In a recent preprint arXiv:2203.15521 G. X. Chan, P. Huang, and X. Wang claim that triple-quantum dot superexchange in a (1, 0, 1) charge configuration exhibits a change of sign (going from positive to negative) as a function of middle dot detuning. Furthermore, their claim is that charge sweet-spots exist for specific values of the inter-dot detuning. Their analysis is based on the Hubbard model and something to what they refer to as the full Configuration-Interaction method. All of this findings were already reported by M. J. Rančić and G. Burkard in Ref. Phys. Rev. B 96, 201304(R) (2017) based on the Hubbard model. No reference to this manuscript was made in Ref. arXiv:2203.15521. I have asked the authors to urgently modify the pre-print and position their work with respect to the previously conducted study - which they rejected to do at the current moment, quoting that pp. it is not their style to modify preprints before they were accepted. This alongside with a very similar style of some figures lead me to the conclusion that they are deliberately misleading the scientific community and trying to adopt other peoples work and ideas as their own.
In a recent preprint arXiv:2203.15521 G. X. Chan, P. Huang, and X. Wang claim that triple-quantum dot superexchange in a (1, 0, 1) charge configuration exhibits a change of sign (going from positive to negative) as a function of middle dot detuning. Furthermore, their claim is that charge sweet-spots exist for specific values of the inter-dot detuning. Their analysis is based on the Hubbard model and something to what they refer to as the full Configuration-Interaction method. All of this findings were already reported by M. J. Rančić and G. Burkard in Ref. Phys. Rev. B 96, 201304(R) (2017) based on the Hubbard model. No reference to this manuscript was made in Ref. arXiv:2203.15521. I have asked the authors to urgently modify the pre-print and position their work with respect to the previously conducted study - which they rejected to do at the current moment, quoting that pp. it is not their style to modify preprints before they were accepted. This alongside with a very similar style of some figures lead me to the conclusion that they are deliberately misleading the scientific community and trying to adopt other peoples work and ideas as their own.
Superexchange is one of the vital resources to realize long-range interaction between distant spins for large-scale quantum computing. Recent experiments have demonstrated coherent oscillations between logical states defined by remote spins whose coupling is given by the superexchange interaction mediated by central spins. Excavating the potential of superexchange requires a full understanding of the interaction in terms of control parameters, which is still lacking in literature. Here, using full configuration interaction calculations, we study a two-electron system in a linear triple-quantum-dot device in which the left and right dots are occupied by a single electron each, whose spin states are defined as qubits. The numerical nature of the full configuration interaction calculations allows access to the microscopic details of the quantum-dot confining potential and electronic wave functions, some of which are overlooked in the celebrated Hubbard model but turn out to be critical for the behavior of superexchange. Following experimental demonstrations of superexchange interactions, we focus on the detuning regime where the charge ground state yields an empty middle dot. We have found that, when the detunings at the left and right dots are leveled, the superexchange can exhibit a nonmonotonic behavior, which ranges from positive to negative values as a function of the middle-dot detuning. We further show that a larger relative detuning between the left and right dots causes the magnitude of the superexchange to increase (decrease) for an originally positive (negative) superexchange. We then proceed to show the results for a much larger left-right dot detuning. Using a Hubbard-like model, we present analytical expressions of the superexchange and have found that they conform well qualitatively with the numerical results. Our results suggest that even a simple configuration of delocalized two-electron states in a linear triple-quantum-dot device exhibits superexchange energy with nontrivial behaviors, which could have important applications in spin-based quantum computing.
Semiconductor quantum dot systems offer a promising platform for quantum computation. And these quantum computation candidates are normally based on spin or charge properties of electrons. In these systems, we focus on quantum computation based on electron spins since these systems has good scalability, long coherence times, and rapid gate operations. And this thesis focuses on building a theoretical description of quantum dot systems and the link between theory and experiments. In many quantum dot systems, exchange interactions are the primary mechanism used to control spins and generate entanglement. And exchange energies are normally positive, which limits control flexibility. However, recent experiments show that negative exchange interactions can arise in a linear three-dot system when a two-electron double quantum dot is exchange coupled to a larger quantum dot containing on the order of one hundred electrons. The origin of this negative exchange can be traced to the larger quantum dot exhibiting a spin triplet-like rather than singlet-like ground state. Here we show using a microscopic model based on the configuration interaction (CI) method that both triplet-like and singlet-like ground states are realized depending on the number of electrons. In the case of only four electrons, a full CI calculation reveals that triplet-like ground states occur for sufficiently large dots. These results hold for symmetric and asymmetric quantum dots in both Si and GaAs, showing that negative exchange interactions are robust in few-electron double quantum dots and do not require large numbers of electrons. Recent experiments also show the potential to utilize large quantum dots to mediate superexchange interaction and generate entanglement between distant spins. This opens up a possible mechanism for selectively coupling pairs of remote spins in a larger network of quantum dots. Taking advantage of this opportunity requires a deeper understanding of how to control superexchange interactions in these systems. Here, we consider a triple-dot system arranged in linear and triangular geometries. We use CI calculations to investigate the interplay of superexchange and nearest-neighbor exchange interactions as the location, detuning, and electron number of the mediating dot are varied. We show that superexchange processes strongly enhance and increase the range of the net spin-spin exchange as the dots approach a linear configuration. Furthermore, we show that the strength of the exchange interaction depends sensitively on the number of electrons in the mediator. Our results can be used as a guide to assist further experimental efforts towards scaling up to larger, two-dimensional quantum dot arrays.
