Decoherence and protection of entanglement of a system of three qubits driven by a classical Gaussian distributed fluctuating field

We consider a system of three uncoupled entangled qubits undergoing a decoherence process (DP) induced by a classical environmental noise portrayed by a Gaussian distributed fluctuating field with either Ornstein–Uhlenbeck (OU) or Gaussian (G) autocorrelation function. The impacts of such a DP on the entanglement of the qubits are analyzed in detail when they are initialized either in the GHZ- or W-type states and interact with the fluctuating field in three different scenarios namely, common, independent and mixed environment(s). We found that: (i) the way the qubits interact with the noise as well as their initial state play an important role towards the protection of entanglement; (ii) there are optimal parameters which permit to delay or totally avoid the disentanglement of the qubits; (iii) irrespective of the qubit-noise coupling (QNC) scenario and the initial prepared state considered, the OU noise is more injurious to the survivorship of entanglement than the G one. Specifically, we show that, irrespective of the QNC scenario and the character of the noise considered, the DP disentangles the qubits more quickly when they are initialized in the W-type states than in the GHZ-type one. Furthermore, we show that when the initial state of the qubits is considered to be a W-type state, the disentanglement occurs more rapidly in the common environment (CE) scenario followed by the mixed environments (MEs) scenario than in the independent environments (IEs) one. However, the situation is completely reversed when a GHZ-type state is considered.     

Thermal entanglement of two interacting qubits in a static magnetic field

We study systematically the entanglement of a two-qubit Heisenberg XY model in thermal equilibrium in the presence of an external arbitrarily-directed static magnetic field, thereby generalizing our prior work [G. Lagmago Kamta, A.F. Starace, Phys. Rev. Lett. 88, 107901 (2002)]. We show that a magnetic field having a component in the xy-plane containing the spin-spin interaction components produces different entanglement for ferromagnetic (FM) and antiferromagnetic (AFM) couplings. In particular, quantum phase transitions induced by the magnetic field-driven level crossings always occur for the AFM-coupled qubits, but only occur in FM-coupled qubits when the coupling is of Ising type or when the magnetic field has a component perpendicular to the xy-plane. When the magnetic field has a component in the xy-plane, the cut-off temperature above which the entanglement of both the FM- and AFM-coupled qubits vanishes can always be controlled using the magnetic field for any value of the XY coupling anisotropy parameter. Thus, by adjusting the magnetic field, an entangled state of two spins can be produced at any finite temperature. Finally, we find that a higher level of entanglement is achieved when the in-plane component of the magnetic field is parallel to the direction in which the XY exchange coupling is smaller.     

Stationary entanglement between two spatially separated atoms driven by a coherent laser field

This paper studies quantum entanglement between two spatially separated atoms driven by a coherent laser field in the dissipative process of spontaneous emission. It is shown that the entanglement strongly depends on the detuning of the laser frequency from atomic transition frequency, the interatomic separation and the Rabi frequency of the coherent laser field. A considerable amount of steady state entanglement can be obtained near Δ=-α (i.e., the dipole--dipole interaction and the detuning cancel out mutually) for small atomic separation and large Rabi frequency of the coherent laser field.     

Coherence-controlled entanglement and nonlocality of two qubits interacting with a thermal reservoir

We investigate the entanglement and the nonlocality of two qubits interacting with a thermal reservoir. It is shown that the time behavior of these quantities exhibits a strong dependence on the initial state of two qubits, and that the entanglement and the nonlocality of two qubits can be manipulated by changing the relative phases and the amplitudes of the polarized qubits.     

Dissipative dynamics of an entangled three-qubit system via non-Hermitian Hamiltonian: Its correspondence with Markovian and non-Markovian regimes

We investigate an entangled three-qubit system in which only one of the qubits experiences the decoherence effect by considering a non-Hermitian Hamiltonian,while the other two qubits are isolated,i.e.,do not interact with environment,directly.Then,the time evolution of the density matrix(for the pure as well as mixed initial density matrix)and the corresponding reduced density matrices are obtained,by which we are able to utilize the dissipative non-Hermitian Hamiltonian model with Markovian and non-Markovian regimes via adjusting the strange of the non-Hermitian term of the total Hamiltonian of the under-considered system.     

