Localization of a macroscopic object induced by the factorization of internal adiabatic motion

To account for the phenomenon of quantum decoherence of a macroscopic object, such as the localization and disappearance of interference, we invoke the adiabatic quantum entanglement between its collective states (such as that of the center-of-mass (CM)) and its inner states based on our recent investigation. Under the adiabatic limit where motion of the CM does not excite the transition of inner states, it is shown that the wave function of the macroscopic object can be written as an entangled state with correlation between adiabatic inner states and quasi-classical motion configurations of the CM. Since the adiabatic inner states are factorized with respect to each component of the macroscopic object, this adiabatic separation can induce the quantum decoherence. This observation thus provides us with a possible solution to the Schr?dinger cat paradox. Received 24 October 2000 and Received in final form 8 March 2001     

Quantum leakage of collective excitations of atomic ensemble induced by spatial motion

We generalize the conception of quantum leakage for the atomic collective excitation states. By making use of the atomic coherence state approach, we study the influence of the atomic spatial motion on the symmetric collective states of 2-level atomic ensemble due to inhomogeneous coupling. In the macroscopic limit, we analyze the quantum decoherence of the collective atomic state by calculating the quantum leakage for a very large ensemble at a finite temperature. Our investigations show that the fidelity of the atomic system will not be good in the case of atom numberN→∞. Therefore, quantum leakage is an inevitable problem in using the atomic ensemble as a quantum information memory. The detailed calculations shed theoretical light on quantum processing using atomic ensemble collective qubit.     

Quantum Decoherence of Macroscopic Object Induced by Inner Environments: an Exactly Solvable Model

We revisit the quantum decoherence problem of the center of mass motion of a macroscopic object,which is modelled as a one-dimensional atom chain.Induced by the coupling of the center of mass (C.M) motion with the inner degrees of freedom,this inner-environment-induced decoherence is reflected by the localization of the C.M wave packet.We show that,the C.M motion is coupled to the inner states only when the chain has interaction with the external potential.This result provides a realistic mechanism for the analysis of the inner-environment-induced localization of a macroscopic object.     

HYPER and Gravitational Decoherence

We study the decoherence process associated with the scattering of stochastic backgrounds of gravitational waves. We show that it has a negligible influence on HYPER-like atomic interferometers although it may dominate decoherence of macroscopic motions, such as the planetary motion of the Moon around the Earth.     

核多体系统输运现象的微观理论

首先回顾了描写核多体系统输运现象的一些主要模型和方法,然后介绍了输运现象微观动力学基础研究上一些新的结果,强调了单粒子运动动力学特征在建立集体输运方程和理解超重核合成机制上的重要作用。能量耗散和熵产生的数值计算结果表明,集体运动耗散过程可分为退相干、弛豫和定态等3 个阶段,弛豫过程通常表现为非常复杂的反常扩散过程。在这些理论工作的基础上,提出了一种自洽地分离核多体系统集体和单粒子变量的可能途径。In this article, I provide a simple review on conventional methods and models on the transport phenomenon of nuclear many-body systems. By exploiting the basic idea of time-dependent projection operator, I recommend a novel method to derive the transport equation for collective motion which is embedded on the microscopic dynamics of timedependent single-particle motion. It is emphasized that the microscopic dynamics of single-particle motion should play an important role for understanding the dynamics of nuclear reaction and the synthesis mechanisms of new superheavy elements. The numerical results of energy dissipation and entropy production indicate that the collective motion passes through three stages, such as dephasing/decoherence, statistical relaxation and stationary state. The statistical relaxation is a complex anomalous diffusion process in general. With the aid of above analysis and results, a possible way to define the collective and single-particle variables for the realistic nuclear many-body systems is proposed.     

Preparation of Macroscopic Entangled Coherent States in Nitrogen-Vacancy Centers Ensembles Coupled to a Superconducting Flux Qubit

We propose a potentially practical scheme for creating macroscopic entangled coherent state between two separate nitrogen-vacancy center spin ensembles placed near a superconducting flux qubit. Through the collective magnetic coupling and the in situ tunability of the flux qubit, the arbitrary entangled coherent states of spin ensembles can be achieved with high success possibilities under the influence from decoherence of the flux qubit and spin ensembles.The experimental feasibility and challenge are justified using currently available technology.     

Bose-Einstein condensates beyond mean field theory: quantum backreaction as decoherence

We propose an experiment to measure the slow log(N) convergence to mean field theory (MFT) around a dynamical instability. Using a density matrix formalism instead of the standard macroscopic wave function approach, we derive equations of motion which go beyond MFT and provide accurate predictions for the quantum break time. The leading quantum corrections appear as decoherence of the reduced single-particle quantum state.     

An intrinsic limit to quantum coherence due to spontaneous symmetry breaking

We investigate the influence of spontaneous symmetry breaking on the decoherence of a many-particle quantum system. This decoherence process is analyzed in an exactly solvable model system that is known to be representative of symmetry broken macroscopic systems in equilibrium. It is shown that spontaneous symmetry breaking imposes a fundamental limit to the time that a system can stay quantum coherent. This universal time scale is t(spon) approximately = 2piNH/(kBT), given in terms of the number of microscopic degrees of freedom N, temperature T, and the constants of Planck (h) and Boltzmann (kB).     

