Dynamical Suppression of Pure Amplitude Damping in Two-State Quantum Systems

Dynamical control of decoherence induced by the environment in a single quantum-bit system is investigated. We choose the suitable perturbations acting on the qubit system to decrease the decoherence due to pure amplitude damping. The scheme proposed here is based on the free-Hamiltonian-elimination technique and the paritykick technique, which concludes two homogeneous classical large-blue-detuned optical fields with different frequencies added to the qubit system. By applying this scheme, the decoherence can be completely suppressed.     

Decoherence of a Double Quantum Dot Charge Qubit Approached with Redfield Equation

In this paper, we investigate the decoherence time of a double quantum dot （DQD） charge qubit in three kinds of baths through solving dynamics of the qubit. The dynamics of the qubit is investigated with Redfield master equation. It is shown that the decoherence time of the qubit in Ohmic bath has the same order of magnitude as the experiments reported. When the environment is modeled with the supra-Ohmic bath the decoherence time of the qubit is shorter than the experimental result. And when modeled with the sub-Ohmic bath the decoherence time of the qubit is longer than the experimental result.     

Entanglement evolution in an anisotropic two-qubit Heisenberg XYZ model with Dzyaloshinskii-Moriya interaction

This paper investigates the entanglement evolution of a two-qubit anisotropic Heisenberg XYZ chain in the presence of Dzyaloshinskii-Moriya interaction.The time evolution of the concurrence is studied for the initial pure entangled states cos θ |00 + sin θ |11 and cos |01 + sin |10 at zero temperature.The influences of Dzyaloshinskii-Moriya interaction D,anisotropic parameter and environment coupling strength γ on entanglement evolution are analysed in detail.It is found that the effect of noisy environment obviously suppresses the entanglement evolution,and the Dzyaloshinskii-Moriya interaction D acts on the time evolution of entanglement only when the initial state is cos |01 + sin |10.Finally,a formula of steady state concurrence is obtained,and it is shown that the stable concurrence,which is independent of different initial states and Dzyaloshinskii-Moriya interaction D,depends on the anisotropic parameter and the environment coupling strength γ.     

Decoherence Induced by Spin Chain Environment

We study the decoherence process of an exact solvable model that consists of a central spin-1/2 coupling to the surrounding anisotropy spin-1/2 chain in transverse fields. The Loschrnidt echo is calculated to study the character of decoherence with different degree of anisotropy. Our results show that the degree of anisotropy γ greatly affects the decoherence process of the central spin-system when the spin chain is in weak transverse fields, but it gives weak effect in the strong transverse field. The decoherence process of the central system changed dramatically along the line of the critical points, and this may be explained as the reflection of quantum phase transitions.     

Effects of inhomogeneous magnetic fields and different Dzyaloshinskii- Moriya interaction on entanglement and teleportation in a two-qubit Heisenberg XYZ chain

In this paper we calculate the thermal entanglement and teleportation in a two-qubit Heisenberg XYZ chain with the different Dzyaloshinskii-Moriya interaction and inhomogeneous magnetic fields. The analytical expres- sions of the concurrence and the average fidelity are obtained for this model. We have shown that the quantum phase transition occurs in the system and the quantum phase transition point depends on the inhomogeneity of magnetic fields. We compare the x-component Dzyaloshinskii-Moriya interaction with the z-component Dzyaloshinskii-Moriya interaction on the effects of quantum teleportation. It is found that we can take Dzyaloshinskii-Moriya interaction as one of the effective control parameters for the teleportation manipulation.     

Effects of inhomogeneous magnetic fields and different Dzyaloshinskii-Moriya interaction on entanglement and teleportation in a two-qubit Heisenberg XY Z chain

In this paper we calculate the thermal entanglement and teleportation in a two-qubit Heisenberg XYZ chain with the different Dzyaloshinskii-Moriya interaction and inhomogeneous magnetic fields. The analytical expressions of the concurrence and the average fidelity are obtained for this model. We have shown that the quantum phase transition occurs in the system and the quantum phase transition point depends on the inhomogeneity of magnetic fields. We compare the x-component Dzyaloshinskii-Moriya interaction with the z-component Dzyaloshinskii-Moriya interaction on the effects of quantum teleportation. It is found that we can take Dzyaloshinskii-Moriya interaction as one of the effective control parameters for the teleportation manipulation.     

