Thermal entanglement in a triple quantum dot system

We present studies of thermal entanglement of a three-spin system in triangular symmetry. Spin correlations are described within an effective Heisenberg Hamiltonian, derived from the Hubbard Hamiltonian, with super-exchange couplings modulated by an effective electric field. Additionally a homogenous magnetic field is applied to completely break the degeneracy of the system. We show that entanglement is generated in the subspace of doublet states with different pairwise spin correlations for the ground and excited states. For the doublets with the same spin orientation one can observe nonmonotonic temperature dependence of entanglement due to competition between entanglement encoded in the ground state and the excited state. The mixing of the states with an opposite spin orientation or with quadruplets (unentangled states) always monotonically destroys entanglement. Pairwise entanglement is quantified using concurrence for which analytical formulae are derived in various thermal mixing scenarios. The electric field plays a specific role – it breaks the symmetry of the system and changes spin correlations. Rotating the electric field can create maximally entangled qubit pairs together with a separate spin (monogamy) that survives in a relatively wide temperature range providing robust pairwise entanglement generation at elevated temperatures.     

Tunable supercurrent in a triangular triple quantum dot system

The supercurrent in a triangular triple quantum dot system is investigated by using the nonequilibrium Green's function method. It is found that the sign of the supercurrent can be changed from positive to negative with increasing the strength of spin-flip scattering, resulting in the π-junction transition. The supercurrent and the π-junction transition are also modulated by tuning the system parameters such as the gate voltage and the interdot coupling. The tunable π-junction transition is explained in terms of the current carrying density of states. These results provide the ways of manipulating the supercurrent in a triple quantum dot system.     

Electronic transport in a side-coupled triple quantum dot array

The electronic transport through a side-coupled triple quantum dot array (QDA) is investigated by means of Green function technique within the tight-binding framework. We obtain the formula of the linear conductance of QDA. The linear conductance spectrum is numerically studied. We discuss the feasibility of applying our structure to the electron spin polarized device and calculate the ratio of the spin polarized current flows.     

Coherent spectroscopy of a strongly driven triple quantum dot molecule

Based on a calculation model,we study the interference phenomena of serially coupled V-type and Λ-type triple quantum dots (CTQDs) driven simultaneously by a strong driving field and a weak probe field.Strongly depending on the configuration of the three-level CTQD,the probe absorption spectra,which are shown in the tunneling current,exhibit various quantum coherence properties.In the case where the two pairs of transitions of the CTQD have a small eigenfrequency difference △ω,the double-coupling effect of the driving field results in two Autler-Townes doublets and one weak Mollow triplet in one spectrum.With the value of △ω increasing,only one Autler-Townes splitting remains due to the single-coupling of the field.We also find that the effect of spontaneous emission of phonons may lead to an obvious background current,which can be used to distinguish which transition is driven by the driving field in experiment.The interesting quantum property of a CTQD revealed in our results suggests its potential applications in quantum modulators and quantum logic devices.     

Fano effect of a parallel-coupled triple Rashba quantum dot system

Using the nonequilibrium Keldysh Green's function technique, the Fano effect of a parallel-coupled triple Rashba quantum dot system is investigated. The conductance as a function of electron energy is numerically calculated. Compared with the case of a parallel-coupled double quantum dot system, two additional Fano resonance peaks occur in the conductance spectrum. By adjusting the structural parameters, the two Fano resonance peaks may change into the resonance peaks. In addition, the influence of Rashba spin-orbit interaction on the conductance is studied.     

Fano effect of a laterally coupled vertical triple quantum dot system

Using the nonequilibrium Green’s function technique,electron transport through a laterally coupled vertical triple quantum dot is investigated.The conductance as a function of electron energy is numerically calculated.The evolution of the conductance strongly depends on the configuration of dot levels and interdot coupling strengths.     

