Decoherence in a system of many two-level atoms

I show that the decoherence in a system of degenerate two-level atoms interacting with a bosonic heat bath is for any number of atoms governed by a generalized Hamming distance (called "decoherence metric") between the superposed quantum states, with a time-dependent metric tensor that is specific for the heat bath. The decoherence metric allows for the complete characterization of the decoherence of all possible superpositions of many-particle states, and can be applied to minimize the overall decoherence in a quantum memory. For qubits which are far apart, the decoherence is given by a function describing single-qubit decoherence times the standard Hamming distance. I apply the theory to cold atoms in an optical lattice interacting with blackbody radiation.     

存在相位退相干时原子与光场相互作用的纠缠演化和保持

应用全量子理论研究了存在相位退相干时单模相干光场与一个二能级原子相互作用系统纠缠的时间演化规律;分别讨论了原子—光场耦合常数、光场的平均光子数以及失谐量的大小对场与原子纠缠的影响.结果表明:随着原子—光场耦合常数的增大和光场平均光子数的增加,系统纠缠的振荡频率都会明显增大.不存在相位退相干时,纠缠的时间演化明显受到失谐量的影响,若选取适当的失谐量,系统的纠缠可长时间保持在最大纠缠态.若考虑相位退相干的影响,则在共振情况下系统纠缠的时间演化是一个逐渐衰减的过程,且最终衰减到零;但若存在适当的失谐量,则在初始一段时间内系统的纠缠也是一个波动幅度逐渐衰减的过程,但随着时间的演化,失谐量抵消了相位退相干的影响,使系统的纠缠不再衰减到零.如果增大失谐量,纠缠在初始一段时间内波动的幅度会相应的减小,并且纠缠趋于稳定的时间也随着失谐量的增大而缩短;当失谐量适当时,系统可保持在纠缠相对较大的状态而无消纠缠态.     

Dynamical Suppression of Pure Amplitude Damping in Two-State Quantum Systems

Dynamical control of decoherence induced by the environment in a single quantum-bit system is investigated. We choose the suitable perturbations acting on the qubit system to decrease the decoherence due to pure amplitude damping. The scheme proposed here is based on the free-Hamiltonian-elimination technique and the paritykick technique, which concludes two homogeneous classical large-blue-detuned optical fields with different frequencies added to the qubit system. By applying this scheme, the decoherence can be completely suppressed.     

Dynamical Suppression of Pure Amplitude Damping in Two-State Quantum Systems

Dynamical control of decoherence induced by the environment in a single quantum-bit system is investigated. We choose the suitable perturbations acting on the qubit system to decrease the decoherence due to pure amplitude damping. The scheme proposed here is based on the free-Hamiltonian-elimination technique and the paritykick technique, which concludes two homogeneous classical large-blue-detuned optical fields with different frequencies added to the qubit system. By applying this scheme, the decoherence can be completely suppressed.     

Quantum decoherence in the theory of open systems

Within the framework of the Lindblad theory for open quantum systems, we determine the degree of quantum decoherence of a harmonic oscillator interacting with a thermal bath. It is found that the system manifests a quantum decoherence which is more and more significant in time. We also calculate the decoherence time scale and analyze the transition from quantum to classical behavior of the considered system. The text was submitted by the author in English.     

二能级系统自发辐射的动力学抑制

研究了在二能级原子系统中有环境诱导的退相干的控制问题.通过对量子位系统施加适当的扰动可以减少由自发辐射引起的退相干.本文提出了一种新的基于频率位移技术和宇称反演技术的机制.这种机制可由两束均匀经典大失谐光场作用于原子来实现.通过应用这种机制,在存在自发辐射的情况下,可以有效地抑制退相干.     

Decoherence and equilibration under nondestructive measurements

The evolution of observable quantities of finite quantum systems is analyzed when the latter are subject to nondestructive measurements. The type and number of measurements characterize the level of decoherence produced in the system. A finite number of instantaneous measurements leads to only a partial decoherence. But infinite number of such measurements yields complete decoherence and equilibration. Continuous measurements result in partial decoherence in finite time, but produce complete decoherence and equilibration as time tends to infinity. Resulting equilibrium states are characterized by representative statistical ensembles that, generally, retain information on initial conditions. Any system, to be observable, necessarily requires the presence of measurements, whose large number leads to the system equilibration and decoherence.     

Localization under the effect of randomly distributed decoherence

Electron transport through disordered quasi one-dimensional quantum systems is studied. Decoherence is taken into account by a spatial distribution of virtual reservoirs, which represent local interactions of the conduction electrons with their environment. We show that the decoherence distribution has observable effects on the transport. If the decoherence reservoirs are distributed randomly without spatial correlations, a minimal degree of decoherence is necessary to obtain Ohmic conduction. Below this threshold the system is localized and thus, a decoherence driven metal-insulator transition is found. In contrast, for homogenously distributed decoherence, any finite degree of decoherence is sufficient to destroy localization. Thus, the presence or absence of localization in a disordered one-dimensional system may give important insight about how the electron phase is randomized.     

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.     

