约瑟夫森电荷量子比特的耗散特性

从环境的两种不同初态(粒子数态和热平衡态)出发,对电荷量子比特的时间演化进行了深入的研究.结果表明:在温度较高(500 mK)时,如果环境初始处在粒子数态,那么翻转算符的衰减时间会比热库处于热平衡态时更长,并且10＞态的布居数算符的渐近值也更小.在温度极低(50 mK)时,电荷量子比特的动力学特性就与环境的初态无关.     

Stabilizing Geometric Phase by Detuning in a Non-Markovian Dissipative Environment

The geometric phase of a two-level atom non-resonantly coupled to a non-Markovian dissipative environment is investigated. Compared to an earlier work [Chen J. J. et al. Phys. Rev. A 81 （2010）022120] in which the non-Markovian effect has a serious correction on geometric phase, we find that the geometric phase can be stabilized by detuning in non-Markovian dissipative decoherence. Moreover, the geometric phase approaches the unitary geometric phase with the increase of detuning for any initial polar angle, which shows that the geometric phase is not only resilient to the Markovian noise but is also resilient to the non-Markovian noise when a large detuning between the qubit and environment is considered. Our results may be helpful for geometric quantum computation.     

Screening Effect in Charge Qubit

We study the influence of screening effect on quantum decoherence for charge qubit and the process of quantum information storage. When the flux produced by the circulating current in SQUID loop is considered, screening effect is formally characterized by a LC resonator. Using large-detuning condition and Fröhlich transformation in the qubit-cavity-resonator system, we calculate the decoherence factor for charge qubit and the effective qubit-cavity Hamiltonian. The decoherence factor owns a factorized structure, it shows that screening effect is a resource of decoherence for charge qubit. The effective Hamiltonian shows that the screening effect results in a frequency shift for charge qubit and a modified qubit-cavity coupling constant induced by a LC resonator.     

Investigation of Decoherence of Superconducting Charge Qubit Entangled with the Environment

In the Born-Markov approximation, a method that calculates the energy relaxation time T ₁ and the decoherence time T ₂ of superconducting qubits is given by solving the set of Bloch-Redfield equations and considering the results of decoherence of a superconducting charge qubit. Compared to the spin-boson model, it not only contains the decoherence being caused by the dissipative environment, but also includes the decoherence being generated by the dissipative elements in a superconducting electronic circuit. Hence, it is good for studying the decoherence of superconducting qubits comprehensively.     

Short-Time Effects of Environment to Josephson Charge Qubit： A Master Equation Approach

In this paper we investigate an environmental effects to Josephson charge qubit （JCQ） when the environment is taken as the Ohmic bath. At first, we derive the master equation from a JCQ-bath model. Then we investigate the coefficients of the equations that describe the shift in frequency, diffusive, decoherence, and so on. It is shown that our result on decoherence agrees with experimental one very well as the time is short enough.     

Polarization as a Berry Phase

The macroscopic polarization of a crystalline dielectric is best defined as a Berry phase of the electronic Bloch wavefunctions, a feature that is best exemplified by the spontaneous polarization of a ferroelectric crystal for which a microscopic theory was previously unavailable.     

Dissipative Evolution of the Qubit State in the Tomographic-Probability Representation

We consider the evolution of qubit states for the Demkov problem in the presence of dephasing processes in the spin tomographic-probability representation. We present an explicit solution of the spin tomogram in terms of the ₁ F ₂ hypergeometric function. We calculate the tomographic Shannon and q entropies through the solution of the master equation in the form of tomographic-probability distribution of the qubit states obtained.     

Preservation of Quantum Fisher Information and Geometric Phase of a Single Qubit System in a Dissipative Reservoir Through the Addition of Qubits

In this paper, we have investigated the preservation of quantum Fisher information (QFI) of a single-qubit system coupled to a common zero temperature reservoir through the addition of noninteracting qubits. The results show that, the QFI is completely protected in both Markovian and non-Markovian regimes by increasing the number of additional qubits. Besides, the phenomena of QFI display monotonic decay or non-monotonic with revival oscillations depending on the number of additional qubits N ??1 in a common dissipative reservoir. If N < N _c (a critical number depending on the reservoirs parameters), the behavior of QFI with monotonic decay occurs. However, if N ≥ N _c , QFI exhibits non-monotonic behavior with revival oscillations. Moreover, we extend this model to investigate the effect of additional qubits and the initial conditions of the system on the geometric phase (GP). It is found that, the robustness of GP against the dissipative reservoir has been demonstrated by increasing gradually the number of additional qubits N ??1. Besides, the GP is sensitive to the initial parameter ??, and possesses symmetric in a range regime [0,2π].     

