两端线型双量子点分子Aharonov-Bohm干涉仪电输运

设计一个两端线型双量子点分子Aharonov-Bohm (A-B)干涉仪. 采用非平衡格林函数技术, 理论研究无含时外场作用下的体系电导和引入含时外场作用下的体系平均电流. 在不考虑含时外场时, 调节点间耦合强度或磁通可以诱导电导共振峰劈裂. 控制穿过A-B干涉仪磁通的有无, 实现了共振峰电导数值在0与1之间的数字转换, 为制造量子开关提供了一个新的物理方案. 同时借助磁通和Rashba自旋轨道相互作用, 获得了自旋过滤. 当体系引入含时外场时, 平均电流曲线展示了旁带效应. 改变含时外场的振幅, 实现了体系平均电流的大小与位置的有效控制, 而调节含时外场的频率, 则可以实现平均电流峰与谷之间的可逆转换. 通过调节磁通与Rashba自旋轨道相互作用, 与自旋相关的平均电流亦得到有效控制. 研究结果为开发利用耦合多量子点链嵌入A-B 干涉仪体系电输运性质提供了新的认知. 上述结果可望对未来的量子器件设计与量子计算发挥重要的指导作用.     

Second quantized scalar QED in homogeneous time-dependent electromagnetic fields

We formulate the second quantization of a charged scalar field in homogeneous, time-dependent electromagnetic fields, in which the Hamiltonian is an infinite system of decoupled, time-dependent oscillators for electric fields, but it is another infinite system of coupled, time-dependent oscillators for magnetic fields. We then employ the quantum invariant method to find various quantum states for the charged field. For time-dependent electric fields, a pair of quantum invariant operators for each oscillator with the given momentum plays the role of the time-dependent annihilation and the creation operators, constructs the exact quantum states, and gives the vacuum persistence amplitude as well as the pair-production rate. We also find the quantum invariants for the coupled oscillators for the charged field in time-dependent magnetic fields and advance a perturbation method when the magnetic fields change adiabatically. Finally, the quantum state and the pair production are discussed when a time-dependent electric field is present in parallel to the magnetic field.     

Quantum ring states in magnetic field and delayed half-cycle pulses

The present work is dedicated to the time evolution of excitation of a quantum ring in external electric and magnetic fields. Such a ring of mesoscopic dimensions in an external magnetic field is known to exhibit a wide variety of interesting physical phenomena. We have studied the dynamics of the single electron quantum ring in the presence of a static magnetic field and a combination of delayed half-cycle pulse pair. Detailed calculations have been worked out and the impact on dynamics by variation in the ring radius, intensity of external electric field, delay between the two pulses, and variation in magnetic field have been reported. A total of 19 states have been taken and the population transfer in the single electron quantum ring is studied by solving the time-dependent Schrödinger equation (TDSE), using the efficient fourth-order Runge–Kutta method. Many interesting features have been observed in the transition probabilities with the variation of magnetic field, delay between pulses and ring dimensions. A very important aspect of the present work is the persistent current generation in a quantum ring in the presence of external magnetic flux and its periodic variation with the magnetic flux, ring dimensions and pulse delay.     

Coherent single electron spin control in a slanting Zeeman field

We consider a single electron in a 1D quantum dot with a static slanting Zeeman field. By combining the spin and orbital degrees of freedom of the electron, an effective quantum two-level (qubit) system is defined. This pseudospin can be coherently manipulated by the voltage applied to the gate electrodes, without the need for an external time-dependent magnetic field or spin-orbit coupling. Single-qubit rotations and the controlled-NOT operation can be realized. We estimated the relaxation (T1) and coherence (T2) times and the (tunable) quality factor. This scheme implies important experimental advantages for single electron spin control.     

Levelstatistics and stochasticity in a driven quantum system

We investigate quantum properties of one anisotropic spin driven by an external time-dependent magnetic field which shows a transition from regular to irregular dynamics with increasing field strength in the classical limit. In particular we study the statistical properties of the quasi-spectrum. Our results support the conjecture that Poisson- and GOE-statistics are to be associated with integrable and nonintegrable systems resp. in the semiclassical limit. Approaching the quantum case we observe significant deviations from GOE statistics.     