Decoherence from a spin chain with Dzyaloshinskii—Moriya interaction

We study the dynamics of quantum discord and entanglement for two spin qubits coupled to a spin chain with Dzyaloshinsky-Moriya interaction.In the case of a two-qubit with an initial pure state,quantum correlations decay to zero at the critical point of the environment in a very short time.In the case of a two-qubit with initial mixed state,it is found that quantum discord may get maximized due to the quantum critical behavior of the environment,while entanglement vanishes under the same condition.Besides,we observed a sudden transition between classical and quantum decoherence when only a single qubit interacts with the environment.The effects of Dzyaloshinsky-Moriya interaction on quantum correlations are considered in the two cases.The decay of quantum correlations is always strengthened by Dzyaloshinsky-Moriya interaction.     

Dipole–dipole interactions enhance non-Markovianity and protect information against dissipation

Preserving non-Markovianity and quantum entanglement from decoherence effect is of theoretical and practical significance in the quantum information processing technologies.In this context, we study a system S that is initially correlated with an ancilla A, which interacts with the environment E via an amplitude damping channel.We also consider dipole–dipole interactions(DDIs) between the system and ancilla, which are responsible for strong correlations.We investigate the impact of DDIs and detuning on the non-Markovianity and information exchange in different environments.We show that DDIs are not only better than detuning at protecting the information(without destroying the memory effect) but also induce memory by causing a transition from Markovian to non-Markovian dynamics.In contrast, although detuning also protects the information, it causes a transition from non-Markovian to the Markovian dynamics.In addition, we demonstrate that the non-Markovianity grows with increasing DDI strength and diminishes with increasing detuning.We also show that the effects of negative detuning and DDIs can cancel out each other, causing a certain loss of coherence and information.     

Correlation dynamics of a qubit–qutrit system in a spin-chain environment with Dzyaloshinsky–Moriya interaction

We study the dynamics of correlations for a hybrid qubit-qutrit system in an XY spin-chain environment with Dzyaloshinsky-Moriya interaction. Our discussion involves a comparative analysis of negativity, quantum discord, and measurement-induced disturbance. It is found that the quantum discord is optimal of the three quantum correlations to de- tect the critical point of quantum phase transition. Only when the qubit interacts with the environment, is the phenomenon of sudden transition between the classical correlation and the quantum discord observed. Moreover, the Dzyaloshinsky- Moriya interaction enhances the decay of quantum correlations.     

Dense coding with a two-qubit Heisenberg XYZ chain under the influence of phase decoherence

We investigate the joint effects of phase decoherence,Dzyaloshinskii-Moriya(DM) interaction and inhomogeneity of the external magnetic field(b) on dense coding in a two-qubit anisotropic Heisenberg XY Z spin chain.Analytical expressions are obtained for the dense coding capacity.It is found that valid dense coding is always possible with this model when the system is initially prepared in the maximum entangled state.Moreover,optimal dense coding can be implemented for this initial state as long as the mean spin-spin coupling constant J + of the XY plane is larger than b and the DM interaction despite the intrinsic decoherence.Non-maximal entangled initial states are found to be undesirable for dense coding with this model.     

Knotted Picture of Single Qubit Quantum Logic Gate

This paper briefly introduces the five types of the surgical operations in knot theory and obtains the expression of single qubit quantum logic gate in terms of these surgical operations.     

Knotted Pictures of Hadamard Gate and CNOT Gate

This paper obtains the knotted pictures of Hadamard gate and CNOT gate in terms of surgical operations described in knot theory.     

Properties of Unsymmetrical Parabolic Confinement Potential Quantum Dot Qubit

On the condition of electric-LO phonon strong coupling in unsymmetrical parabolic confinement potential quantum dot （QD）, we obtain the eigenenergies of the ground state and the first-excited state, the eigenfunctions of the ground state, and the first-excited state by using variational method of Pekar type. This system in QD may be employed as a two-level quantum system-qubit. When the electron is in the superposition state of the ground state and the first-excited state, we obtain the time evolution of the electron density. The relations both the probability density of electron and the period of oscillation with the electron-LO-phonon coupling strength, the confinement strengths in the xy-plane and the z-direction are discussed.     

Progress in superconducting qubits from the perspective of coherence and readout

Superconducting qubits are Josephson junction-based circuits that exhibit macroscopic quantum behavior and can be manipulated as artificial atoms.Benefiting from the well-developed technology of microfabrication and microwave engineering,superconducting qubits have great advantages in design flexibility,controllability,and scalability.Over the past decade,there has been rapid progress in the field,which greatly improved our understanding of qubit decoherence and circuit optimization.The single-qubit coherence time has been steadily raised to the order of 10 to 100μs,allowing for the demonstration of high-fidelity gate operations and measurement-based feedback control.Here we review recent progress in the coherence and readout of superconducting qubits.     

