Amplitude-Damping Decoherence Suppression of Two-Qubit Entangled States by Weak Measurements

Taming decoherence is a critical issue in quantum information science. We here investigate amplitude-damping decoherence suppression of two-qubit entangled states by weak quantum measurements. It is shown that the weak measurements can effectively suppress the decoherence for different initial entangled states. More interestingly, we show that the weak measurements have different effects on the entanglement protection for two entangled states which are equivalent under a local unitary operation. This result implies that the entanglement protection effect could be modulated according to different demands.     

多粒子纠缠的保护方案

量子纠缠是量子信息的重要物理资源. 然而当量子系统与环境相互作用时, 会不可避免地产生消相干导致纠缠下降, 因此保护纠缠不受环境的影响具有重要意义. 振幅衰减是一种典型的衰减机制. 如果探测环境保证没有激发从系统中流出, 即视为对系统的一种弱测量. 本文基于局域脉冲序列和弱测量, 提出了一种可以保护多粒子纠缠不受振幅衰减影响的有效物理方案, 保护的对象是在量子通信和量子计算中发挥重要作用的Cluster态和Maximal slice态.     

Robustness of two-qubit and three-qubit states in correlated quantum channels

We investigate how the correlated actions of quantum channels affect the robustness of entangled states. We consider the Bell-like state and random two-qubit pure states in the correlated depolarizing, bit flip, bit-phase flip, and phase flip channels. It is found that the robustness of two-qubit pure states can be noticeably enhanced due to the correlations between consecutive actions of these noisy channels, and the Bell-like state is always the most robust one. We also consider the robustness of three-qubit pure states in correlated noisy channels. For the correlated bit flip and phase flip channels, the result shows that although the most robust and most fragile states are locally unitary equivalent, they exhibit different robustness in different correlated channels, and the effect of channel correlations on them is also significantly different. However, for the correlated depolarizing and bit-phase flip channels, the robustness of two special three-qubit pure states is exactly the same. Moreover, compared with the random three-qubit pure states, they are neither the most robust states nor the most fragile states.     

Entanglement and decoherence: fragile and robust entanglement

The destruction of entanglement of open quantum systems by decoherence is investigated in the asymptotic long-time limit. For this purpose a general and analytically solvable decoherence model is presented which does not involve any weak-coupling or Markovian assumption. It is shown that two fundamentally different classes of entangled states can be distinguished and that they can be influenced significantly by two important environmental properties, namely, its initially prepared state and its size. Quantum states of the first class are fragile against decoherence so that they can be disentangled asymptotically even if coherences between pointer states are still present. Quantum states of the second type are robust against decoherence. Asymptotically they can be disentangled only if also decoherence is perfect. A simple criterion for identifying these two classes on the basis of two-qubit entanglement is presented.     

Quantum discord dynamics of two-qubit system in a quantum spin environment

We study the dynamics of quantum discord of two-qubit system in a quantum spin environment at finite temperature in the thermodynamics limit. Special attention is paid to the difference between the entanglement and quantum discord when considering the influences of the environment temperature and the initial system states. We show that in the same range of the physical parameters, when the system states behave no entanglement or entanglement sudden death, the quantum discord keeps nonzero. So the quantum discord is more robust than entanglement under this decoherence environment. Furthermore, we also illustrate that we can tune the parameters related to the system and the environment to suppress the decay of quantum discord.     

Comparison of entanglement trapping among different photonic band gap models

We investigate the roles of different qubit-environment decoherence models on the entanglement trapping of two qubits. By considering three environmental models （the single photonic band gap model, the common photonic band gap model, and the two independent photonic band gaps model）, we note that the final values of entanglement trapping are determined by these different models. We also give the conditions of obtaining the larger entanglement trapping by comparing two-qubit entanglement dynamics in different decoherence models. Moreover, the comparison of entanglement trapping between two Bell-like states in the same decoherence model are also carried out.     

