Quantum Discord of two Coupled Qubits with Initial System-reservoir Correlation

Recently, the effect of initial qubit-reservoir correlation for two qubits in a common reservoir on the entanglement dynamics has been studied by Yan and Xia (Acta Sinica Quantum Optica 20, 16 2014). We extend their results and investigate how the initial qubit-reservoir correlation and dipole-dipole interaction between two qubits effect on dynamics of quantum discord in Markovian and non-Markovian regimes, respectively. The results show that in general, the larger initial qubit-reservoir correlation and dipole-dipole interaction can retard the decay of quantum discord. Besides, a combination of relatively strong dipole-dipole interaction and non-Markovian effect can efficiently protect quantum discord. Finally,thecomparisonbetweenevolutionsofquantumdiscordandentanglementisalsoconsidered.     

Pulsed and monochromatic excitation of semiconductor quantum wells under the conditions of quantum confinement

The reflectance and absorbance of light by quantum wells whose width is comparable to the light wavelength have been calculated. The difference in the refractive indices of the materials of the quantum well and the barriers has been taken into account. Pulsed irradiation with an arbitrary shape of the exciting pulse has been considered, and the existence of two closely spaced discrete excitation levels has been assumed. This pair of levels can correspond to two magnetopolaron states in a quantizing magnetic field directed perpendicular to the plane of the quantum well. The ratio between the magnitudes of nonradiative and radiative dampings of electronic excitations is arbitrary. The final results have been obtained without invoking the approximation in which the Coulomb interaction of electrons and holes is negligible.     

Observation of a collective mode of an array of transmon qubits

Arrays of transmon qubits coupled to a λ/2 superconducting coplanar waveguide resonator have been studied by microwave spectroscopy. The emergence of a collective mode has been discovered for a cluster of N > 5 qubits, whose coupling constant to the electromagnetic field in the resonator is √N times greater compared to a single qubit. In addition, the emergence of collective multiphoton transitions exciting higher levels of a qubit cluster has been demonstrated and the interaction of an individual qubit with such a cluster has been investigated.     

Non-Markovian entanglement dynamics of two spin-1/2 particles embedded in two separate spin star baths with tunable external magnetic fields

The exact entanglement dynamics of two spin qubits in two independent spin star baths via a Heisenberg XY interaction in the thermodynamic limit has been investigated by using an operator technique. After the Holstein-Primakoff transformation, the transformed Hamiltonian is effectively equivalent to the Hamiltonian of a Jaynes-Cummings model. The results show that the dynamics of the entanglement exhibits strong non-Markovian behavior and depends on the environmental temperature, the coupling strength between the center spin and the bath, the detuning controlled by a locally applied external magnetic field, as well as the initial state of the two qubits.     

具有次近邻相互作用的五量子比特XXZ海森伯自旋链的热纠缠

研究具有次近邻相互作用五量子比特XXZ海森伯自旋链在磁场作用下的热纠缠性质,利用数值计算求出最近邻两量子比特和次近邻两量子比特的共生纠缠度(concurrence),分别记为C_(12)和C_(13).研究结果表明,阻挫参数对配对热纠缠具有重要影响,而且阻挫参数的变化对C_(12)和C_(13)的影响也各不相同;温度、磁场、Dzyaloshinkii-Moriya相互作用以及各向异性参数对配对热纠缠有着不同程度的影响;通过选择适当的模型参数,可以有效地调节和提高五量子比特XXZ海森伯自旋链的配对热纠缠.     

Avoiding the decay of entanglement for coupling two-qubit system interacting with a non-Markov environment

The entanglement evolution of the coupled qubits interacting with a non-Markov environment is investigated in terms of concurrence.The results show that the entanglement of the quantum systems depends not only on the initial state of the system but also on the coupling between the qubit and the environment.For the initial state （|00 ± |11） /2^1/2,the coupled qubits will always been in the maximum entangled state under an asymmetric coupling.For the initial state （|01 ± |10） /2^1/2,in contrast,the entangling degree of the coupled qubits is always equal to unity and does not depend on the evolving time under the symmetric coupling.We find that the stronger the interaction between the qubits is,the better the struggle against the entanglement sudden death is.     

