perspectives of exploiting a multielectron quantum dot as a mediator of the exchange interaction. We investigate interaction between a single spin and the multelectron quantum dot in nine different charge occupancies and identify ground state spin in all cases. For even-occupied spin-1/2 multielectron quantum dot a variation of the gate voltage by a few milivolts in the vicinity of the charge transition leads to sign change of the exchange interaction with a single neighboring electron.

Finally, we demonstrate the exchange coupling between distant electrons mediated by the evenoccupied spin-0 multielectron quantum dot. The exchange interaction strength can be controlled up to several gigahertz frequencies. Small level spacing and many body effects give raise to the positions in the gate voltage space that are characterized by decreased susceptibility to charge noise which can be used to implement high fidelity, long-range two-qubit gates.

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