点间耦合强度对三耦合量子点系统微分电导的影响

本文利用非平衡格林函数运动方程方法,研究了与两个电极耦合在一起的三耦合量子点系统的微分电导及量子干涉的AB振荡问题.通过理论计算发现,由于量子点上的局域态密度的不同从而导致系统电导或隧穿性质的不同,而且量子点间耦合强度、量子点能级等都会对输运性质产生影响. 关键词： 量子点 非平衡格林函数 运动方程 局域态密度     

Evidence of quantum interference in transport properties of a triple quantum dot T-shape system

We consider the transport and the noise characteristic in the case of a triple quantum dots T-shape system where two of the dots form a two-level system and the other works in a detector-like setup. Our theoretical results are obtained using the equation of motion method for the case of zero and finite on-site Coulomb interaction in the detector dot. We present analytic results for the electronic Green’s functions in the system’s component quantum dots, and we used numerical calculations to evaluate the system’s transport properties. The transport trough the T-shaped system can be controlled by varying the coupling between the two-level system dots or the coupling between the detector dot and the exterior electrodes. The system’s conductance presents Fano dips for both strong (fast detector) and weak coupling (slow detector) between the detector dot and the external electrodes. Due to stronger electronic correlations the noise characteristics in the case of a slow detector are much higher. This setup may be of interest for the practical realization of qubit states in quantum dots systems.     

Persistent currents in a mesoscopic ring with a side connected quantum dot

We study the persistent current circulating along a mesoscopic ring with a dot side-coupled to it when threaded by a magnetic field. A cluster including the dot and its vicinity is diagonalized and embedded into the rest of the system. The result is numerically exact. We show that in the Kondo regime, the current can be a smooth or a strongly dependent function of the gate potential according to the structure of occupation of the highest energetic electrons of the system.     

Rashba-induced transverse pure spin currents in a four-terminal quantum dot ring

By applying a local Rashba spin–orbit interaction to an individual quantum dot of a four-terminal four-quantum-dot ring and introducing a finite bias between the longitudinal terminals, we theoretically investigate the charge and spin currents in the transverse terminals. It is found that when the quantum dot levels are separate from the chemical potentials of the transverse terminals, notable pure spin currents appear in the transverse terminals with the same amplitude but opposite polarization directions. In addition, the polarization directions of such pure spin currents can be inverted by altering the structure parameters, i.e., the magnetic flux, the bias voltage, and the values of quantum dot levels with respect to the chemical potentials of the transverse terminals.     

Detection of single spin decoherence in a quantum dot via charge currents

We consider a quantum dot attached to leads in the Coulomb blockade regime that has a spin 1 / 2 ground state. We show that, by applying an ESR field to the dot spin, the stationary current in the sequential tunneling regime exhibits a new resonance peak whose linewidth is determined by the single spin decoherence time T2. The Rabi oscillations of the dot spin are shown to induce coherent current oscillations from which T2 can be deduced in the time domain. We describe a spin inverter which can be used to pump current through a double dot via spin flips generated by ESR.     

Chiral splitting of Kondo peak in triangular triple quantum dot

New characteristics of the Kondo effect, arising from spin chirality induced by the Berry phase in the equilibrium state, are investigated. The analysis is based on the hierarchical equations of motion (HEOM) approach in a triangular triple quantum-dot (TTQD) structure. In the absence of magnetic field, TTQD has four-fold degenerate chiral ground states with degenerate spin chirality. When a perpendicular magnetic field is applied, the chiral interaction is induced by the magnetic flux threading through TTQD and the four-fold degenerate states split into two chiral state pairs. The chiral excited states manifest as chiral splitting of the Kondo peak in the spectral function. The theoretical analysis is confirmed by the numerical computations. Furthermore, under a Zeeman magnetic field B, the chiral Kondo peak splits into four peaks, owing to the splitting of spin freedom. The influence of spin chirality on the Kondo effect signifies an important role of the phase factor. This work provides insight into the quantum transport of strongly correlated electronic systems.     