Decoherence-free generation of many-particle entanglement by adiabatic ground-state transitions

We discuss a decoherence insensitive method to create many-particle entanglement in a spin system with controllable collective interactions and propose an implementation in an ion trap. An adiabatic change of parameters allows a transfer from separable to a large variety of entangled eigenstates. We show that the Hamiltonian can have a supersymmetry permitting an explicit construction of the ground state at all times. Of particular interest is a transition in a nondegenerate ground state with a finite energy gap since here the influence of collective as well as individual decoherence mechanisms is substantially reduced. A lower bound for the energy gap is given.     

Conditions for Collective Decoherence

An efficient decoherence-reducing strategy exists if two qubits can be made decohered collectively. This paper serves to derive the explicit conditions for coJJective decoherence from a practical decoherence model. We describe two kinds of collective decoherence. In each case, a corresponding decoherence-reducing strategy is proposed.     

Dynamics and Recovery of Genuine Multipartite Einstein–Podolsky–Rosen Steering and Genuine Multipartite Nonlocality for a Dissipative Dirac System via the Unruh Effect

The dynamics behaviors of genuine multipartite Einstein–Podolsky–Rosen steering (GMS) and genuine multipartite nonlocality (GMN) are investigated herein, and how the lost GMS and GMN under a mixed decoherence system can be recovered is explored. Explicitly, the decoherence system can be modeled by that a tripartite Werner‐type state suffers from the non‐Markovian regimes and one subsystem of the tripartite is under a non‐inertial frame. The conditions for steerable and nonlocal states can be obtained with respect to the tripartite Werner‐type state established initially. GMS and GMN are very fragile and vulnerable under the influence of the collective decoherence. GMS and GMN will vanish with growing intensity of the Unruh effect and the non‐Markovian reservoir. Besides, all achievable GMN's states are steerable, while not every steerable state (GMS's state) can achieve nonlocality. This means that the steering–nonlocality hierarchy is still tenable and GMN's states are a strict subset of the GMS's states in such a scenario. Subsequently, an available methodology to recover the damaged GMS and GMN is proposed. It turns out that the lost GMS and GMN can be effectively restored, and the ability of GMS and GMN to suppress the collective decoherence can be enhanced.     

On the Decoherence of Macroscopic Bodies

We consider the field, either gravitational or electric, associated to a macroscopic source. Tracing over the field's degrees of freedom we show that the reduced density matrix diagonalizes on the position basis for macroscopic separations. The non diagonal reduced density matrix elements are quenched by a factor which is independent of the body being at rest or in motion. This may provide an explanation of the classical behavior of everyday objects not dissimilar to the one based on decoherence by environment. We discuss a few examples which indicate that the electric field even in the case of a totally neutral body is more effective, through a dipole contribution, than the gravitational field.     

Measurement of decoherence of electron waves and visualization of the quantum-classical transition

Controlled decoherence of free electrons due to Coulomb interaction with a truly macroscopic environment, the electron (and phonon) gas inside a semiconducting plate, is studied experimentally. The quantitative results are compared with different theoretical models. The experiment confirms the main features of the theory of decoherence and can be interpreted in terms of which-path information. In contrast to previous model experiments on decoherence, the obtained interferograms directly visualize the transition from quantum to classical.     

Effective size of certain macroscopic quantum superpositions

Several experiments and experimental proposals for the production of macroscopic superpositions naturally lead to states of the general form /phi(1)>( multiply sign in circle N)+/phi 2 >( multiply sign in circle N), where the number of subsystems N is very large, but the states of the individual subsystems have large overlap, // 2=1-epsilon 2. We propose two different methods for assigning an effective particle number to such states, using ideal Greenberger-Horne-Zeilinger states of the form /0>( multiply sign in circle n)+/1>( multiply sign in circle n) as a standard of comparison. The two methods are based on decoherence and on a distillation protocol, respectively. Both lead to an effective size n of the order of N epsilon 2.     

Suppression of decoherence in a wave packet via nonlinear resonance

We propose a simple approach for suppressing decoherence of a wave packet excited in an anharmonic oscillator. We show that when a resonant external field forces the oscillator to follow the driving force, motion around the resonant trajectory inside a stable resonant island can be made almost completely immune to the environment. As an example, we study suppression of decoherence due to coupling to thermally populated rotations in vibrational wave packets in a Na2 molecule.     

Stable macroscopic quantum superpositions

We study the stability of superpositions of macroscopically distinct quantum states under decoherence. We introduce a class of quantum states with entanglement features similar to Greenberger-Horne-Zeilinger (GHZ) states, but with an inherent stability against noise and decoherence. We show that in contrast to GHZ states, these so-called concatenated GHZ states remain multipartite entangled even for macroscopic numbers of particles and can be used for quantum metrology in noisy environments. We also propose a scalable experimental realization of these states using existing ion-trap setups.     

Study of decoherence with a generalized master equation

The behavior of macroscopic system is investigated with a generalized master equation. Making use of a coarse-grained position operator based on the SO(3,1), we can derive that the macroscopic nature of this operator induces decoherence leading to classical behavior.     

基于无消相干子空间的量子避错码设计

针对量子系统的联合消相干模型,可以找到一些不受消相干错误影响的系统状态,这种状态被称为相干保持态,所有相干保持态构成的空间就称为无消相干子空间(decoherencefreesubspace,缩写为DFS).利用DFS的特性可以实现自动容错的量子避错码.首先提出一种DFS的定义,并且以定理的形式证明其他DFS的定义都是等价的.然后给出了DFS的存在性定理.最后利用群论的方法设计一种构造DFS的简单方法 关键词： 相干保持态 无消相干子空间 量子避错码 容错量子计算