Entanglement Dynamics of Three Superconducting Charge Qubits Coupled to a Cavity

We investigate the entanglement dynamics of a quantum system consisting of three superconducting charge qubits (SCQs) interacting with a microwave field. For separable and entangled states of the SCQs, the evolutions are studied under various photon numbers of cavity field. The results show that the amplitude and period of the bipartite entanglement square concurrences can be controlled by the choice of initial states of SCQs and photon numberof cavity field, respectively. This simple model of a quantum register allows us to understand the dynamic process of the quantum storage of information carried by charge qubit.     

Influence of Intrinsic Decoherence on Entanglement of Superconducting Charge Qubit in a Resonant Cavity

Taking the intrinsic decoherence effect into account, we investigate the entanglement dynamics of a superconducting charge qubit in a single-mode optical cavity. Concurrence, as the measure of entanglement of the coupled field-junction system, is calculated. In comparison, we also consider the entanglement of the system by using the entanglement parameter based on the ratio between mutual entropy and partial Von-Neumann entropy to investigate how the intrinsic decoherence affects the entanglement of the coupling system. Our results show that the evolution of the entanglement parameter has the behaviour similar to the concurrence and it is thus the well measure of entanglement for the mixed state in such a coupling system.     

Entropy Exchange and Entanglement in Superconducting Charge Qubit Inside a Resonant Cavity with Intrinsic Decoherence

By considering the intrinsic decoherence effect, we investigate the entropy exchange and entanglement in the interacting system of a superconducting charge qubit coupled to a single-mode optical cavity. We found that although the intrinsic decoherence leads to an irreversible evolution of the interacting system due to a suppression of coherent quantum features through the decay of off-diagonal matrix elements of the density operator, and has an apparently influence on the partial entropies of the two-component subsystems, it dose not destroy entropy exchange behavior. In addition, the lower bound of the concurrence, as the measure of entanglement of the coupling system, is calculated. It is shown that the evolution of entanglement is sensitive to the change of the intrinsic decoherence.     

Entanglement Dynamics Induced by a Squeezed Coherent Cavity Coupled Nonlinearly with a Qubit and Filled with a Kerr-Like Medium

An analytical solution for a master equation describing the dynamics of a qubit interacting with a nonlinear Kerr-like cavity through intensity-dependent coupling is established. A superposition of squeezed coherent states is propped as the initial cavity field. The dynamics of the entangled qubit-cavity states are explored by negativity for different deformed function of the intensity-dependent coupling. We have examined the effects of the Kerr-like nonlinearity and the qubit-cavity detuning as well as the phase cavity damping on the generated entanglement. The intensity-dependent coupling increases the sensitivity of the generated entanglement to the phase-damping. The stability and the strength of the entanglement are controlled by the Kerr-like nonlinearity, the qubit-cavity detuning, and the initial cavity non-classicality. These physical parameters enhance the robustness of the qubit-cavity entanglement against the cavity phase-damping. The high initial cavity non-classicality enhances the robustness of the qubit-cavity entanglement against the phase-damping effect.     

Statistical Properties of a System Consisting of a Superconducting Qubit Coupled to an Optical Field Inside a Transmission Line

The present work investigates the entropy and the excitation inversion of a coupled system that consists of a qubit represented by a Cooper pair box(CPB) interacting with a transmission line working as a circuit quantum electrodynamics(CQED). The proposed scheme uses the Buck-Sukumar model with a time dependent frequency in the presence of losses to study the evolution of the entropy and the excitation inversion of the system. We have shown that the CQED is much more sensitive to the presence of losses than the CPB. The results also show that it is possible to monitor properties of the subsystems through appropriate choices of the time-dependent parameters.     

Decoherence Dynamics of a Superconducting Charge Qubit Coupled to a Boson Bath and a Spin Bath

By virtue of a superconducting charge qubit, we derive the off-diagonal matrix operator and investigate the decoherence of the system in different regimes coupled to, respectively, the boson bath and the spin bath. It is found that the two different baths make a bit of difference on the decay of the system at low but finite temperature and the decoherence of the system is most closely linked with the regime as well as the coupling strength. Therefore, by optimizing some reasonable parameters, we can suppress appropriately the decoherence of a given quantum system.     