Quantum entanglement and quantum operation

It is a simple introduction to quantum entanglement and quantum operations. The authors focus on some applications of quantum entanglement and relations between two-qubit entangled states and unitary operations. It includes remote state preparation by using any pure entangled states, nonlocal operation implementation using entangled states, entanglement capacity of two-qubit gates and two-qubit gates construction. Supported by the National Fundamental Research Program of China (Grant No. 2001CB309306), the National Natural Science Foundation of China (Grant Nos. 60621064 and 10674127) and the Innovation Funds from Chinese Academy of Sciences     

Quantum Remote Control of Unitary Operations on a Qubit of Pure Entangled States

We consider the remote implementation of an arbitrary unitary operation on one qubit of a pure two-qubit entangled state with 100% efficiency via entanglement swapping in detail, then directly generalize this protocol from two-qubit to N-qubit entangled states. The overall classical information and distributed entanglement cost required for this quantum remote control protocol is less than the bi-directional quantum state teleportation method.     

Quantum logic networks for probabilistic teleportation

By eans of the primitive operations consisting of single-qubit gates.two-qubit controlled-not gates,Von Neuman measurement and classically controlled operations.,we construct efficient quantum logic networks for implementing probabilistic teleportation of a single qubit,a two-particle entangled state,and an N-particle entanglement.Based on the quantum networks,we show that after the partially entangled states are concentrated into maximal entanglement,the above three kinds of probabilistic teleportation are the same as the standard teleportation using the corresponding maximally entangled states as the quantum channels.     

Protecting Distribution Entanglement for Two-Qubit State Using Weak Measurement and Reversal

Protection of entanglement from disturbance of the environment is an essential task in quantum information processing. We investigate the effect of the weak measurement and reversal (WMR) on the protection of the entanglement for an arbitrarily entangled two-qubit pure state from these three typical quantum noisy channels, i.e., amplitude damping channel, phase damping channel and depolarizing quantum channel. Given the parameters of the Bell-like initial qubits’ state |ψ〉 = a|00〉 + d|11〉, it is found that the WMR operation indeed helps for protecting distributed entanglement from the above three noisy quantum channels. But for the Bell-like initial qubits’ state |?〉 = b|01〉 + c|10〉, the WMR operation only protects entanglement in the amplitude damping channel, not for the phase damping and depolarizing quantum channels. In addition, we discuss how the concurrence and the success probability behave with adjusting the weak or the reversal weak measurement strength.     

Entanglement Dynamics of Two Qubits Coupled to a Noise Environment

We study the time evolution of two two-state systems （two qubits） initially in the pure entangled states or the maximally entangled mixed states interacting with the individual environmental noise. It is shown that due to environment noise, all quantum entangled states are very fragile and become a classical mixed state in a short-time limit. But the environment can affect entanglement in very different ways. The type of decoherence process for certain entangled states belongs to amplitude damping, while the others belong to dephasing decoherenee.     

Entanglement dynamics of two interacting qubits under the influence of local dissipation

We investigate the dynamics of entanglement given by the concurrence of a two-qubit system in the non-Markovian setting. A quantum master equation is derived, which is solved in the eigenbasis of the system Hamiltonian for X-type initial states. A closed formula for time evolution of concurrence is presented for a pure state. It is shown that under the influence of dissipation non-zero entanglement is created in unentangled two-qubit states which decay in the same way as pure entangled states. We also show that under real circumstances, the decay rate of concurrence is strongly modified by the non-Markovianity of the evolution.     

One-Way Information Deficit and Geometry for a Class of Two-Qubit States

The work deficit, as introduced by Jonathan Oppenheim et al. [Phys. Rev. Lett. 89 (2002) 180402] is a good measure of the quantum correlations in a state and provides a new standpoint for understanding quantum non-locality. In this paper, we analytically evaluate the one-way information deficit (OWID) for the Bell-diagonal states and a class of two-qubit states and further give the geometry picture for OWID. The dynamic behavior of the OWID under decoherence channel is investigated and it is shown that the OWID of some classes of X states is more robust against the decoherence than the entanglement.     

