Interdot Coupling Research Articles

Tuning single-electron charging and interactions between compressible Landau level islands in graphene

Interacting and tunable quantum dots (QDs) have been extensively exploited in condensed matter physics and quantum information science. Using a low-temperature scanning tunneling microscope (STM), we both create and directly image a new type of coupled QD system in graphene, a highly interacting quantum relativistic system with tunable density. Using detailed scanning tunneling spectroscopy (STS) measurements, we show that Landau quantization inside a potential well enables novel electron confinement via the incompressible strips between partially filled Landau levels (LLs), forming isolated and concentric LL QDs. By changing the charge density and the magnetic field we can tune continuously between single- and double-concentric LL QD systems within the same potential well. In the concentric QD regime, single-electron charging peaks of the two dots intersect, displaying a characteristic avoidance pattern. At moderate fields, we observe an unconventional avoidance pattern that differs significantly from that observed in capacitively coupled double-QD systems. We find that we can reproduce in detail this anomalous avoidance pattern within the framework of the electrostatic double-QD model by replacing the capacitive interdot coupling with a phenomenological charge-counting system in which charges in the inner concentric dot are counted in the total charge of both islands. The emergence of such strange forms of interdot coupling in a single potential well, together with the ease of producing such charge pockets in graphene and other two-dimensional (2D) materials, reveals an intriguing testbed for the confinement of 2D electrons in customizable potentials.

Controlling collision properties of solitons in five-level M-type triple quantum dot electromagnetically induced transparency medium by inter-dot tunneling coupling

<sec>Experimentally, the triple-quantum-dots system can be produced on a GaAs <inline-formula><tex-math id="M7">\begin{document}$ \left[ {001} \right]$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M7.png"/></alternatives></inline-formula> substrate by molecular beam epitaxy or in-situ atomic layer precise etching, thus enabling a triangle triple quantum dot (QD) aligned along the <inline-formula><tex-math id="M8">\begin{document}$ \left[ {1\bar 10} \right]$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M8.png"/></alternatives></inline-formula> direction. According to this, we first propose a five-level M-type triple QD electromagnetically induced transparency (EIT) model which consists of a triple QD molecule interacting with a weakly linearly polarized probe field with two orthogonal polarization components under the action of a magnetic field parallel to the light propagation direction. Subsequently, by using the multiple-scale method combined with the Fourier integration method, the propagation characteristics of the optical solitons and the collision characteristics of two solitons in the system are studied. It is shown that the optical solitons can form and propagate stably in this system under the action of quantum inter-dot tunneling coupling whose formation mechanism is different from the soliton-forming mechanism in ultra-cold atomic, single QD, and double QD EIT system. This is because the necessary condition for forming a soliton is to use a strong light beam to modulate a weak light beam, whether it is in an ultra-cold atom system, or a single quantum dot EIT medium or a double quantum dot EIT medium. In a word, the formation of soliton in previous EIT systems needs an additional strong controlling field, while the five-level M-type triple QD EIT system is dependent on the inter-dot tunneling.</sec><sec>Since the solitons can propagate stably, the collision properties of the solitons may be studied in this system. Finally, by applying Fourier integration method, it is found that the collision behaviors of two solitons are determined by their initial phase difference. When their initial phase difference is 0, the collision behavior between the solitons is periodic elastic collision. While their initial phase difference is separately <inline-formula><tex-math id="M9">\begin{document}$ {\rm{\pi }}/4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M9.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M10">\begin{document}$ \text{π}/2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M10.png"/></alternatives></inline-formula>, and <inline-formula><tex-math id="M11">\begin{document}$ \text{π}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200141_M11.png"/></alternatives></inline-formula>, the collision behaviors exhibit separation phenomenon due to repulsive effect. Interestingly, the collision characteristics of two solitons are controlled by the inter-dot tunneling strength. With the increase of inter-dot tunneling strength, the collision period of two solitons with the initial phase difference of 0 decreases, and the repulsive force of two solitons with the initial phase difference being separately π/4, π/2 and π increases. This provides some theoretical basis for experimentally controlling the soliton dynamical properties in semiconductor quantum dot devices.</sec>

Optical properties of nanocrystal films: blue shifted transitions as signature of strong coupling.

We present a theoretical study at the atomistic level of the optical properties of semiconductor nanocrystal films. We investigate the dependence of the absorption coefficient on size, inter-dot separation, surface stoichiometry and morphology, temperature, position of the Fermi level and light polarization. Our results show that, counter-intuitively, huge blue shifts are expected in some intra-band transitions for strongly coupled arrays, in contrast with the predicted and observed red shift of the band gap absorption in such systems. Furthermore, we find that the energies of such transitions can be tuned within a range of several hundreds of meV, just by engineering the inter-dot separation in the film through the choice of appropriately sized capping ligands. Finally we discuss the application of this effect to nanocrystal-based intermediate-band solar cells.