两能级原子与单模腔场共振耦合系统中热态纠缠现象

借助于共生纠缠度, 研究了旋转波和非旋转波近似下共振Jaynes-Cummings模型中二能级原子与单光子腔场的热态纠缠现象. 结果表明：该共振耦合系统中, 存在影响原子与单光子腔模纠缠的临界温度, 当环境温度高于这一温度时, 原子与腔场的纠缠特性消失. 若只考虑实光子跃迁, 这一临界温度仅与原子和腔场的耦合强度有关, 随耦合系数的增大而升高; 若将虚光场一并考虑, 只要实光子跃迁强度与虚光子跃迁强度不相等, 在一定的温度下能很好的保持原子与腔场的纠缠相干性, 当二者相等时, 即使在绝对零度下, 原子与腔场也难以形成纠缠形式.     

Superconducting Qubit Systems as a Platform for Studying Effects of Nonstationary Electrodynamics in a Cavity

It has been shown that superconducting qubit systems, having high tunability, can be used as a platform for the experimental study of various effects of nonstationary quantum electrodynamics in a cavity. In particular, the dynamic Lamb effect can be implemented owing to a nonadiabatic change in the effective coupling between the subsystem of qubits and a cavity. This effect is manifested in the excitation of a qubit (atom) at the change in the Lamb shift of its levels. It is remarkable that the effect of energy dissipation in such parametrically excited systems can be very nontrivial: dissipation in one of the subsystems of the hybrid system can enhance quantum effects in the other subsystem. This refers to various phenomena such as parametric qubit excitation, generation of photons from vacuum, and creation and confinement of finite entanglement of qubits.     

Preparation of Macroscopic Entangled Coherent States in Nitrogen-Vacancy Centers Ensembles Coupled to a Superconducting Flux Qubit

We propose a potentially practical scheme for creating macroscopic entangled coherent state between two separate nitrogen-vacancy center spin ensembles placed near a superconducting flux qubit. Through the collective magnetic coupling and the in situ tunability of the flux qubit, the arbitrary entangled coherent states of spin ensembles can be achieved with high success possibilities under the influence from decoherence of the flux qubit and spin ensembles.The experimental feasibility and challenge are justified using currently available technology.     

Mixedness,Coherence and Entanglement in a Family of Three-Qubit States

We consider a family of states describing three-qubit systems. We derived formulas showing the relations between linear entropy and measures of coherence such as degree of coherence, first- and second-order correlation functions. We show that qubit–qubit states are strongly entangled when linear entropy reaches some range of values. For such states, we derived the conditions determining boundary values of linear entropy parametrized by measures of coherence.     

Entanglement and Mixedness of Locally Cloned Non-Maximal W-State

In this work we describe a protocol by which two of three parties generate two bipartite entangled state among themselves without involving third party, from a non maximal W-state or W-type state |X〉=α|001〉₁₂₃+β|010〉₁₂₃+γ|100〉₁₂₃,α ²+β ²+γ ²=1 shared by three distant partners. Also we have considered the case β=γ, to obtain a range for α ², for which the local output states are separable and non local output states are inseparable. We also find out the dependence of the mixedness of inseparable states with their amount of inseparability, for that range of α ².     

Thermal Entanglement of the Two-Qubit Heisenberg Spin Chain Coupled to a Single-Mode Cavity Field

Thermal entanglement of a two-qubit Heisenberg spin chain coupled to a single-mode cavity field is investigated. It is found that (1) thermal entanglement without the rotating-wave approximation (RWA) is explicitly smaller than that obtained with the RWA, which means that the counter-rotating terms have a large impact on thermal entanglement, therefore they cannot be neglected; (2) the case (ω≪Ω) is more beneficial for enhancing thermal entanglement than the resonance case (ω=Ω), the near-resonant case (ω≈Ω) and the case (ω≫Ω); (3) for thermal entanglement, there is a competition process between the exchange coupling J (the direct-coupling between the two two-level atoms) and the coupling constant g (which deduces the indirect effect between the two two-level atoms); the critical value of g increases with the spin coupling strength J.     

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.     