Different types of integrability and their relation to decoherence in central spin models

We investigate the relation between integrability and decoherence in central spin models with more than one central spin. We show that there is a transition between integrability ensured by the Bethe ansatz and integrability ensured by complete sets of commuting operators. This has a significant impact on the decoherence properties of the system, suggesting that it is not necessarily integrability or nonintegrability which is related to decoherence, but rather its type or a change from integrability to nonintegrability.     

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.     

Time Correlation and Decoherence in a Two-Particle Interferometer

A two-particle interferometer is theoretically analyzed to show how decoherence induced by interactions with the environment affects time correlations, a process we call time correlation decoherence. Specifically, on the basis of simple mathematical analysis, we show how the interaction between a bipartite entangled system and a photon bath representing the environment can efface the oscillations in the coincidence detection rate of the interferometer. We discuss the dependence of this kind of decoherence on the photon energy and density.     

三量子位系统的消相干和退纠缠

基于多谐振子热库环境模型,采用求解非马尔可夫近似主方程方法,研究了三量子位系统由于相对位相变化引起的消相干和退纠缠.计算得到了初始态为GHZ-class态、W-class态和WGHZ-class态在演化过程中的消相干和退纠缠时间尺度.结果表明,对初始GHZ-class态,相干性全部丧失,退纠缠完全发生,纠缠度变为0;对初始W-class态,相干性和纠缠度被保持;对初始WGHZ-class态,消相干部分发生,纠缠首先随时间变化最后达到一个渐近值.     

Coherence stability in three-level systems

We study the decoherence rate for estimating the time at which the coherence instability of a quantum pure state is onset. We analyze the coherence stability of pure states of a three-level quantum system under the effect of a bosonic reservoir and driven by two Raman classical fields. By assuming the boson systems to be in thermal states we find for a symmetric V-system a set of three states free from decoherence and, for a symmetric cascade-system, a two-dimensional subspace whose states are stable against the considered decoherence mechanism.     

Behavior of quantum coherence of Ξ-type four-level atom under bang-bang control

In this paper, we have studied the bang-bang (BB) decoupling scheme to suppress the phase decoherence, the amplitude decoherence and the general decoherence in a four-level Ξ-type atom system. The corresponding dynamical decoupling groups are given for designing the decoupling pulse sequences to suppress these three kinds of decoherence, respectively. Results show that in a proper time scale, the decoupling operations suppress the decoherence effectively. Especially in the ideal limits, it can suppress the decoherence completely. We also give the time scale in which the BB control works well. Numerical simulations show that, the larger cycle times N, the better effect of the BB decoupling operations under a fixed time scale.     

Universal decoherence in solids

Symmetry implications for the decoherence of quantum oscillations of a two-state system in a solid are studied. When the oscillation frequency is small compared to the Debye frequency, the universal lower bound on the decoherence due to the atomic environment is derived in terms of the macroscopic parameters of the solid, with no unknown interaction constants.     

Quantum Correlation in Circuit QED Under Various Dissipative Modes

Dynamical evolutions of quantum correlations in circuit quantum electrodynamics (circuit-QED) are investigated under various dissipative modes. The influences of photon number, coupling strength, detuning and relative phase angle on quantum entanglement and quantum discord are compared as well. The results show that quantum discord may be less robust to decoherence than quantum entanglement since the death and revival also appears. Under certain dissipative mode, the decoherence subspace can be formed in circuit-QED due to the cooperative action of vacuum field. Whether a decoherence subspace can be formed not only depends on the form of quantum system but also relates closely to the dissipative mode of environment. One can manipulate decoherence through manipulating the correlation between environments, but the effect depends on the choice of initial quantum states and dissipative modes. Furthermore, we find that proper relative phase of initial quantum state provides one means of suppressing decoherence.     

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.     

Coherence theory and coherence phenomena in a closed spin‐1/2 system

A simplified Heisenberg spin model is studied in order to examine the idea of decoherence in closed quantum systems. For this purpose, we present a quantifiable definition to quantum coherence Ξ, and discuss in some detail a general coherence theory and its elementary results. As expected, decoherence is understood as a statistical process that is caused by the dynamics of the system, similar to the growth of entropy. It appears that coherence is an important measure that helps to understand quantum properties of a system, e.g., the decoherence time can be derived from the coherence function Ξ(t), but not from the entropy dynamics. Moreover, the concept of decoherence time is applicable in closed and finite systems. However, in most cases, the decay of off‐diagonal elements differs from the usual exp(‐t/τ_d) behaviour. For concreteness, we report the form of decoherence time τ_d in a finite Heisenberg model with respect to the number of particles N, density n_ρ, spatial dimension D and ? in a η/r^?‐type of potential.     

Sudden Change of Quantum Discord in Qutrit-Qutrit System Under Depolarising Noise

The effects of decoherence on quantum discord of two-qutrit system under the influence of multilocal, collective and global depolarising noises are investigated. Sudden change of quantum discord with the decoherence parameter and the initial state parameter occurs for the qutrits being coupled to multilocal or global depolarising noises. On the other hand, decoherence cannot induce sudden change of quantum discord when the system is collectively coupled to the depolarising noise. In addition, the local unitary transformation can alter the region in which the phenomenon of sudden change exists.