Ground State Transitions in Vertically Coupled Four-Layer Single Electron Quantum Dots

We study a four-electron system in a vertically coupled four-layer quantum dot under a magnetic field by the exact diagonalization of the Hamiltonian matrix. We find that discontinuous ground-state energy transitions are induced by an external magnetic field. We find that dot-dot distance and electron-electron interaction strongly affect the low-lying states of the coupled quantum dots. The inter-dot correlation leads to some sequences of possible disappearances of ground state transitions, which are present for uncoupled dots.     

Ground State Transitions in Vertically Coupled Four-Layer Single Electron Quantum Dots

We study a four-electron system in a vertically coupled four-layer quantum dot under a magnetic field by the exact diagonalization of the Hamiltonian matrix. We find that discontinuous ground-state energy transitions are induced by an external magnetic field. We find that dot-dot distance and electron-electron interaction strongly affect the low-lying states of the coupled quantum dots. The inter-dot correlation leads to some sequences of possible disappearances of ground state transitions, which are present for uncoupled dots.     

Berry Phase of Composite System Induced by Time-Dependent Interaction

The Berry phase in a composite system induced by the time-dependent interaction is discussed. We choose two coupled spin-1/2 systems as the composite system： one of the subsystems is subjected to a static magnetic field, and the coupling parameters between two spins are controllable in time. We show that the time-dependent interaction can induce the Berry phase in a similar way as that a spin-1/2 system （qubit） is driven by an effective time-dependent magnetic field. Furthermore, using two consecutive cycles with opposite directions of both the static magnetic field as well as opposite signs of the coupling parameters, a nontrivial two-qubit unitary transformation purely based on Berry phases can be constructed.     

Four-Electron Systems in a Coupled Double-Layer Quantum Dots

Making use of the method of few-body physics, the energy spectrum of a four-electron system consisting in a vertically coupled double-layer quantum dot as a function of the strength ofa magnetic field is investigated. Discontinuous ground-state transitions induced by an external magnetic field are shown. We find that, in the strong coupling case, the ground-state transitions depend not only on the external magnetic field B but also on the distance d between double-layer quantum dots. However, in the case of weak coupling, the ground-state transitions occur in the new sequence of the values of the magic angular momentum. Hence, the interlayer separation d and electron-electron interaction strongly affect the ground state of the coupled quantum dots.     

Quantum Phase Transition and Thermal Entanglement in the Isotropic XXX Model

We investigate the quantum phase transition (QPT) and the pairwise thermal entanglement in the three-qubit Heisenberg XXX chain with Dzyaloshinskii--Moriya (DM) interaction under a magnetic field. The ground states of the system exist crossing points, which shows that the system exhibits a QPT. At a given temperature, the entanglement undergoes two sudden changes (platform-like behavior) as the DM interaction or external magnetic field increases. This special property can be used as the entanglement switch, which is also influenced by the temperature. We can modulate the DM interaction or external magnetic field to control the entanglement switch.     

Quantum–classical correspondence and the role of the dipole function in molecular dissociation

We consider the quantum and classical dissociation dynamics of heteronuclear diatomic molecules induced by infrared laser pulses. The field–molecule interaction is given by the product of the time-dependent electric field and the molecule permanent dipole. We investigate the influence of the dipole function in molecular dissociation. We show that the dissociation can be suppressed at certain external field frequencies for a nonlinear and finite-range dipole function. The correspondence between quantum and classical results is established by relating classical Fourier amplitudes to discrete–continuum quantum matrix elements.     

Nanomechanical analog of a laser: amplification of mechanical oscillations by stimulated zeeman transitions

We propose a magnetomechanical device that exhibits many properties of a laser. The device is formed by a nanocantilever and dynamically polarized paramagnetic nuclei of a solid sample in a strong external magnetic field. The corresponding quantum oscillator and effective two-level systems are coupled by the magnetostatic dipole-dipole interaction between a permanent magnet on the cantilever tip and the magnetic moments of the spins, so that the entire system is effectively described by the Jaynes-Cummings model. We consider the possibility of observing transient and cw lasing in this system, and show how these processes can be used to improve the sensitivity of magnetic resonance force microscopy.     