Josephson Effect in a T‐Shaped Triple‐Quantum‐Dot Structure: π‐Junction Behavior and Two‐Stage Kondo Effect

By means of the exact diagonalization approach, the interplay between the Josephson effect and the two‐stage Kondo physics in a T‐shaped triple‐quantum‐dot (TQD) junction is theoretically investigated. It is found that by adjusting the interdot couplings, the Josephson current exhibits the rich 0–π transition behaviors. These results originate from the mutual competition and cooperation of Cooper‐pair correlation and two‐stage Kondo correlation in the system. Moreover, due to the occurrence of two‐stage Kondo effect, the Josephson phase transition can be adjusted in a precise manner. It is believed that this work can help to understand the supercurrent phase transition in the QD‐based Josephson junctions.

Antiresonance in electron transport through a four quantum dots system

A four quantum dots structure is designed. The influence of the interdot coupling strength, energy levels of quantum dots, magnetic flux and intradot Coulomb interactions on the conductance is discussed. An antiresonance point exactly emerges at the location of the energy level of side-coupled quantum dots (SCQD). The conversion between resonance band and antiresonance band can be achieved as parallel-coupled di-quantum dots with or without SCQD, which suggests a physical scheme of an effective quantum switch. The Breit–Wigner resonant and Fano antiresonant peaks may merge into one resonant band. By means of the mathematical formula, we analyze how the resonance band is formed. By varying intradot Coulomb interaction, the resonance band may evolve into a Fano antiresonance with an ultra-narrow width. This study provides theoretical basis for the design of quantum computing devices.

Electric field tuning of excitonic fine-structure splitting in asymmetric InAs/InP nanowire quantum dot molecules

We use an atomistic model to show that coupling between two nanowire quantum dots, forming an artificial molecule, can be used to control bright exciton splitting in these systems with external electric field and even reduce it to zero or reverse it. Importantly, strong interdot coupling allows this reduction to occur without a simultaneous detrimental reduction of excitonic optical activity, which is inherently present in weakly coupled systems. Our results indicate that nanowire quantum dot molecules could form a promising platform for quantum dot--based entanglement generation.

Dynamics of quantum discord between two quantum dots placed near a nanoring surface plasmons

We investigate the dynamics of quantum discord between two colloidal quantum dots placed in the vicinity of nanoring with surface plasmons (SPs). We also analytically solve the corresponding master equation and show the influence of coupling strength and interdot distance on the dynamics of quantum discord. It is found that for an initial separable state, maximum quantum discord can be generated at a suitable interdot distance by the scattering of SPs in the metal nanoring resonator. Furthermore, for the initial correlated state, we observe that the decay and revivals of the quantum discord can be controlled by properly choosing the interdot distance and coupling strengths.

Enhanced Spin Seebeck Efficiency in Closed Triple Quantum Dots Ring with Spin-Dependent Interdot Couplings

Enhanced Spin Seebeck Efficiency in Closed Triple Quantum Dots Ring with Spin-Dependent Interdot Couplings

Voltage-controllable multifunctional spin polarizer based on side-coupled quantum dots

Abstract Quantum dot system is considered as the prime candidate for nanospintronics devices, within which to generate spins one by one is the long term goal for quantum information and quantum computation. In this paper, with the aid of the Wilson’s numerical renormalization group method, we find a side-coupled double quantum dot molecule could be used to implement a multifunctional spin polarizer in the strongly correlated regime without switching the direction of the magnetic field. The spin polarizer reveals almost 100 % efficiencies, and the polarizing scenarios could be controlled easily by the initial charge state through external gate voltage, leading to various kinds of spin polarizers, e.g., unidirectional, bidirectional, and dual spin polarizers. We demonstrate the Zeeman effect and the inter-dot transport processes are responsible for the spin-polarized transport. Our scheme is robust for any inter-dot coupling and may also be useful for quantum dots connected with magnetic leads.

Anti-Resonance in a Laterally Coupled Triple-Quantum-Dot Chain

Employing the nonequilibrium Green’s function technique, electron transport characteristics through a laterally coupled triple-quantum-dot chain are investigated. The conductance versus electron energy is numerically calculated. Two anti-resonance points change into two anti-resonance bands with increasing the size of triple-quantum-dot chain. The widths of two anti-resonance bands are the same and can be regulated via adjusting the inter-dot tunneling coupling strength in the main chain or its attachment. Spin-polarized window emerges due to Zeeman splitting as an external magnetic flux is introduced. Our studies demonstrate that the system can be used as a quantum switch or an efficient spin filtering.

