Entangled Subspaces for Two Coupled Qubits in a Normal Environment

By investigating the effect of environmental perturbations on two initially coupled qubits, we find that the interactions between the qubits and between the qubits and the environment are not only the source of decoherence, but also the power of avoiding disentanglement. It is shown that there are the entangled subspaces for four kinds of different coupling ways between the qubits, in which the qubits preserve entanglement all the time. Thus, any new coherent source does not be introduced to preserve entanglement in the entangled subspaces.     

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.     

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.     

Quantum State Transfer with Tunable Couplings for Superconducting Qubits

Currently, scalability and decoherence are still the two crucial problems for realizing large-scale quantum computation. To transfer information efficiently in the field of quantum computer and quantum communication, we must solve the two problems reasonably. Here, we propose an efficient scheme for the implementation of quantum states transfer in different couplings of superconducting devices, which are coupled weakly over a long distance. It is found that the isotropic Heisenberg coupling can offer the perfect operation for arbitrary states transfer with the shortest time. The scheme is feasible because the couplings have been demonstrated in actual experiments recently.     

基于量子Zeno效应的二位防错方案

讨论了量子Ｚｅｎｏ效应中测量的特点,指出这种测量只是使系统塌缩到原态的一种操作;通过用控制非门的偶数次操作,提出了一种理论上更简单、更易实现的二位防错方案。     

Robust Quantum Gates in Decoherence-Free Subspaces with Josephson Charge Qubits

We propose a new and feasible scheme to implement quantum gates in decoherence-free subspaces (DFSs) with Josephson charge qubits situated in a circuit QED architecture. Based on the resonator-assisted interaction, the controllable interqubit couplings occur only by tuning the individual flux biases, by which we obtain the DFS-encoded universal quantum gates. Compared with the non-DFS situation, we numerically consider the robustness of the DFS-encoded scheme that can be insensitive to the collective noises. Thus the protocol may perform the fault-tolerant quantum computing with Josephson charge qubits.     

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.     

Robust Quantum Computing in Decoherence-Free Subspaces with Double-Dot Spin Qubits

We present a scheme for implementing robust quantum gates in decoherence-free subspaces （DFSs） with double-dot spin qubits. Through the resonator-assisted interaction, the controllable interqubit couplings can be achieved only by adjusting the qubit transition frequencies. We construct a set of logic gates on the DFS-encoded qubits to eliminate the collective noise effects, and thus the gate fidelities can be enhanced remarkably. This proposal may offer a potential approach to realize the robust quantum computing with spin qubits.     

Entanglement Dynamics of Strongly-AC-Driven Superconducting Qubits in Circuit QED

We study the entanglement dynamics between two strongly-AC-driven superconducting charge qubits coupled collectively to a zero temperature, dissipative resonator and find an unusual feather that the competing of creation and annihilation of entanglement can lead to entanglement increasing, sudden death and revival. We also calculate the dependence of the death time on the initial state of the system.     

超导量子比特实验的开端

我们对超导量子比特领域的科学背景、历史起源和早期发展做简要评述.莱格特(Anthony J. Leggett)为这个领域打下了理论基础.克拉克(John Clarke)和他的两个学生马丁尼(John Martinis)和德沃雷(Michel H. Devoret)最早通过偏电流约瑟夫森结,首次观察到约瑟夫森结的量子行为.后来德沃雷实现了电荷量子比特叠加态、电荷-磁通混合量子比特的拉比共振和其他演化及投影测量.中村泰信(Yasunobu Nakamura)首先实现电荷量子比特的量子叠加和拉比振荡,还参与莫伊(J. E. Mooij)组实现了磁通量子比特的拉比振荡和读出.     

Multiplexing Read-Out of Charge Qubits by a Superconducting Resonator

Wavelength division multiplexing(WDM) is widely used in modern optics and electronics.For future quantum computers,the integration of readout is also vitally important.Here we incorporate an idea of WDM to demonstrate multiplexing readout of charge qubits by using a single integrated on-chip superconducting microwave resonator.Two distant qubits formed by two graphene double quantum dots(DQDs) are simultaneously readout by an interconnected superconducting resonator.This readout device is found to have 2 MHz bandwidth and1.1×10~(-4) e/(Hz)~(1/2) charge sensitivity.Different frequency gate-modulations,which are used selectively to change the impedance of the qubits,are applied to different DQDs,which results in separated sidebands in the spectrum.These sidebands enable a multiplexing readout for the multi-qubits circuit.This architecture can largely reduce the amount of detectors and can improve the prospect for scaling-up of semiconductor qubits.     

