We present a theoretical proposal of a tunable charge qubit, hosted in triple quantum dots. The manipulation is solely performed by changing the heights of the two potential barriers between the three dots, while the energy of all three dots are fixed. We have found that when the relative height of the two barriers are changed, the direction of the axis of rotation in performing single-qubit gates can be varied. On the other hand, the corresponding rotation speed can be tuned by raising or lowering the two barriers at the same time. Our proposal therefore allows for tunability of both the rotation axis and rotating speed for a charge qubit via all-electrical control, which may facilitate realization of quantum algorithms in these devices. arXiv:1805.04221v2 [cond-mat.mes-hall]
This paper presents a simple yet effective strategy to suppress the leakage in a three‐quantum‐dot charge qubit system by having the dots in a triangle as opposed to the linear geometry commonly conceived. It is found that the tunnel coupling between the two outmost dots is amplified in triangular triple dots, which consequently reduces leakage by separating the leaked state and qubit states. It has been found that the leakage can be suppressed by as much as five orders of magnitude when the dots form an equilateral triangle, with further improvement of control fidelities possible if composite pulses are applied.
We consider two typical approximations that are used in the microscopic calculations of doublequantum dot spin qubits, namely, the Heitler-London (HL) and the Hund-Mulliken (HM) approximations, which use linear combinations of Fock-Darwin states to approximate the two-electron states under the double-well confinement potential. We compared these results to a case in which the solution to a one-dimensional Schrödinger equation was exactly known and found that typical microscopic calculations based on Fock-Darwin states substantially underestimate the value of the exchange interaction, which is the key parameter that controls the quantum dot spin qubits. This underestimation originates from the lack of tunneling of Fock-Darwin states, which is accurate only in the case with a single potential well. Our results suggest that the accuracies of the current two-dimensional molecular-orbit-theoretical calculations based on Fock-Darwin states should be revisited since underestimation could only deteriorate in dimensions that are higher than one. arXiv:1801.00902v1 [quant-ph]
Recent studies reveal that a double‐quantum‐dot system hosting more than two electrons may be superior in certain aspects as compared to the traditional case in which only two electrons are confined (a singlet–triplet qubit). The electron–phonon dephasing occurring in a GaAs multi‐electron double‐quantum‐dot system is studied, in a biased case in which the singlet state is hybridized, as well as in an unbiased case in which the hybridization is absent. It is found that while the electron–phonon dephasing rate increases with the number of electrons confined in the unbiased case, this does not hold in the biased case. A merit figure is defined as a ratio between the exchange energy and the dephasing rate, and have shown that in experimentally relevant range of the exchange energy, the merit figure actually increases with the number of electrons in the biased case. The results show that the multi‐electron quantum‐dot system has another advantage in mitigating the effect of electron–phonon dephasing, which is previously under‐appreciated in the literature.
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