Suppression of Phase and Amplitude Damping Decoherence in a Three-Level Atom in Λ-Configuration Using Bang-Bang Controls

In this paper, we present decoupling bang-bang operations for the suppression of general decoherence, both amplitude and phase damping, in a three-level system in Λ-configuration. We give a program to design a sequence of periodic twinborn pulses to suppress the decoherence in such the system.     

Absorption Spectra of a Three-Level Atom Embedded in a PBG Reservoir

We introduce the ‘decay rate＇ terms into the density matrix equations of an atom embedded in a photonic band gap （PBG） reservoir successfully. By utilizing the master equations, the probe absorption spectra and the refractivity properties of a three-level atom in the PBG reservoir are obtained. The interaction between the atom and the PBG reservoir as well as the effects of the quantum interference on the absorption of the atom has also been taken into account. It is interesting that two different types of the anomalous dispersion relations of refractivity are exhibited in one dispersion line. The methodology used here can be applied to theoretical investigation of quantum interference effects of other atomic models embedded in a PBG reservoir.     

Effect of Intrinsic Decoherence of Milburn＇s Model on Entanglement of Two-Qutrit States

We analyze the time evolution of entanglement of two-qutrit system within the framework of Milburn's model of intrinsic decoherence. The entanglement evolution relies not only on the parameters of system, but also on the concrete states either pure or mixed. The linear entropy used to measure the extent to which the intrinsic decoherence affects quantum states is evaluated.     

Solvent Suppression in Intermolecular Magnetic Resonance Spectra Multiple-Quantum Coherence Nuclear with Only z-axis Gradients

The solvent peak in the intermolecular multiple-quantum coherence spectra can be suppressed by either applying pulse field gradients or spinning sample along the magic angle direction （φ = 54.7）. However, these two methods also suppress the signals of the solute. We design two pulse sequences with only z-axis gradients to suppress the solvent peak without reducing the intensity of solute signals. Compared to the former pulse sequence, the latter pulse sequence is insensitive to the imperfection of pulse flip angles. When the flip angles of the second pulse sequence are purposely deviated 1/10 from the optimal values, the solvent peak is still weak. Theoretical expressions, experimental observations and computer simulations demonstrate that the two methods can be used to effectively suppress solvent peak in intermolecular multiple-quantum coherence spectra.     

Suppression of Amplitude Decoherence in Arbitrary n-Level Atom in ≡-Configuration with Bang-Bang Controls

In this paper, we give Bang-Bang （BB） decoupling schemes to suppress the amplitude decoherence in the five-and six-level atom systems in ≡-configuration. We generalize this scheme to the arbitrary level atom system in ≡-configuration. The corresponding decoupling operators are given explicitly.     

Influence of Intrinsic Decoherence on Nonclassical Effects of System of a Three-Level Atom in Ξ Configuration

With the help of an SU(3) dynamical algebraic structure, we find an exact solution of the Milburn equation for the system of a three-level atom in the Ξ configuration interacting with one quantized field mode with arbitrary detuning. The exact solution is then used to discuss the influence of the intrinsic decoherence on the nonclassical properties of the system, such as collapes and revivals of the atomic populations, oscillations of the photon number distribution, and squeezing of the radiation field.     

Spin Pumping from a Quantum Dot in the Presence of Decoherence

We study the pumped spin current of an interacting quantum dot tunnel coupled to a single lead in the presence of electron spin resonance （ESR） field. The spin decoherence in the dot is included by the Bffttiker approach. Using the nonequilibrium Green＇s function technique, we show that ESR-induced spin flip can generate finite spin current with no charge transport. Both the Coulomb interaction and spin decoherence decrease the amplitude of spin current. The dependence of pumped spin current on the intensity and frequency of ESR field, and the spin decoherence is discussed.     

Entanglement Dynamics of a Three-Level Atom in Ξ Configuration

Dynamical behaviour of entanglement between the cavity mode and a three-level atom in the configuration is studied by making use of log-negativity. The influence of initial temperature of the thermal field and detuning on the entanglement is investigated. It is found that the maximal values of entanglement decreases with the temperature of the thermal field and can be controlled by the defaming.     

Average Dissipative and Dipole Forces on a Three-Level Atom in a Laguerre-Gaussian Beam

By means of the optical Bloch equations based on the atomic density matrix elements, the general expressions of the average dissipative force, dipole force and the mechanical torque acting on a A-configuration three-level atom in a linearly-polarized Laguerre-Gaussian beam (LGB) with an angular momentum of lh are derived, and the general properties of the average dissipative and dipole force on the three-level atom in the linearly-polarized LGB are analysed. We find a resonant property (with two resonant peaks) of the dissipative force and a non-resonant property (with two pairs of non-resonant peaks) of the dipole force on the three-level atom, which are completely different from those on the two-level atom. Our study also shows that all of general expressions on the three-level atom will be simplified to those on the two-level atom in the approximation of large detuning.     