Dynamic localization of two electrons in AC-driven triple quantum dots and quantum dot shuttles

We analyze the dynamic localization of two interacting electrons induced by alternating current electric fields in triple quantum dots and triple quantum dot shuttles. The calculation of the long-time averaged occupation probability shows that both the intra-and inter-dot Coulomb interaction can increase the localization of electrons even when the AC field is not very large. The mechanical oscillation of the quantum dot shuttles may keep the localization of electrons at a high level within a range if its frequency is quite a bit smaller than the AC field. However, the localization may be depressed if the frequency of the mechanical oscillation is the integer times of the frequency of the AC field. We also derive the analytical condition of two-electron localization both for triple quantum dots and quantum dot shuttles within the Floquet formalism.     

Quantization of persistent currents in quantum dot at strong magnetic fields

We investigate equilibrium electron currents and magnetization in an ideal two-dimensional disc of radius R placed in a strong magnetic field H. The most striking results emerge when the conditions for the existence of edge and bulk states are met, namely . When the Fermi energy is locked on a Landau level, the current as a function of electron density is quantized in units of , where ω_c is the cyclotron frequency. We argue that this effect survives against weak disorder. It is also shown that the persistent current has an approximately periodic dependence on 1/H.     

Charge and spin currents tunnelling through a toroidal carbon nanotube side-coupled with a quantum dot

We have investigated the mesoscopic transport through the system with a quantum dot (QD) side-coupled to a toroidal carbon nanotube (TCN) in the presence of spin-flip effect. The coupled QD contributes to the mesoscopic transport significantly through adjusting the gate voltage and Zeeman field applied to the QD. The compound TCN-QD microstructure is related to the separate subsystems, the applied external magnetic fields, as well as the combination of subsystems. The spin current component I_z^s is independent on time, while the spin current components I_x^s and I_y^s evolve with time sinusoidally. The rotating magnetic field induces novel levels due to the spin splitting and photon absorption procedures. The suppression and enhancement of resonant peaks, and semiconductor-metal phase transition are observed by studying the differential conductance through tuning the source-drain bias and photon energy. The magnetic flux induces Aharonov-Bohm oscillation, and it controls the tunnelling behavior due to adjusting the flux. The Fano type of multi-resonant behaviors are displayed in the conductance structures by adjusting the gate voltage V_g and the Zeeman field applied to the QD.     

Fabrication of quantum dot transistors incorporating a single self-assembled quantum dot

We report on the fabrication and the characterization of quantum dot transistors incorporating a single self-assembled quantum dot. The current–voltage characteristics exhibit clear staircase structures at room temperature. They are attributed to electron tunneling through the quantized energy levels of a single quantum dot.     

Light absorption by a two-dimensional quantum dot superlattice

The monochromatic light absorption in an ideal two-dimensional quantum dot superlattice (QDSL) is considered theoretically. Calculations of the absorption coefficient are done in both the absence and presence of a homogeneous DC electric field with rational and irrational orientations. The explicit dependencies of the absorption coefficient on the frequency of the light, the QDSL parameters and the strength of the electric field are found. Some numerical results for GaAs—Ga_0.7Al_0.3As QDSL are obtained.     

Generation of one-photon states by a quantum dot

A technique for preparation of a one-photon wave packet through action of a classical electromagnetic field on a semiconducting quantum dot is proposed. We demonstrate that the Coulomb repulsion between charge carriers allows one to select the frequency, amplitude, and duration of an electromagnetic pulse so that one electron will transfer from an upper size-quantized level of the valence band to a lower size-quantized level of the conduction band with a probability close to unity. As a result of radiative recombination of the produced electron-hole pair, exactly one photon is emitted (a one-photon wave packet). This source of one-photon states can be used in quantum systems of data transmission and in quantum computers. Zh. éksp. Teor. Fiz. 112, 1257–1272 (October 1997)