Decoherence from a spin chain with Dzyaloshinskii—Moriya interaction

We study the dynamics of quantum discord and entanglement for two spin qubits coupled to a spin chain with Dzyaloshinsky-Moriya interaction.In the case of a two-qubit with an initial pure state,quantum correlations decay to zero at the critical point of the environment in a very short time.In the case of a two-qubit with initial mixed state,it is found that quantum discord may get maximized due to the quantum critical behavior of the environment,while entanglement vanishes under the same condition.Besides,we observed a sudden transition between classical and quantum decoherence when only a single qubit interacts with the environment.The effects of Dzyaloshinsky-Moriya interaction on quantum correlations are considered in the two cases.The decay of quantum correlations is always strengthened by Dzyaloshinsky-Moriya interaction.     

Dzyaloshinskii-Moriya相互作用和内禀消相干对基于两量子比特Heisenberg自旋系统的量子密集编码的影响

通过求解Milburn方程, 研究了内禀消相干条件下包含Dzyaloshinskii-Moriya (DM) 相互作用的两量子比特Heisenberg自旋系统实现的量子密集编码最佳传输容量的演化特性, 分析了不同方向DM相互作用、不同初态、各向异性以及内禀消相干因子等参数对最佳编码容量的影响. 研究表明: 初态的选择对系统密集编码最佳传输容量的影响很大, 不同类型初态下密集编码容量的依赖参数不完全相同; 当系统初态处于c|01>+ d|10> 形式的非最大纠缠时, 引入较弱的DM相互作用z分量可提高最佳编码容量; 相位消相干可抑制最佳编码容量的涨落并使其在长时间演化下趋于稳定. 研究还发现: 内禀消相干下, 通过选取合适的最大纠缠初态, 系统密集编码的最佳传输容量能够保持理想极大值2; 而且无论引入哪个方向的DM相互作用, 基于两量子比特Heisenberg自旋系统的最佳编码容量总可优于经典通信的传输容量.     

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.     

Decoherence Sensing of Entangled-Coherent-State Channels via Unambiguous Quantum State Filtering

Unambiguous quantum state filtering is applied to evaluation of the decoherence sensing of entangled quantum channels consisting of N-mode entangled coherent states. It is found that quantum entanglement can enhance the performance of decoherence sensing while the increase of the mode numbers in the entangled probe field can slightly improve the sensing performance only in the weak field regime.     

Information under Lorentz Transformation

In the context of quantum information, we investigate extensively some important classes of a general form of a two-qubit system under Lorentz transformation. It is shown Lorentz transformation causes a decay of entanglement and consequently information loses. On the other hand, it generates entangled states between systems prepared initially in a separable states. The partial entangled states are more robust under Lorentz transformation than maximally entangled states. Therefore the rate of information lose is larger for maximum entangled states compared with that for partially entangled states.     

High Fidelity Symmetric Telecloning and Entanglement Distribution of Spin Quantum States by Weak Measurement and Reversal

We propose a physical realization of robust symmetric telecloning scheme for spin quantum states by employing the weak measurement and reversal (WMR) operation. Using proper WMR, the ultrahigh telecloning fidelity and long distance of quantum state transfer with certain success probability can be achieved. More interestingly, the lowest average telecloning fidelity can attain 80 %, which is almost independent of the spin chain length. We also study the properties of entanglement distribution via the spin chain for arbitrary two-qubit entangled pure states as inputs and find that the WMR operation indeed helps for protecting distributed entanglement.     

在不同初态下Dzyaloshinskii-Moriya相互作用及内禀退相干对海森伯系统的量子纠缠的影响

研究了存在内禀退相干时,对于不同的系统初态,具有DM相互作用和各向异性的三粒子XXZ海森伯模型的对纠缠动力学特性.得出了一些结论:系统的对纠缠度与各向异性参数?无关,但内禀退相干对系统的纠缠有明显的抑制作用;在内禀退相干存在时,若系统初态为纠缠态,选择合适的DM相互作用的参数,系统的对纠缠有一个非零的稳定值;系统初态为分离态时,系统的对纠缠会随时间震荡衰减,并且每次震荡会出现纠缠突然死亡现象,系统的对纠缠最终达到解纠缠状态.因此,选择合适的系统初态和DM相互作用参数可以有效地控制系统的对纠缠.