Abstract
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.
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.
| Original language | English |
|---|
| Publisher | The Niels Bohr Institute, Faculty of Science, University of Copenhagen |
|---|---|
| Number of pages | 199 |
| Publication status | Published - 2017 |