Sudden death and revival qubits coupled collectively of entanglement of two to a thermal reservoir

In this paper, we investigate the entanglement of two qubits coupled collectively to a common thermal environment and find that the the collective decay can lead to a revival of the entanglement that has already been destroyed. We also show that the ability of the system to revival entanglement relies on the mean photon number of the thermal environment and the degree of entanglement of the initial state.     

与腔场耦合的两个量子点中激子的纠缠动力学

利用共生纠缠度研究了单模腔场内两个耦合量子点中激子的纠缠动力学行为.结果表明:无论腔场初始制备于奇相干态还是偶相干态,两个量子点间直接耦合作用均能减弱激子的纠缠度.在腔场初始为奇相干态时,激子的纠缠度随场模强度的增加而减小;偶相干态时,激子的纠缠度呈现一个转折变化.此外,也研究了单模腔场内平均光子数与激子准最大相干纠缠态的关联.     

Resonance fluorescence spectrum of two atoms,coherently driven by a strong resonant laser field

In Lehmberg's approach, we consider the resonance fluorescence spectrum of two radiatively interacting atoms. In the strong field limit we have obtained analytical solutions for the spectrum of the symmetric and antisymmetric modes without decoupling approximation. Our solutions are valid for all values of the distance r₁₂ separating the atoms. The spectrum of the symmetric modes contains additional sidebands in 2Ω (Ω is the Rabi frequency) with amplitude dependent on (a/Ω)², where a is a parameter dependent on r₁₂. The antisymmetric part of the spectrum has no additional sidebands in 2Ω. For small distances r₁₂ (a=1) our results for the symmetric modes are identical with those of Agarwal et al. apart from the so-called scaling factor. For large distances r₁₂ (a=0) the spectra of the symmetric and antisymmetric modes are identical with the well-known one-atom spectrum.     

Sudden death and revival of entanglement of two qubits coupled collectively to a thermal reservoir

In this paper, we investigate the entanglement of two qubits coupled collectively to a common thermal environment and find that the the collective decay can lead to a revival of the entanglement that has already been destroyed. We also show that the ability of the system to revival entanglement relies on the mean photon number of the thermal environment and the degree of entanglement of the initial state.     

Structure formation of entanglement entropy in a system of two superconducting qubits coupled with an LC-resonator

This paper investigates theoretically the evolutions of the entanglement entropy of a system of two coupled-charge-qubits interacting with an LC-resonator.It is found that when the initial states of the two qubits are prepared in a given superposition excited state,the evolution of the von Neumann entropy of the system depends significantly on the coupling strength between the two Josephson charge qubits.With the variation of the coupling strength,the evolution of the entanglement entropy of the system forms some structures,especially the periodically bistable properties,which are the first discovered for such a system to our knowledge.It is found that the relative entropy entanglement of the system is also sensitive to the variation of the coupling strength between the two charge qubits,some novel ’collective oscillations’ of the relative entropy are found for the system.     

Dynamics of impurity: Local and non-local information for two non-identical qubits

From the separability point of view the problem of two ions interact with a single cavity mode is investigated. The density matrix is calculated and used to discuss the entanglement and to examine the dynamics of the local and non-local information. Our examination concentrated on the variation in the mean photon number and the ratio of the coupling parameters. Furthermore, we have also assumed that the atomic system is initially in the ground states as well as in the intermediate states. It has been shown that the local information is transferred to non-local information when the impurity of one qubit or both is maximum.     

Conservation issue of pairwise quantum discord and entanglement of two coupled qubits in a two-mode vacuum cavity

The conservation issues of pairwise quantum discord and entanglement of two qubits coupled to a two-mode vacuum cavity are investigated by considering the dipole-dipole interaction between two qubits.It is found that the sum of the square of the pairwise quantum discords and the sum of the square of the pairwise concurrences are both conserved in the strong dipole-dipole interaction limit.However,in the middle dipole-dipole and weak dipole-dipole interaction limits,the sum of the square of the pairwise concurrences is still conserved while the sum of the square of the pairwise discords is not.The crucial reason for this is that the quantum discords are not equivalent if the measurements are performed on different subsystems in a general situation.So it is very important for quantum computation depending on the quantum discord to select the target performed by the measurements.     