Entanglement and Zeeman interaction in diluted magnetic semiconductor quantum dot

We present theoretically the Zeeman coupling and exchange-induced swap action in spin-based quantum dot quantum computer models in the presence of magnetic field. We study the valence and conduction band states in a double quantum dots made in diluted magnetic semiconductor. The latter have been proven to be very useful in building an all-semiconductor platform for spintronics. Due to a strong p–d exchange interaction in diluted magnetic semiconductor (Cd_0.57Mn_0.43Te), the relative contribution of this component is strongly affected by an external magnetic field, a feature that is absent in nonmagnetic double quantum dots. We determine the energy spectrum as a function of magnetic field within the Hund–Mulliken molecular-orbit approach and by including the Coulomb interaction. Since we show that the ground state of the two carriers confined in a vertically coupled quantum dots provide a possible realization for a gate of a quantum computer, the crossing between the lowest states, caused by the giant spin splitting, can be observed as a pronounced jump in the magnetization of small magnetic field amplitude. Finally, we determine the swap time as a function of magnetic field and the inter dot distance. We estimate quantitatively swap errors caused by the field, establishing that error correction would, in principle, be possible in the presence of nonuniform magnetic field in realistic structures.     

Response of a system of strongly interacting particles to an inhomogeneous external field

We show that a system of N strongly interacting quantum particles in a parabolic confining potential can be unstable under the action of a time-dependent quadrupole external field. The instability leads to the generation or amplification of dipole oscillations. Parameters of the instability are independent of the number of particles and the inter-particle interaction.     

Spin precession and electron spin polarization wave in [001]-grown quantum wells

We theoretically study the spatial behaviors of spin precessions modulated by an effective magnetic field in a two-dimensional electron system with spin-orbit interaction. Through analysis of interaction between the spin and the effective magnetic field, we find some laws of spin precession in the system, by which we explain some previous phenomena of spin precession, and predict a controllable electron spin polarization wave in [001]-grown quantum wells. The shape of the wave, like water wave, mostly are ellipse-like or circle-like, and the wavelength is anisotropic in the quantum wells with two unequal coupling strengths of the Rashba and Dresselhaus interactions, and is isotropic in the quantum wells with only one spin orbit interaction.     

Kondo effect for electron transport through an artificial quantum dot

We have calculated the transport properties of electron through an artificial quantum dot by using the numerical renormalization group technique in this paper. We obtain the conductance for the system of a quantum dot which is embedded in a one-dimensional chain in zero and finite temperature cases. The external magnetic field gives rise to a negative magnetoconductance in the zero temperature case. It increases as the external magnetic field increases. We obtain the relation between the coupling coefficient and conductance. If the interaction is big enough to prevent conduction electrons from tunnelling through the dot, the dispersion effect is dominant in this case. In the Kondo temperature regime, we obtain the conductivity of a quantum dot system with Kondo correlation.     

Holonomic quantum computation based on the scalar Aharonov–Bohm effect for neutral particles and linear topological defects

We discuss holonomic quantum computation based on the scalar Aharonov–Bohm effect for a neutral particle. We show that the interaction between the magnetic dipole moment and external fields yields a non-abelian quantum phase allowing us to make any arbitrary rotation on a one-qubit. Moreover, we show that the interaction between the magnetic dipole moment and a magnetic field in the presence of a topological defect yields an analogue effect of the scalar Aharonov–Bohm effect for a neutral particle, and a new way of building one-qubit quantum gates.     

Thermal Quantum Discord in Pure Dzyaloshinskii-Moriya Model with Magnetic Field

We investigate the effects of the directions of Dzyaloshinskii-Moriya (DM) interaction vector and magnetic field on the quantum discord in the pure DM model. For different directions of DM vector, we find that there are different optimal parameter components of magnetic field. Moreover, we find that the optimal parameter components rules are the same for the Hamiltonian H₁ and H₂. According to the rules, for a certain axial DM vector, we can get the maximal quantum discord by adjusting the direction of the external magnetic field, which is feasible under the current experimental technology.