Side-gated, enhancement mode, InAs nanowire double quantum dot devices—toward controlling transverse electric fields in spin-transport measurements

A double quantum dot system with a definitive transverse electric field in the plane of the sample is defined by combining a facile side-gating technique with enhancement mode InAs nanowires. Positive bias on the plunger gates enhance quantum dot segments along the nanowire, negative bias on barrier gates deplete regions, and situating gates biased at opposite polarities on opposing sides of the nanowire allows an electric field to be engineered. With sufficiently biased barrier regions stable bias triangle features are observed in the weak interdot coupling regime. The singlet-triplet energy splitting ΔST in Pauli spin-blockaded features is studied as a function of an external magnetic field applied perpendicular to the sample plane. We interpret an apparent absence of mixing between singlet and triplet states as an indication that the spin–orbit field is oriented out of the sample plane due to the induced electric field. Finally, we discuss the potential of combining advanced gating architectures with enhancement mode nanowires to control the orientation of the spin–orbit field—a prospect that could enable multiple, nanowire-based spin-qubits to be operated on a single chip with a fixed-angle external magnetic field applied.

Spin Seebeck Effect in a Multiple Quantum Dot Molecule with Spin-Dependent Interdot Coupling

We study the spin Seebeck effect in a circularly connected triple quantum dot (TQD) structure taking the spin-dependent interdot coupling and magnetic flux into consideration. Particular attention is paid on the generation and manipulation of the 100% spin-polarized and pure spin thermopowers, which denote the arisen spin voltage in response of an infinitely small temperature gradient applied across the system. This can be realized by adjusting the peaks’ positions in the spin-up and spin-down thermopowers with the help of the spin polarization of the interdot couplings. At low temperature, a large value of pure spin thermopower is obtained even under very weak spin polarization of the interdot coupling. Strong spin polarization of it is favorable for 100% spin-polarized thermopower whose magnitude can reach as large as that of the charge one. We also find that a sign change of the considered two quantities can be realized by adjusting the magnetic flux penetrating through the TQDs. The present results could be useful in designing high-efficiency pure spin energy conversion and spin filter devices.

Tuning Methods for Semiconductor Spin Qubits

We present efficient methods to reliably characterize and tune gate-defined semiconductor spin qubits. Our methods are designed to target the tuning procedures of semiconductor double quantum dot in GaAs heterostructures, but can easily be adapted to other quantum-dot-like qubit systems. These tuning procedures include the characterization of the inter-dot tunnel coupling, the tunnel coupling to the surrounding leads and the identification of the various fast initialization points for the operation of the qubit. Since semiconductor-based spin qubits are compatible with standard semiconductor process technology and hence promise good prospects of scalability, the challenge of efficiently tuning the dot's parameters will only grow in the near future, once the multi-qubit stage is reached. With the anticipation of being used as the basis for future automated tuning protocols, all measurements presented here are fast-to-execute and easy-to-analyze characterization methods. They result in quantitative measures of the relevant qubit parameters within a couple of seconds, and require almost no human interference.

Spin-Polarized Transport and Spin Seebeck Effect in Triple Quantum Dots with Spin-Dependent Interdot Couplings

We study the spin-dependent electronic and thermoelectric transport through a structure composed of triple quantum dots (TQDs) coupled to two metallic leads in the presence of spin-dependent interdot couplings, which is reliable by applying a static magnetic field on the tunnel junctions between different dots. When the TQDs are serially connected, a 100 % spin-polarized conductance and thermopower emerge even for very small spin-polarization of the interdot coupling as the dots are weakly coupled to each other. Whereas if the TQDs are connected in a ring shape, the Fano antiresonance will result in sharp peaks in the conductance and thermopower. In the presence of spin-dependent interdot couplings, the peaks of the spin-up and spin-down thermopowers will shift to opposite directions in the dot level regime, resulting large either 100 % spin-polarized or pure spin thermopowers. The latter generally arises at low temperatures and is robust against the level detuning, the dot-lead coupling, and the system equilibrium temperature.

Tuning the Emission Directionality of Stacked Quantum Dots

The emission directionality of stacks of coupled quantum dots (QDs) is investigated within the framework of 8-band k·p-theory including strain and strain-induced piezoelectricity up to second order. Using an artificial cuboidal QD, we show that the degree of radiation anisotropy can be tuned from −33% to nearly +60% via the structure’s vertical aspect ratio. We then demonstrate that these findings can be transferred to stacked InGaAs QDs whose emission directionality is tailored (i) via the interdot coupling strength given by the separating barrier width and (ii) the number of stacked QDs. Our results enable the design and optimization of top and edge emitters based on stacked QDs.

