Dynamics of super-quantum discord and direct control with weak measurement in open quantum system

Super-quantum discord(SQD) with weak measurement is regarded as a kind of quantum correlation in quantum information processing. We compare and analyze the dynamical evolutions of SQD, quantum discord(QD), and quantum entanglement(QE) between two qubits in the correlated dephasing environmental model. The results indicate that(i) owing to the much smaller influence of weak measurement on the coherence of the system than that of von Neumann projection measurement, SQD with weak measurement is larger than QD, and(ii) dynamical evolution of QD or QE monotonically goes to zero with time, while SQD monotonically tends to a stable value and a freezing phenomenon occurs. The stable value after freezing mainly depends on the measurement strength and the purity of the initial quantum state.     

双自旋系统中的量子失协问题研究

利用量子失协的几何度量方案研究了双自旋海森堡模型中的量子关联特性, 得到了一般情形下两量子态量子失协度的解析表达式, 讨论了量子位之间的耦合强度、温度和外加磁场强度等对量子关联大小的影响, 并给出了对应的量子关联调控方案. 此外还发现在低温下量子失协存在突变的现象. 结果表明, 在双自旋的海森堡模型体系下, 可以通过对系统参数(如温度、耦合强度、磁场强度等)的调节来实现对量子关联大小的有效调控, 这将会对在量子信息科学中精确控制量子失协和实现量子态的隐形传输以及量子逻辑门的设计提供一定的借鉴和指导意义.     

Non-Markovian relaxation in an open quantum system

Non-Markovian dynamics in open quantum systems is characterized by a time-non-locality in the equation of motion valid for the reduced density operator. An expansion of this density matrix equation with respect to Laguerre polynomials is used to tackle the time-non-locality. The applicability and the numerical limitations of the method are discussed in detail. In order to illuminate the characteristics of non-Markovian dynamics the reference example is studied of a single quantum degree of freedom moving in a harmonic potential and being embedded in a heat bath. If interpreted as the photoinduced dynamics of nuclear motion in polyatomic molecules we can suggest two clear signatures of non-Markovian dynamics observable in ultrafast optical experiments, firstly a pronounced and somewhat irregular oscillatory behavior of the vibrational level populations, and secondly a separation of the vibrational wavepacket into a double-structure. Received 12 April 2000 and Received in final form 2 September 2000     

Comparison and control of the robustness between quantum entanglement and quantum correlation in an open quantum system

We investigate the influence of environmental decoherence on the dynamics of a coupled qubit system and quantum correlation.We analyse the relationship between concurrence and the degree of initial entanglement or the purity of initial quantum state,and also their relationship with quantum discord.The results show that the decrease of the purity of an initial quantum state can induce the attenuation of concurrence or quantum discord,but the attenuation of quantum discord is obviously slower than the concurrence’s,correspondingly the survival time of quantum discord is longer.Further investigation reveals that the robustness of quantum discord and concurrence relies on the entanglement degree of the initial quantum state.The higher the degree of entanglement,the more robust the quantum discord is than concurrence.And the reverse is equally true.Birth and death happen to quantum discord periodically and a newborn quantum discord comes into being under a certain condition,so does the concurrence.     

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.     

量子Discord简介及其在特定系统中的计算

量子Discord作为一种比量子纠缠更准确地刻画系统量子关联的物理量,在量子信息理论中备受关注.本文首先简略介绍量子Discord的基本概念,然后以横场Ising系统为例,讨论了量子Discord的一种计算方法.最后根据横场Ising系统的量子Discord得到了表征其特征的三维图像,并讨论了系统中两格点间的关联特性.     