Recent experiments on semiconductor quantum dots have demonstrated the ability to utilize a large quantum dot to mediate superexchange interactions and generate entanglement between distant spins. This opens up a possible mechanism for selectively coupling pairs of remote spins in a larger network of quantum dots. Taking advantage of this opportunity requires a deeper understanding of how to control superexchange interactions in these systems. Here, we consider a triple-dot system arranged in linear and triangular geometries. We use configuration interaction calculations to investigate the interplay of superexchange and nearest-neighbor exchange interactions as the location, detuning, and electron number of the mediating dot are varied. We show that superexchange processes strongly enhance and increase the range of the net spin-spin exchange as the dots approach a linear configuration. Furthermore, we show that the strength of the exchange interaction depends sensitively on the number of electrons in the mediator. Our results can be used as a guide to assist further experimental efforts towards scaling up to larger, two-dimensional quantum dot arrays.
Long-range interaction between distant spins is an important building block for the realization of a large quantum-dot network in which couplings between pairs of spins can be selectively addressed. Recent experiments on the coherent oscillation of logical states between remote spins facilitated by intermediate electron states were the first step for large-scale quantum information processing. Reaching this ultimate goal requires extensive studies on the superexchange interaction in different quantum-dot spatial arrangements and electron configurations. Here, we consider a linear triple quantum dot with two antiparallel spins in the outer dots forming the logical states while varying numbers of electrons in the middle dot form a mediator, which facilitates the superexchange interaction. We show that the superexchange is enhanced when the number of mediating electrons increases. In addition, we show that forming a four-electron triplet in the mediator dot further enhances the superexchange strength. Our work can be a guide to scale up the quantum-dot array with controllable and dense connectivity.
The resonant exchange (RX) qubit is a promising variant of the exchange-only spin qubit in a triple quantum dot which responds to a narrow-band resonant frequency. But the advantage of a permanently applied exchange splitting for spin control generally entails an increased susceptibility to charge noise. We have investigated the influence of electrical charge noise on a resonant exchange (RX) qubit by taking into account uncorrelated noise in each quantum dot, giving rise to two independent noisy bias parameters e and ∆ [1]. Calculating the energy splitting of the two qubit states as a function of these two bias detuning parameters, we have identified sweet spots, where the qubit is least susceptible to noise. Our investigation shows that the sweet spots exist within the low-bias regime, in which the bias detuning parameters have the same magnitude as the hopping parameters between the dots. By calculating and comparing the charge dephasing rates at the various operating points of the RX qubit, we identify a new favorable operating regime for the RX qubit in the case of weak noise, based on these double sweet spots. In contrast, spin noise can be mitigated using exchange-based dynamical decoupling sequences that have been optimized using two different strategies, Uhrig dynamical decoupling (UDD) and optimized filter function dynamical decoupling (OFDD) [2]. Finally, we give a brief outlook towards the possibility of long-distance coupling between resonant exchange qubits mediated by a microwave cavity [3]. [1] M. Russ and G. Burkard, Phys. Rev. B 91, 235411 (2015). [2] N. Rohling and G. Burkard, arXiv:1510.04098. [3] M. Russ and G. Burkard, Phys. Rev. B (accepted) [arXiv:1508.07122].
The energy landscape of a single electron in a triple quantum dot can be tuned such that the energy separation between ground and excited states becomes a flat function of the relevant gate voltages.These so-called sweet spots are beneficial for charge coherence since the decoherence effects caused by small fluctuations of gate voltages or surrounding charge fluctuators are minimized.We propose a new operation point for a triple quantum dot charge qubit, a so-called CQ 3 -qubit, having a third-order sweet spot.We show strong coupling of the qubit to single photons in a frequency tunable high-impedance SQUID-array resonator.In the dispersive regime, we investigate the qubit linewidth in the vicinity of the proposed operating point.In contrast to the expectation for a higher-order sweet spot, we there find a local maximum of the linewidth.We find that this is due to a non-negligible contribution of noise on the quadrupolar detuning axis not being in a sweet spot at the proposed operating point.While the original motivation to realize a low-decoherence charge qubit was not fulfilled, our analysis provides insights into charge decoherence mechanisms relevant also for other qubits.