Electron transport through two one-dimensional multi-quantum dot arrays coupled to four leads

Employing the non-equilibrium Green’s function technique, we have obtained the formula for dc current of two one-dimensional multi-quantum dot arrays, which couple to each other via tunneling coupling between two quantum dots connected to four leads, respectively. The retarded Green’s function is a staircase type, terminating at the four leads. Furthermore, the four quantum dots case is demonstrated. The influence of inter-dot coupling strength and quantum dot energy level on the transmission probability for TL, TM and TN branches is investigated. A non-resonant band is observed. By adjusting energy levels of quantum dots, a resonance emerges in the region of the non-resonance band. The system can be used as a quantum switch.

Enhancement of charge and spin Seebeck effect in triple quantum dots coupling to ferromagnetic and superconducting electrodes

We theoretically study the thermoelectric transport properties through a triple quantum dots (QDs) device with the central QD coupled to a ferromagnetic lead, a superconducting one, and two side QDs with spin-dependent interdot tunneling coupling. The thermoelectric coefficients are calculated in the linear response regime by means of nonequilibrium Green's function method. The thermopower is determined by the single-electron tunneling processes at the edge of superconducting gap. Near the outside of the gap edge the thermopower is enhanced while thermal conductance is suppressed, as a result, the charge figure of merit can be greatly improved as the gap appropriately increases. In the same way, charge figure of merit also can be greatly improved near the outside of the gap edge by adjusting interdot tunneling coupling and asymmetry coupling of the side QDs to central QD. Moreover, the appropriate increase of the interdot tunneling splitting and spin polarization of ferromagnetic lead not only can improve charge thermopower and charge figure of merit, but also can enhance spin thermopower and spin figure of merit. Especially, the interdot tunneling splitting scheme provides a method of controlling charge (spin) figure merit by external magnetic field.

Automated tuning of inter-dot tunnel coupling in double quantum dots

Semiconductor quantum dot arrays defined electrostatically in a 2D electron gas provide a scalable platform for quantum information processing and quantum simulations. For the operation of quantum dot arrays, appropriate voltages need to be applied to the gate electrodes that define the quantum dot potential landscape. Tuning the gate voltages has proven to be a time-consuming task, because of initial electrostatic disorder and capacitive cross-talk effects. Here, we report on the automated tuning of the inter-dot tunnel coupling in gate-defined semiconductor double quantum dots. The automation of the tuning of the inter-dot tunnel coupling is the next step forward in scalable and efficient control of larger quantum dot arrays. This work greatly reduces the effort of tuning semiconductor quantum dots for quantum information processing and quantum simulation.

The influence of side-coupled quantum dots on thermoelectric effect of parallel-coupled double quantum dot system

The thermoelectric transport properties of a parallel-coupled double quantum dot (PCDQD) system with side-coupled quantum dots (QDs) is investigated by using the Keldysh non-equilibrium Green's function technique. The thermoelectric quantities, including the thermal conductance, thermopower, and thermoelectric figure of merit denoted by ZT, are sensitive to the inter-dot coupling strength. With the help of side-coupled QD, unusual double Fano resonances are created in the conductance spectra to largely enhance the thermoelectric effect at low-temperature. Benefited from the coexistence of local bipolar effect and Fano resonance, the ZT can be improved by one-fold higher than that of original PCDQD system. Moreover, when the asymmetry parameter α, which indicates the geometric arrangement of coupled QDs with a given lead, takes appropriate value, the optimization of ZT can be achieved at high temperature. Our work suggests that the side-coupled QDs scheme holds promise for the designing of high-efficiency thermoelectric conversion devices.

Optimal control of universal quantum gates in a double quantum dot

We theoretically investigate electron spin operations driven by applied electric fields in a semiconductor double quantum dot (DQD). Our model describes a DQD formed in semiconductor nanowire with longitudinal potential modulated by local gating. The eigenstates for two electron occupation, including spin-orbit interaction, are calculated and then used to construct a model for the charge transport cycle in the DQD taking into account the spatial dependence and spin mixing of states. The dynamics of the system is simulated aiming at implementing protocols for qubit operations, that is, controlled transitions between the singlet and triplet states. In order to obtain fast spin manipulation, the dynamics is carried out taking advantage of the anticrossings of energy levels introduced by the spin-orbit and interdot couplings. The theory of optimal quantum control is invoked to find the specific electric-field driving that performs qubit logical operations. We demonstrate that it is possible to perform within high efficiency a universal set of quantum gates $\{$CNOT, H$\otimes$I, I$\otimes$H, T$\otimes$I, and T$\otimes$I$\}$, where H is the Hadamard gate, T is the $\pi/8$ gate, and I is the identity, even in the presence of a fast charge transport cycle and charge noise effects.