Dynamics of quantum discord in a two-qubit system under classical noise

We study the quantum discord dynamics of two noninteracting qubits that are, respectively, subject to classical noise. The results show that the dynamics of quantum discord are dependent on both the coupling between the qubits and classical noise, and the average switching rate of the classical noise. In the weak-coupling Markovian region, quantum discord exhibits exponent decay without revival, and can be well protected by increasing the average classical noise switching rate. While in the strong-coupling non-Markovian region, quantum discord reveals slowly decayed oscillations with quick revival by decreasing the average switching rate of the classical noise. Thus, our results provide a new method of protecting quantum discord in a two-qubit system by controlling the coupling between the qubits and classical noise, and the average switching rate of the classical noise.     

双原子Jaynes-Cummings模型下的量子失协和超量子失协

在量子信息的研究中,量子关联起着至关重要的作用,而弱测量对揭示量子关联有重要作用.在双原子Jaynes-Cummings(JC)模型下,分析了量子失协和超量子失协两种量子关联随时间演化过程中的差异性,以及测量强度参数、平均光子数在决定量子关联和影响其动态演化的一般作用.结果表明,弱测量可以更好地揭示量子关联,不仅可以更好地揭示量子关联的复活,还可以获得更大的关联最大值;平均光子数会有效影响两种关联的消失与复活,平均光子数越大,量子关联的消失与复活越快;平均光子数较大时,使用旋波近似得到的弱测量下的超量子失协比强测量下的量子失协与实际误差更小.     

Dynamics of quantum discord in the purification process

We investigate the dynamics of quantum discord during the purification process. In the case of Werner states, it is shown that quantum discord is increased after a round of purification protocol. Furthermore, quantum mutual information and classical correlation is also increased during this process. We also give an analytic expression for a class of higher dimensional states which have additive quantum discord.     

Asymmetry of bipartite quantum discord

It is known from the analysis of the density matrix for bipartite systems that the quantum discord (as a measure of quantum correlations) depends on the particular subsystem chosen for the projective measurements. We study asymmetry of the discord in a simple physical model of two spin-1/2 particles with the dipole-dipole interaction governed by the XY Hamiltonian in the inhomogeneous magnetic field. The dependence of the above discord asymmetry on the Larmor frequencies at both T = 0 (the ground state) and T > 0 has been investigated. It is demonstrated, in particular, that the asymmetry is negligible for high temperatures but it may become significant with a decrease in the temperature.     

Non-adiabatic geometric phases and dephasing in an open quantum system

We analyze the influence of a dissipative environment on geometric phases in a quantum system subject to non-adiabatic evolution. We find dissipative contributions to the acquired phase and modification of dephasing, considering the cases of both weak short-correlated noise and slow quasi-stationary noise. Motivated by recent experiments, we find the leading non-adiabatic corrections to the results, known for the adiabatic limit.     

Tavis-Cummings模型中的几何量子失协特性

采用几何量子失协的计算方法,通过改变两原子初始状态、腔内光子数和偶极-偶极相互作用强度,研究了Tavis-Cummings模型中的几何量子失协特性.结果表明:几何量子失协都是随时间周期性振荡的,选取适当的初态可以使两原子一直保持失协状态,增加腔内光子数和偶极相互作用对几何量子失协有积极的影响.     

Dynamics of quantum discord in photonic crystals

We study the system-reservoir dynamics of quantum correlations in the decoherence phenomenon within a two-qubit composite system interacting with a common photonic band-gap (PBG) environment. We compare the dynamics of entanglement with that of quantum discord. By analytical and numerical analyses we find that, the quantum discord can maintain a constant value in the long-time limit even when entanglement suddenly disappears. We also show that the detuning conditions play a crucial role in controlling quantum correlations of the two-qubit system. In PBG environment, the stationary quantum discord can be attained in well-controlled conditions. Our results have lots of potential applications to quantum information processing in nanostructured materials.     