All-optical manipulation of electron spins in carbon-nanotube quantum dots

We demonstrate theoretically that it is possible to manipulate electron or hole spins all optically in semiconducting carbon nanotubes. The scheme that we propose is based on the spin-orbit interaction that was recently measured experimentally; we show that this interaction, together with an external magnetic field, can be used to achieve optical electron-spin state preparation with a fidelity exceeding 99%. Our results also imply that it is possible to implement coherent spin rotation and measurement using laser fields linearly polarized along the nanotube axis, as well as to convert spin qubits into time-bin photonic qubits. We expect that our findings will open up new avenues for exploring spin physics in one-dimensional systems.     

Simulation of sublinear dose dependence of thermoluminescence with the inclusion of the competitive interaction of trapping centers

The sublinearity of dose characteristics of the thermoluminescence has been calculated in terms of the model of competitive interaction of electron and hole trapping centers. It has been found that the sub-linearity is caused by the nonradiative electron recombination with hole deep traps during irradiation and recording of the thermoluminescence upon linear heating of a dosimetric phosphor. It has been shown that the proposed sublinearity mechanism can be used to explain the dose characteristics of the thermoluminescence of anion-defective aluminum oxide.     

Modulation of Entanglement for Coupled Superconducting Qubits Under Non-Markovian Environment

The evolution of entanglement decoherence is investigated for a coupled superconducting qubit under non-Markovian environment by utilizing a commensal entanglement degree. The results show that, owing to the memory feedback effect of environment, the entanglement degree of the coupled qubits at the thermal equilibrium always monotonously tends to zero so that entanglement sudden death occurs briefly in the non-Markovian process. Different from the Markovian process, stronger the dissipation is, faster the entanglement sudden death is. We find that, furthermore, the interaction between the qubits results generally in reduction of entanglement degree in the quantum system. With some special initial states or initial phase angles, however, the influence of the interaction between qubits on the system entanglement degree can be avoided.     

Non-Markovian disentanglement dynamics of two-qubit system in common environment

The disentanglement of a pair of identical qubits, sharing a dissipative environment, is investigated. We proposed a new non-Markovian analytical method to study the model, which takes into account the counter-rotating term in the qubit-environment interaction. It is shown that when the qubits sufficiently separate from each other, the model is equivalent to each subsystem interacts independently with its local environment. The effects of the dissipative environment and the interqubit distance on the time evolution of the concurrence for four bell states are examined in detail. And point out some new features appearing in the case of the two qubits close together.     

Kraus Operators for a Pair of Interacting Qubits: a Case Study

The Kraus form of the completely positive dynamical maps is appealing from the mathematical and the point of the diverse applications of the open quantum systems theory. Unfortunately, the Kraus operators are poorly known for the two-qubit processes. In this paper, we derive the Kraus operators for a pair of interacting qubits, while the strength of the interaction is arbitrary. One of the qubits is subjected to the x-projection spin measurement. The obtained results are applied to calculate the dynamics of the entanglement in the qubits system. We obtain the loss of the correlations in the finite time interval; the stronger the inter-qubit interaction, the longer lasting entanglement in the system.     

Synchronizing Two Superconducting Qubits through a Dissipating Resonator

A system consisting of two qubits and a resonator is considered in the presence of different sources of noise, bringing to light the possibility of making the two qubits evolve in a synchronized way. A direct qubit–qubit interaction turns out to be a crucial ingredient, as well as the dissipation processes involving the resonator. The detrimental role of the local dephasing of the qubits is also taken into account.     

Geometric Phase of Double Quantum Dot Qubits System with Coulomb Interaction

In quantum computing the geometric phase is a valuable tool to achieve fault tolerant. And quantum dot system is a candidate for constructing quantum processor. In this paper we investigate the geometric phase of a double qubits system interaction with a quantum point contact device. The qubits were constructed by two coupled double quantum dots systems. The coulomb interaction between the two subsystem have been considered. By using the definition which introduced by Tong, we calculate the geometric phases of each double quantum dots subsystem.     

Eliminating interactions between non-neighboring qubits in the preparation of cluster states in quantum molecules

We propose a scheme to eliminate the effect of non-nearest-neighbor qubits in preparing cluster state with double-dot molecules. As the interaction Hamiltonians between qubits are Ising-model and mutually commute, we can get positive and negative effective interactions between qubits to cancel the effect of non-nearest-neighbor qubits by properly changing the electron charge states of each quantum dot molecule. The total time for the present multi-step cluster state preparation scheme is only doubled for one-dimensional qubit chain and tripled for two-dimensional qubit array comparing with the time of previous protocol leaving out the non-nearest-neighbor interactions.     

