Control Of Continuous Variable Entanglement And Transport In Quantum Dot

In the recent years, continuous variable entanglement has attracted a lot of attention in quantum optics and quantum information processing in continuous variable regime. Any attempt to exploit the entanglement have to, however, face the corruption of the entanglement by unavoidable decoherence. As a result, how to produce the robust, steady, and high entanglement is an interesting research issue. On the other hand, with the development of micro-nanofabrication, people can experimentally produce the quantum dots, which are also called artificial atoms and are analogous to the real atoms. Since the features of the quantum dots are controllable, the study of external influences on quantum dots becomes an important topic. In the last part of our paper, we have studied the optical phase control of electron transport in coupled quantum dots. Furthermore, we have analyzed theoretically the pumped spin current in a two-level quantum dot system, where the dot is irriated by circularly polarized laser field.Firstly, we propose a scheme for the unconditional preparation of Cluster and Greenberger-Horne-Zeilinger (GHZ) entangled states of effective bosonic modes realized in four physically separated atomic ensembles. Subsequently, we investigated the enhancement of the intracavity entanglement of a nonlinear coupler via homodyne-mediated quantum feedback. Two feedback loops are used to modulate the strengths of the lasers driving the cavity modes so as to control the variances of operators X₁^θ+X₂^θand Y₁^θ-Y₂^θrespectively, where X_j^θand Y_j^θare the two quadrature operators at the phase angleθof the jth (j = 1,2) mode in the cavity. The feedback master equation is driven and solved analytically. It is found that the feedback can effectively enhance the entanglement, two-mode squeezing, and purity of the two-mode field produced from the nonlinear coupler. Furthermore the phase angle at which the feedback is adopted is critical to determine whether the quantum feedback can improve the entanglement or not. If the quantum feedback is added at the phase angle ofθ=0, no improvement of the entanglement can take place. We have found that the optimum case to improve the entanglement is the one that the quantum feedback is added at the phase angle ofθ=0.25Ï€. On the other hand, we have studied the coherent field phase control of transport through a closed three-level structure in an asymmetric double quantum dots system. The results are separately discussed in two different transport regimes, respect to the location of the chemical potentialμ_R. In the regime ofμ_R<ε₂, we discuss the phase modulation on current for energy differenceΔ. It is shown that the two-peak structure appears and the location and height of the current peak can be modulated by choosing different relative phasesθ₁ andθ₂. Importantly, the peak of current can be nearly tuned to zero because the electron is nearly trapped into the ground state of the left QD, due to a complete destructive interference determined by relative phasesθ₁ andθ₂. This effect can serve as an optical phase controlled current switch. In the other regime ofε₂<μ_R<ε₃, we find that the two coupled quantum dots can be trapped into a dark state |D>=1/(2^1/2)(e^-iθ2|1>-e^-iθ1|2>), which is decoupled from the light, and here we chooseΩ-1=Ω₂. That the electron is trapped in a decoupled coherent superposition corresponds to the effect of the current disappearance. Finally, we have theoretically analyzed the pumped spin current and its shot noise spectra in a two-level quantum dot system, where the dot is irradiated by circularly polarized laser field. The device operates at a certain chemical potential configuration in the absence of the magnetic field and the ferromagnetic spin injection. The spin-resolved shot noise can be modulated by the laser field nonmonotonously and always indicates a sub-Poissonian type.