Figure 1 (a) The AlGaAs/GaAs quantum dot device used throughout the experiment, whose crystallographic alignment is indicated at the lower-left corner. The orange circle marks the location of the quantum dot, and the crossed boxes indicate ohmic contacts. The cobalt micromagnet is enclosed by the red dotted lines, whose field direction is qualitatively plotted by the red transparent arrow. The magnetic field is applied in-plane to the 2DEG, as depicted in the upper left corner. (b) The charge stability diagram near (1,1) and (2,0) regimes. The location of the EST (PSB) regime is marked by the green box (blue triangle). (c) The Rabi oscillation, which is plotted after a series of Rabi pulse applications whose parking position was at either EST (green) or PSB (blue). When parked at EST, the oscillation frequency increased with the lab time, indicating that the serial Rabi pulse application increased ∆Bz. On the contrary, the parking at PSB showed the exact opposite behavior. (d) The change of the Rabi oscillation frequency as a function of lab time. The PSB- and EST-parking scheme shows the opposite behavior.
Figure 2 (a) The color plot of dP change under a single Rabi pulse. The clear reversal appears. The orange dotted lines mark the location for the left line-cut plot. (b) The line-cut plot of (a). Neither of line-cut shows dP reversal, indicating that T(2, 0) included dynamics is crucial in the T+ pumping effect in PSB regime. (c) The color plot of dP change under a single Rabi pulse with zero pulse width. The orange dotted lines mark the location for the left line-cut plot. (d) The line-cut of Fig (c).
Computational Simulation
This page contains detailed information about the simulation of experimentally observed parking-position dependent dynamic nuclear polarization effect.
Measurement of Superlinear tendency of Capacitive Coupling between Singlet-Triplet Qubits
Due to their relatively long coherence time and high scalability, spin qubit in a
semiconductor quantum dot is considered to be a promising candidate for realizing a
quantum computer. Among the various types of host materials, GaAs has been one
of the most successful materials in realizing a quantum dot spin qubit, although the
nucleus spin in GaAs results in relatively fast decoherence limiting the usability of the
GaAs. Fortunately, the dynamic nuclear polarization technique based on the hyperfine
mediated adiabatic spin flip-flop enabled the control of the nuclear spin. Furthermore,
the unidirectional T+-pumping effect even by nonadiabatic Rabi pulse has also been
reported, whose theoretical explanation has yet to be provided.
This thesis presents the bidirectional nuclear spin polarization effect of a nonadiabatic
Rabi pulse, whose polarization direction depends on the parking position. In
addition, we provide the theoretical explanation of the polarization effect of a nonadiabatic
Rabi pulse by the simulation result based on the excited-level spin mixing.
Specifically, We modeled the system using a modified Hubbard model for a two quantum dot system. The
results showed the triplet population change is consonant with the previously reported
T+-pumping effect of a PSB-parked Rabi pulse only when the excited levels are incorporated.
Moreover, the simulated population change also agreed with the observed
bidirectional polarization induced by a Rabi pulse, although long-term dynamics such
as the nuclear spin diffusion must be considered to explain the experimental observation
fully, especially in the EST regime. More information can be found at my master's thesis