Constraint Dynamics and Tracking Control to Coherence of a Thermal Dissipative Qubit

Constraint dynamics and tracking control strategy to stabilize the coherence of a decoherent system is applied to a dissipative qubit system at a finite temperature. By using a control field dependent on the dynamical state of the qubit via the constraint equations, we show that the coherence of the qubit can be preserved within a finite time duration by the feedback effect of the qubit system. It is also shown that the temperature plays a negative role to this coherence control strategy.     

Quantum Efficiency of Charge Qubit Measurements Using a Single Electron Transistor

The quantum efficiency, which characterizes the quality of information gain against information loss, is an important figure of merit for any realistic quantum detectors in the gradual process of collapsing the state being measured. In this work we consider the problem of solid-state charge qubit measurements with a single-electron-transistor (SET). We analyze two models: one corresponds to a strong response SET, and the other is a tunable one in response strength. We find that the response strength would essentially bound the quantum efficiency, making the detector non-quantum-limited. Quantum limited measurements, however, can be achieved in the limits of strong response and asymmetric tunneling. The present study is also associated with appropriate justifications for the measurement and backaction-dephasing rates, which were usually evaluated in controversial methods.     

Entanglement and Berry Phase in a Parameterized Three-Qubit System

In this paper, we construct a parameterized form of unitary \(\breve {R}_{123}(\theta _{1},\theta _{2},\varphi )\) matrix through the Yang-Baxterization method. Acting such matrix on three-qubit natural basis as a quantum gate, we can obtain a set of entangled states, which possess the same entanglement value depending on the parameters ?? ₁ and ?? ₂. Particularly, such entangled states can produce a set of maximally entangled bases Greenberger-Horne-Zeilinger (GHZ) states with respect to ?? ₁ = ?? ₂ = π/2. Choosing a useful Hamiltonian, one can study the evolution of the eigenstates and investigate the result of Berry phase. It is not difficult to find that the Berry phase for this new three-qubit system consistent with the solid angle on the Bloch sphere.     

Entropy Squeezing in Coupled Field-Superconducting Charge Qubit with Intrinsic Decoherence

We investigate the entropy squeezing in the system of a superconducting charge qubit coupled to a single mode field. We find an exact solution of the Milburn equation for the system and discuss the influence of intrinsic decoherence on entropy squeezing. As a comparison, we also consider the variance squeezing. Our results show that in the absence of the intrinsic decoherence both entropy and variance squeezings have the same periodic properties of time,and occur at the same range of time. However, when the intrinsic decoherence is considered, we find that as the time going on the entropy squeezing disappears fast than the variance squeezing, there exists a range of time where entropy squeezing can occur but variance squeezing cannot.     

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.     

Entropy Squeezing in Coupled Field-Superconducting Charge Qubit with Intrinsic Decoherence

We investigate the entropy squeezing in the system of a superconducting charge qubit coupled to a single mode field. We find an exact solution of the Milburn equation for the system and discuss the influence of intrinsic decoherence on entropy squeezing. As a comparison, we also consider the variance squeezing. Our results show that in the absence of the intrinsic decoherence both entropy and variance squeezings have the same periodic properties of time, and occur at the same range of time. However, when the intrinsic decoherence is considered, we find that as the time going on the entropy squeezing disappears fast than the variance squeezing, there exists a range of time where entropy squeezing can occur but variance squeezing cannot.     

Implementing a Multi-Qubit Quantum Phase Gate Encoded by Photonic Qubit

A protocol is proposed to implement a three-qubit phase gate for photonic qubits in a three-mode cavity. The idea can be extended to directly implement a N-qubit phase gate. We also show that the interaction time remains unchanged with the increasing number of qubits. In addition, the influence of cavity decay and atomic spontaneous emission on the gate fidelity and photon loss probability is also discussed by numerical calculation.     

耗散反应产物的电荷扩散与能量相干

在输运理论的框架内研究了¹⁹F+⁵¹V耗散反应中能量相干宽度随出射产物中质比N/Z变化的关系,得到了反应系统已经达到电荷平衡的结论,提取了相应的电荷扩散系数. 分析表明电荷扩散过程造成了的分布宽度随反应时间线性增大的现象. 还探讨了各个同位素出射道的相关对激发函数振荡结构涨落幅度的作用.     

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.