Influence of Multi-Photon Process on Entanglement of Interacting System of the Qubit and Thermal Field

We study the system of a single qubit couples to a single mode thermal field according to a multi-photon Jaynes-Cummings-type interaction with phase decoherence. Both the time evolving entanglement and the stationary state entanglement are calculated by adopting the log-negativity as a measure. It is found that the multi-photonprocess can enhance the stationary state entanglement of this system and can enlarge the range of the parameter Δ/g and the mean photon number of initial thermal field in which the stationary state is distillable.     

Quantum Logic Network for Probabilistic Teleportation of Two-Particle State in a General Form

A simplification scheme of probabilistic teleportation of two-particle state in a general form is given.By means of the primitive operations consisting of single-qubit gates,two-qubit controlled-not gates,Von Neumann measure-ment and classically controlled operations,we construct and efficient quantum logical network for implementing the new scheme of probabilistic teleportation of a two-particle state in a general form.     

Information Erasure and Recovery in Quantum Memory

We show that information in quantum memory can be erased and recovered perfectly if it is necessary. The fact that the final states of environment are completely determined by the initial states of the system allows an erasure operation to be realized by a swap operation between the system and an ancilla. Therefore, the erased information can be recovered. When there is an irreversible process, e.g. an irreversible operation or a decoherence process, in the erasure process, the information would be erased perpetually. We present that quantum erasure will also give heat dissipation in the environment. A classical limit of quantum erasure is given to coincide with Landauer‘s erasure principle.     

One Atom in an Optical Cavity： Entanglement and Applications to Quantum Computation

We propose a scheme to implement two-bit .quantum phase gates and one-bit unitary gates by using the two- mode two-photon Jaynes Cummings model. The entanglement between the atom and cavity is also investigated in the presence of phase decoherence. It is found that there is stationary entanglement that is sensitive with the detuning     

Quantum Nonlocality in Atom—Field Coupling System via Degenerate Raman Processes with Cavity Damping

The property of quantum nonlocality in the atom-field coupling system via degenerate Raman processes with cavity damping is investigated.It is found that if the cavity dissipation is very weak,the atom-field state can exhibit quantum nonlocality periodically.The rapidity of the loss of the quantum nonlocality depends on the amplitude of the initial coherent state and the cavity damping constant.The stronger the field is and the larger the damping constant could be ,the more rapidly the nonlocality decreases.     

Quantum Phase Transition and Ferromagnetic Spin Correlation in Parallel Double Quantum Dots

We investigate electronic transport through a parallel double quantum dot （DQD） system with strong on-site Coulomb interaction, as well as the interdot tunnelling. By applying numerical renormalization group method, the ground state of the system and the transmission probability at zero temperature are obtained. For a system of quantum dots with degenerate energy levels and small interdot tunnel coupling, the spin correlations between the DQDs is ferromagnetic, and the ground state of the system is a spin-1 triplet state. The linear conductance will reach the unitary limit （2e^2/h） due to the Kondo effect at low temperature. As the interdot tunnel coupling increases, there is a quantum phase transition from ferromagnetic to anti-ferromagnetic spin correlation in DQDs and the linear conductance is strongly suppressed.     

Pairwise Entanglement and Quantum Phase Transitions in Spin Systems

We examine several well-known quantum spin models and categorize the behaviour of pairwise entanglement at quantum phase transitions. A unitied picture on the connection between the entanglement and quantum phase transition in spin systems is presented.     

Information-Disturbance Tradeoff in Estimating the Unknown Three-Qubit GHZ State

In a recent paper, Sacchi （Phys. Rev. Lett. 96 （2006） 220502） studied the information-disturbance tradeoff in estimating an unknown two-qubit maximally entangled state. In this study, we explore the tradeoff in estimating 13 an unknown three-qubit GHZ state. The optimal estimation process supplies a fidelity of 13/54 and the tradeoff interpolates smoothly between non-informative measurement and optimal estimation process.     

Two-Time Diffusion Process in the Porous Medium

We find that there are two time scales t and ε ln t in the asymptotic behaviour of diffusion process in the porous medium, which give us a new insight to the anomalous dimension in this problem. Further we construct an iterative method to calculate the anomalous dimension and obtain an improved result.     

Squeezing and Entanglement in Continuous Variable Systems

Based on total variance of a pair of Einstein-Podolsky-Rosen (EPR) type operators, the generalized EPR entangled states in continuous variable systems are defined. We show that such entangled states must correspond to two-mode squeezing states whether these states are Gaussian or not and whether they are pure or not. With help of the relation between the total variance and the entanglement, the degree of such entangIement is also defined. Through analysing some specific cases, we see that this method is very convenient and easy in practical applications. In addition, an entangled state with no squeezing is studied, which reveals that there certainly exists something unknown about entanglement in continuous variable systems.     

Entangled States Used in Remote Information Concentration and Their Properties

We analyse a process of remote information concentration achieved by the W state. The result turns out to be neither as good as performed by the GHZ state nor as by the Smolin bound entangled state. Based on this particular phenomenon, the properties of the three entangled states are realized.