Quantum-Chemical Calculations of the Excited Electronic States of the Prodan Molecule and Its Complexes in Water

A quantum-chemical calculation of the prodan molecule and its complexes in water for the geometry of the ground and fluorescent states is carried out. To describe the fluorescent state, changes in the electronic state (population) on bonds and atoms during transition of the molecule into an excited state are taken into account. A model of interaction of the prodan molecule with a polar proton-donor solvent (water) is suggested. It is shown that interaction with the ionic forms (H₃O₊ provides an explanation for the sensitivity of prodan to the solvent (displacement of the fluorescence bands). The nature of the electronic excited states of the prodan molecule and its complexes has been investigated. The constants of the rates of radiative and nonradiative processes and the fluorescence quantum yields have been calculated. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 3, pp. 330–334, May–June, 2005.     

Evolution of entanglement between qubits ultra-strongly coupling to a quantum oscillator

We investigate the dynamics of two qubits coupled with a quantum oscillator by using the adiabatic approximation method. We take account of the interaction between the qubits and show how the entanglement is affected by the interaction parameter. The most interesting result is that we can prolong the entanglement time or improve the entanglement degree by using an appropriate interaction parameter. As the generation and preservation of entanglement of qubits are crucial for quantum information processing, our research will be useful.     

Surface plasmons at the surface of liquid mercury

We have evaluated the surface plasmon dispersion relation in the presence of the inhomogeneous surface zone which has been proposed for liquid mercury. Assuming that photon-plasmon coupling occurs through the mechanism of frustrated total reflection, we find the dispersion relation to be sensitive to the existence of such a zone, providing retardation effects are taken into account. The results, it is believed, define nonradiative surface plasmon absorption as a useful technique for the study of conducting surfaces.     

A Theory of Electronic Energy Transfer in Complex Molecular Systems

The Förster theory for energy transfer is critically reviewed in the context of the present-day theory of nonradiative transitions, and the fundamental tenets of the Förster theory are shown to be erroneous. A new theory of electronic energy transfer is constructed taking into account the electronic transition theory. The intermolecular interaction between donor and acceptor molecules is assumed to perturb electron states of isolated molecules before the donor-molecule excitation. A distinguishing characteristic of the intermolecular interaction is spatial delocalization of the wave functions of electron states of the interacting molecules. It is this fact that has made it possible to realize ordinary photophysical processes between electron states of various molecules in a bimolecular system. In the experiments under study, the result of the intermolecular nonradiative photoprocess is given as evidence of electronic energy transfer from the donor molecule to the acceptor molecule.     

具有Dzyaloshinskii-Moriya相互作用的四量子比特海森堡XXZ模型中的热纠缠

研究四量子比特海森堡XXZ模型中配对纠缠的性质,在该系统中引入了Dzyaloshinskii-Moriya (DM)相互作用,通过求解配对纠缠度来讨论最近邻和次近邻两量子比特之间的热纠缠性质. 研究结果表明:对于铁磁和反铁磁两种情形而言,次近邻两量子比特之间不存在配对热纠缠;但在最近邻两量子比特情况时,DM相互作用和各向异性参数Δ对配对热纠缠和临界温度T_c都具有重要的影响,且随着温度T的增加,配对纠缠度逐渐减小直至消失. 因此,选择和调整合适的DM相互作用和各向异性参数,可以有效地控制和提高配对热纠缠. 关键词： 配对纠缠 XXZ模型')" href="#">XXZ模型 Dzyaloshinskii-Moriya相互作用     

Programmable Quantum Processor with Quantum Dot Qubits

The realization of controllable couplings between any two qubits and among any multiple qubits is the critical problem in building a programmable quantum processor(PQP). We present a design to implement these types of couplings in a double-dot molecule system, where all the qubits are connected directly with capacitors and the couplings between them are controlled via the voltage on the double-dot molecules. A general interaction Hamiltonian of n qubits is presented, from which we can derive the Hamiltonians for performing operations needed in building a PQP, such as gate operations between arbitrary two qubits and parallel coupling operations for multigroup qubits. The scheme is realizable with current technology.