Dynamics of quantum zeno and anti-zeno effects in an open system

We provide a general dynamical approach for the quantum Zeno and anti-Zeno efects in an open quantum system under repeated non-demolition measurements.In our approach the repeated measurements are described by a general dynamical model without the wave function collapse postulation.Based on that model,we further study both the short-time and long-time evolutions of the open quantum system under repeated non-demolition measurements,and derive the measurement-modified decay rates of the excited state.In the cases with frequent ideal measurements at zero-temperature,we re-obtain the same decay rate as that from the wave function collapse postulation(Nature,2000,405:546).The correction to the ideal decay rate is also obtained under the non-ideal measurements.Especially,we find that the quantum Zeno and anti-Zeno efects are possibly enhanced by the non-ideal natures of measurements.For the open system under measurements with arbitrary period,we generally derive the rate equation for the long-time evolution for the cases with arbitrary temperature and noise spectrum,and show that in the long-time evolution the noise spectrum is efectively tuned by the repeated measurements.Our approach is also able to describe the quantum Zeno and anti-Zeno efects given by the phase modulation pulses,as well as the relevant quantum control schemes.     

Dynamics of quantum zeno and anti-zeno efects in an open system

We provide a general dynamical approach for the quantum Zeno and anti-Zeno efects in an open quantum system under repeated non-demolition measurements.In our approach the repeated measurements are described by a general dynamical model without the wave function collapse postulation.Based on that model,we further study both the short-time and long-time evolutions of the open quantum system under repeated non-demolition measurements,and derive the measurement-modified decay rates of the excited state.In the cases with frequent ideal measurements at zero-temperature,we re-obtain the same decay rate as that from the wave function collapse postulation(Nature,2000,405:546).The correction to the ideal decay rate is also obtained under the non-ideal measurements.Especially,we find that the quantum Zeno and anti-Zeno efects are possibly enhanced by the non-ideal natures of measurements.For the open system under measurements with arbitrary period,we generally derive the rate equation for the long-time evolution for the cases with arbitrary temperature and noise spectrum,and show that in the long-time evolution the noise spectrum is efectively tuned by the repeated measurements.Our approach is also able to describe the quantum Zeno and anti-Zeno efects given by the phase modulation pulses,as well as the relevant quantum control schemes.     

Quantum control based on machine learning in an open quantum system

Designing robust control schemes in n-level open quantum system is significant for quantum computation. Here, we investigate two quantum control strategies based on supervised machine learning to suppress the quantum noise in an open quantum system. One is controlling state distance and the other is governing the average of a Hermitian operator. In this process, the dynamics of the system is mapped to a neural network where the control fields correspond to the weights. Besides, the system is transformed into the coherence Bloch space without using superoperator thus the complications are reduced largely. As an example, the two control protocols are demonstrated in a two-level and four-level systems, respectively. By applying these examples, the results show that the state of the system transfers to the target state and the average of a Hermitian operator to its minimum value in a given time despite disturbed by various types of noise.     

Experimental investigation of an active open optical resonator in a turbulent atmosphere

The spectral densities of the time fluctuations of the amplitude of steady-state generation at two wavelengths ₁=0.63 and ₂=1.15 under conditions of turbulent fluctuations of the refractive index in the waveguide channel of an open optical resonator are studied. The frequency dependences of the spectral densities are analyzed at various states of atmospheric turbulence, including the state produced in an aerodynamic chamber. For wavelength ₂ the fluctuations of the water-vapor content form a random absorption field. The distinctive features of the frequency dependence of the spectral density are discussed and related to the time cross-correlation of random fields of the passive impurity of the water-vapor concentration, the temperature, and the turbulent velocity of the flow. The asymptotic behavior of the frequency dependences of the spectral density is analyzed from the standpoint of the self-similarity of the stochastic characteristics of the system consisting of the waveguide channel and the medium.Odessa Hydrometeorological Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 2, pp. 111–118, February, 1991.     

Geometric discord of tripartite quantum systems

We study the quantification of geometric discord for tripartite quantum systems. Firstly, we obtain the analytic formula of geometric discord for tripartite pure states. It is already known that the geometric discord of pure states reduces to the geometric entanglement in bipartite systems, the results presented here show that this property is no longer true in tripartite systems. Furthermore, we provide an operational meaning for tripartite geometric discord by linking it to quantum state discr...