Speeding up the Generation of Entangled State between a Superconducting Qubit and Cavity Photons via Counterdiabatic Driving

Optimal generation of entangled states is of critical significance for robust quantum information processing. An effective scheme is presented for speeding up the generation of an entangled state between a superconducting qubit and microwave photons via counterdiabatic driving. At a magic bias point, the first three levels of a charge-phase quantum circuit constitute an effective qutrit. An entangled state based on adiabatic population transfer is first achieved. By the technique of shortcuts to adiabaticity, a counterdiabatic driving is applied to the qutrit, which then accelerates the entanglement generation significantly. Moreover, with the accessible decoherence rates, the rapid operations in a shortcut way are highly robust when compared with adiabatic manipulations. The scheme could offer a promising approach toward optimal preparation of entangled states with superconducting artificial atoms in circuit quantum electrodynamics, experimentally.     

Entropy Correlations and Entanglement of a Superconducting Charge Qubitin a Resonant Cavity in the Presence of Noise

We study the dynamics of the entropy correlations and entanglement in a system of interaction of a superconducting charge qubit with a single-mode resonant cavity subject to noise considered as two-state random phase telegraph noise. We show that although the noise has an apparent suppressing effect on the evolution of the entropies of the qubit and the field and also on the entanglement in the system, the entropy exchange between the qubit and the
field is independent of it during the time evolution of the system.     

Preparation of Entanglement and Schrodinger Cat States of Superconducting Quantum Interference Devices Qubits via Coupling to a Microwave Cavity

An alternative scheme is proposed for the generation of n-qubit W states of superconducting quantum interference devices （SQUID） in cavity QED. In this scheme, Raman coupling of two lower flux states of SQUID system is achieved via a microwave pulse and the cavity mode. Conditioned on no photon leakage from the cavity, the n-qubit W state can be generated whether the effective coupling parameters of the SQUID to cavity mode and classical microwave fields are the same or different. Our strictly numerical simulations of the time evolution of the system including decay show that the success probability of our scheme is almost unity and the interaction time is on the order of 10-9 s. The scheme can also be used to generate the Schrodinger cat states of multi-SQUID.     

Unconventional Geometric Phase-Shift Gates Based on Superconducting Quantum Interference Devices Coupled to a Single-Mode Cavity

We present a scheme to realize geometric phase-shift gate for two superconducting quantum interference device （SQUID） qubits coupled to a single-mode microwave field. The geometric phase-shift gate operation is performed in two lower flux states, and the excited state [2〉 would not participate in the procedure. The SQUIDs undergo no transitions during the gate operation. Thus, the docoherence due to energy spontaneous emission based on the levels of SQUIDs are suppressed. The gate is insensitive to the cavity decay throughout the operation since the cavity mode is displaced along a circle in the phase space, acquiring a phase conditional upon the two lower flux states of the SQUID qubits, and the cavity mode is still in the original vacuum state. Based on the SQUID qubits interacting with the cavity mode, our proposed approach may open promising prospects for quantum iogic in SQUID-system.     

Unconventional Geometric Phase-Shift Gates Based on Superconducting Quantum Interference Devices Coupled to a Single-Mode Cavity

We present a scheme to realize geometric phase-shift gate for two superconducting quantum interference device (SQUID) qubits coupled to a single-mode microwave field. The geometric phase-shift gate operation is performed transitions during the gate operation. Thus, the docoherence due to energy spontaneous emission based on the levels of SQUIDs are suppressed. The gate is insensitive to the cavity decay throughout the operation since the cavity mode is displaced along a circle in the phase space, acquiring a phase conditional upon the two lower flux states of the SQUID qubits, and the cavity mode is still in the original vacuum state. Based on the SQUID qubits interacting with the cavity mode, our proposed approach may open promising prospects for quantum logic in SQUID-system.     

Quantum Interface between a Superconducting Qubit and Spin Ensembles

An experimentally feasible strong coupling system between a spin ensemble and a superconducting qubit is studied. The coupling strength can be exponentially enhanced by applying the squeezing transformations to the system. By means of the two spin ensembles commonly coupled to a superconducting qubit, a set of universal nonadiabatic holonomic single‐qubit quantum gates can be realized in a decoherence‐free subspace. Furthermore, this proposal is robust with respect to decay of the system parameters, and it is experimentally feasible with currently available technology.