Temperature Effects of Parabolic Linear Bound Potential and Coulomb Bound Potential Quantum Dot Qubit

On the condition of electric-LO phonon strong coupling in a parabolic quantum dot, we obtain the eigenenergy and the eigenfunctions of the ground state and the first-excited state using the variational method of Pekar type. This system in a quantum dot may be employed as a two-level quantum system-qubit. When the electron is in the superposition state of the ground state and the first-excited state, we obtain the time evolution of the electron density. The relations of the probability density of electron on the temperature and the electron-LO-phonon coupling constant and the relations of the period of oscillation on the temperature, the electron-LO-phonon coupling constant, the Coulomb binding parameter and the confinement length are derived. The results show that the probability density of electron oscillates with a period when the electron is in the superposition state of the ground and the first-excited state, and show that there are different laws that the probability density of electron and the period of oscillation change with the temperature and the electron-LO-phonon coupling constant when the temperature is lower or higher. And it is obtained that the period of oscillation decreases with increasing the Coulomb bound potential and increases with increasing the confinement length not only at lower temperatures but also at higher temperatures.     

H2^＋-Like Impurities Confined by Spherical Quantum Dots： a Candidate for Charge Qubits

We have calculated the electron energy of the ground and lower excited states for H2^＋-like impurity states confined in finite spherical quantum dots in GaAs. Based on the characteristics of energy levels, we have proposed a scheme for realizing charge qubit composed by the the ground and the first excited states of this confined double donor system for the first time. In the proposed scheme the charge qubit is coded in terms of the located electronic states.     

Efficient One-Step Generation of Cluster State with Charge Qubits in Circuit QED

t We propose theoretical schemes to generate highly entangled cluster state with superconducting qubits in a circuit QED architecture. Charge qubits are located inside a superconducting transmission line, which serves as a quantum data bus. We show that large clusters state can be efficiently generated in just one step with the longrange Ising-like unitary operators. The quantum operations which are generally realized by two coupling mechanisms： either voltage coupling or current coupling, depend only on global geometric features and are insensitive not only to the thermal state of the transmission line but also to certain random operation errors. Thus high-fidelity one-way quantum computation can be achieved.     

Efficient Atomic One-Qubit Phase Gate Realized by a Cavity QED and Identical Atoms System

We present a scheme to implement a one-qubit phase gate with a two-level atom crossing an optical cavity in which some identical atoms are trapped. One can conveniently acquire an arbitrary phase shift of the gate by properly choosing the number of atoms trapped in the cavity and the velocity of the atom crossing the cavity. The present scheme provides a very simple and efficient way for implementing one-qubit phase gate.     

Quantum Decoherence of Charge Qubit Coupled to Nonlinear Nanomechanical Resonator

When the nonlinearity of nanomechanical resonator is not negligible,the quantum decoherence of charge qubit is studied analytically.Using nonlinear Jaynes–Cummings model,one explores the possibility of being quantum data bus for nonlinear nanomechanical resonator,the nonlinearity destroys the dynamical quantum information-storage and maintains the revival of quantum coherence of charge qubit.With the calculation of decoherence factor,we demonstrate the influence of the nonlinearity of nanomechanical resonator on engineered decoherence of charge qubit.     

Exchange coupling and helical spin order in the triangular lattice antiferromagnet CuCrO2 using first principles

The magnetic and electronic properties of the geometrically frustrated triangular antiferromagnet CuCrO2 are investigated by first principles through density functional theory calculations within the generalized gradient approxi- mations （GGA）＋U scheme. The spin exchange interactions up to the third nearest neighbours in the ab plane as well as the coupling between adjacent layers are calculated to examine the magnetism and spin frustration. It is found that CuCrO2 has a natural two-dimensional characteristic of the magnetic interaction. Using Monte Carlo simulation, we obtain the Neel temperature to be 29.9 K, which accords well with the experimental value of 24 K. Based on non- collinear magnetic structure calculations, we verify that the incommensurate spiral-spin structure with （110） spiral plane is believable for the magnetic ground state, which is consistent with the experimental observations. Due to intra-layer geometric spin frustration, parallel helical-spin chains arise along the a, b, or a＋ b directions, each with a screw-rotation angle of about I20°. Our calculations of the density of states show that the spin frustration plays an important role in the change of d-p hybridization, while the spin-orbit coupling has a very limited influence on the electronic structure.