Markovian master equation for a two-level atom in a strong field and/or a tailored reservoir

The master equation for a two-level atom driven by a strong classical field and damped into a tailored reservoir with nonflat density of modes is derived under the Born-Markov approximation. To derive the master equation, the dressing transformation on the atomic operators is performed first, and, next, the dressed operators are coupled to the reservoir and the corresponding damping rates are calculated. The effects of a strong field and/or structured reservoir are seen as nonstandard terms in the master equation, some of which are reminiscent of terms known for squeezed vacuum reservoirs. The master equation leads to the generalized optical Bloch equations that can easily be solved for the steady state and, together with the quantum regression theorem, allow for analytical expressions for the fluorescence, as well as absorption spectra.     

Dynamics of a three-level V-type atom driven by a cavity photon and microwave field

We discuss the dynamics of a three-level V-type atom driven simultaneously by a cavity photon and microwave field by examining the atomic population evolution. Owing to the coupling effect of the cavity photon, periodical oscillation of the population between the two upper states and the ground state takes place, which is the well-known vacuum Rabi oscillation. Meanwhile, the population exchange between the upmost level and the middle level can occur due to the driving action of the external microwave field. The general dynamic behavior is the superposition of a fast and a slow periodical oscillation under the cooperative and competitive effect of the cavity photon and the microwave field. Numerical results demonstrate that the time evolution of the population is strongly dependent on the atom–cavity coupling coefficient g and Rabi frequency ?_e that reflects the intensity of the external microwave field. By modulating the two parameters g and ?_e, a large number of population transfer behaviors can be achieved.     

Entanglement dynamics of two qubits coupled by Heisenberg XY interaction under a nonuniform magnetic field in a spin bath

This paper investigates the entanglement dynamics of a Heisenberg XY model for a two-spin system in the presence of a nonuniform magnetic field.The master equations and the concurrence evolution equations for the initial α state are derived and analysed.It is shown that for the symmetric initial α state,only the nonuniform field can play a role in entanglement dynamics while the uniform field and the bath will not play such a role.For the asymmetric α state,the nonuniform field leads to the beat pattern oscillation of the concurrence evolution.The inhomogeneity of the field can enhance the entanglement by suppressing the decoherence effects of both the spin-orbit interaction and the spin bath.     

Magnetization and specific heat of classical ferromagnetic chains in a field: Crossover between two regimes

A theoretical approach, previously introduced to threat the statistical mechanics of a classical ferromagnetic chain in a field, has been extended in a systematic way to give the crossover from low-field to spin-wave regime. The results are in quantitative agreement with numerical transfer matrix data.     

Anisotropy and magnetic field effects on the entanglement of a two qubit Heisenberg XY chain

We investigate the entanglement of a two-qubit anisotropic Heisenberg XY chain in thermal equilibrium at temperature T in the presence of an external magnetic field B along the z axis. By means of the combined influences of anisotropic interactions and a magnetic field B, one is able to produce entanglement for any finite T, by adjusting the magnetic field strength. This contrasts with the isotropic interaction or the B = 0 cases, for which there is no entanglement above a critical temperature T(c) that is independent of the external B field.     

Dynamics and correlated performance of a photo-triggered discharge-pumped HF laser using SF6 with hydrogen or ethane

3 , has been performed in Ne/SF₆/H₂ and Ne/SF₆/C₂H₆ mixtures. Parameters involved have been the storage line capacitance and the circuit inductance, the capacitors charging voltage, the RH-molecule type and partial pressure, and the X-ray dose for the preionization. High laser performance has been achieved with C₂H₆: an output energy up to 3 J corresponding to a specific energy of 9.6 J/l at an efficiency of 4.7%, which strengthens the advantage of the photo-triggering technique to energize high-power HF lasers. However the optimum performance achieved with H₂, 5.75 J/l and 3.5%, are lower. It is shown, through a time-resolved study of the electrical discharge and spatial dynamics correlated to laser power and energy measurements, that discharge instabilities are responsible for the poor laser performance of the mixture with H₂. These instabilities, which lead to arc development, are characteristics of the discharge in Ne/SF₆. It is demonstrated for the first time that addition of a heavy hydrocarbon, such as C₂H₆, to that mixture induces the discharge stabilization so that the laser emission arises in a homogeneous active medium. This effect allows us to achieve better laser performance than with H₂. Received: 17 March 1998/Revised version: 13 July 1998