Entanglement Generation for Two Coupled Multi-excitation Fields Interacting with Qubits

We study the entanglement dynamics for two coupled multi-excitation cavity fields, each independently interacting with a two-level atom. We find that even when the atom-field system is initially not entangled, the fields can become heavily entangled through postselecting of the atomic state. The field entanglement entropy is dependent on the total excitation number but independent of the coherent photon hopping strength.     

Robust Quantum Computation with Superconducting Charge Qubits via Coherent Pulses

To achieve robust gate operations on superconducting charge qubits, we theoretically propose a feasible scheme to realize geometric quantum computation via coherent pulses. Only by adiabatically tuning the microwave pulses applied to the gate capacitance can the Berry phases associated with the system be acquired, from which we construct a universal set of geometric gates. Combining the geometric approach with the coherent pulse technique, robust quantum operations aimed at combating noise errors may be implemented experimentally.     

Entropic Inequalities for Two Coupled Superconducting Circuits

We discuss the known construction of two interacting superconducting circuits based on Josephson junctions, which can be precisely engineered and easily controlled. In particular, we use the parametric excitation of two circuits realized by an instant change of the qubit coupling to study entropic and information properties of the density matrix of a composite system. We obtain the density matrix from the initial thermal state and perform its analysis in the approximation of small perturbation parameter and sufficiently low temperature. We also check the subadditivity condition for this system both for the von Neumann entropy and deformed entropies and check the dependence of mutual information on the system temperature. Finally, we discuss the applicability of this approach to describe the two coupled superconducting qubits as harmonic oscillators with limited Hilbert space.     

Entanglement of Two Superconducting Charge Qubits Induced by Quantized Radiation Field

We consider two superconducting charge qubits coupled by a single-mode quantized field. We suppose that the two superconducting charge qubits be initially prepared in the mixed and separable state. As time evolves, we will investigate the entanglement between two superconducting charge qubits induced by the quantized field.     

Controllably Coupling Superconducting Charge and Flux Qubits by Using Nanomechanical Resonator

We propose an effective mechanism to couple superconducting charge and flux qubits by using a quantized nanomechanical resonator. The coupling between the charge and flux qubits can be controlled by the external flux of the charge qubit. Under the strong coupling limit, an iSWAP gate can be generated by this scheme. The experimental feasibility in our scheme is also presented.     

Suppressing the Dielectric Loss in Superconducting Qubits through Useful Geometry Design

Dielectric loss from different interfacial layers in the superconducting circuit and from external environment may cause superconducting qubit decoherence. Compared to modeling the entire device at once with a numerical solver, quantitatively formulating the dielectric loss can both describe all loss mechanisms and make the optimization more transparent. In this paper, we first analyze the expression formula of dielectric loss, and obtain a design scheme that can reduce the dielectric loss of qubits. That is, we replace the straight junction wires with the tapered junction wires. Based on this scheme, we perform a simulation to optimize the design of junction wires. Finally, a real experiment is conducted to verify our design. The results show that both the T1 time and T2 time of qubits are significantly improved.     

Regulation of Entanglement and Geometric Quantum Discord of Hybrid Superconducting Qubits for Circuit QED

We investigate the dynamic evolution behaviors of entanglement and geometric quantum discord (GQD) of hybrid qubits in a circuit QED system. Under certain initial conditions, interactions between qubits and that between the qubit and the cavity can suppress the decay of entanglement and GQD of qubits. Under the initial condition |ψ _Q (0)〉 _I =sinα|↓↑〉+cosα|↑↓〉, such decay was avoided by increasing the coupling strength ratio between qubits and that between the qubit and the cavity. Under the aforementioned condition, the survival time of entanglement and GQD was prolonged by decreasing the coupling strength ratio between qubits and that between the qubit and the cavity.     

Enforcing Thermal Entanglement of Three Coupled Qubits by Dissipation

The stationary state entanglement in a chain with three spins is reported. Each of spins couples to its own separate bosonic reservoir. The master equation for such spins’ system is derived under the Born-Markovian approximation. The result presents that the coupling between the middle spin and its bosonic bath in some special temperature region reinforce the entanglement between the spins. By analyzing the heat current between the middle spin and its bosonic bath, we find a tight relationship between the direction of heat current from the middle spin to its bosonic bath and the reinforcement of the entanglement. The entanglement increases with the heat current between the middle spin and its bosonic bath almost linearly.     

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.