测不准关系和最小不确定态

利用力学量算符的厄密性和希尔伯特状态矢量模的非负性，重新给出了关于测不准关系的数 学证明.简单的证明过程不仅揭示了测不准关系某些经常被忽视的特征，而且还可以直接给 出最小不确定态的充要条件.在此基础上，我们提出了对易子为非零常数的任意一对力学量 的最小不确定态问题，并且采用玻色型产生和湮没算符给出了它们的压缩态的明显表达式. 关键词： 海森堡测不准原理 最小不确定态 压缩态     

双光子过程中任意初态原子的信息熵压缩

运用量子信息熵理论研究了双光子过程中任意初态二能级原子与相干场相互作用的信息熵压缩，讨论了系统初态对原子信息熵压缩的影响. 并且比较了分别从基于信息熵不确定关系和海森伯不确定关系出发得出的结果. 结果表明通过选择适当初始的原子分布角，原子的混合度和相干场的位相角，可以分别控制原子信息熵压缩的偶极矩分量数、压缩频率、压缩幅度和压缩方向. 当原子反转为零时，基于海森堡不确定关系的方差压缩定义不再有效，而信息熵压缩实现了对原子压缩效应的高灵敏量度. 关键词： 二能级原子 双光子过程 信息熵压缩 方差压缩     

双模最小关联混合态作为量子信道实现量子隐形传态的保真度

基于Braunstein和Kimble提出的B-K方案以双模最小关联混合态作为量子信道实施对未知量子态的隐形传送，并以传送相干态为例进行了研究.结果表明：双模最小关联混合态作为一种广义的Einstein-Podolsky-Rosen型纠缠态在实现量子隐形传态中能很好地担当量子信道的角色，在纠缠度和压缩度选择适当的条件下被传送未知量子态的保真度可以达到1.这是比双模压缩真空态更优越的量子信道. 关键词： 量子隐形传态 双模最小关联混合态 保真度     

相干态的海森堡不确定关系验证实验

由于量子理论和实验的微观特性,导致海森堡不确定性原理的实验验证十分困难.针对这一问题,本文从光量子化入手,介绍了量子正交算符的海森堡不确定关系,通过平衡零拍技术探测了量子正交算符的方差,验证海森堡不确定性原理.实验结果表明:光场正交算符在相干态中具有最小不确定关系.通过该实验,学生在掌握海森堡不确定性原理的同时,了解量...     

波粒二象性和量子不确定性之间的关系:纯态到混合态

研究了波粒二象性和量子不确定性之间的关系,可见度表达的是粒子的波动性,可预测性(predictability)表达的是粒子性,还有它们的起伏,这些量可以通过一个表达式联系起来.单光子纯态和混合态的测量,在实验上[PRA,85,022106]已经得到证明,但并没有给出中间态的情况,在此重点研究了纯态和混合态之间的状态,测量结果仍满足可见度、可预测性和它们的起伏的关系.     

运动原子与光场依赖强度纠缠下最佳熵压缩态的制备和控制

运用量子信息熵理论,研究了运动二能级原子与光场依赖强度纠缠下最佳熵压缩态的制备和控制;比较了分别从基于信息熵不确定关系和海森堡不确定关系出发得出的结果;分析了制备原子最佳熵压缩态的充要条件,并进行了数值验证.考察了场模结构参数对最佳熵压缩态的影响.结果表明,信息熵压缩是对原子压缩效应的高灵敏量度;控制原子与场的相互作用时间,斩断原子和场的纠缠,选择原子的相干性,调节系统的相对位相可制备原子最佳熵压缩态;控制场模结构参数,可获得持续的原子最佳熵压缩态. 关键词： 依赖强度耦合 场模结构参数 最佳熵压缩态     

热场动力学理论中的Husimi分布函数及Wehrl熵的研究

利用量子相空间技术和信息熵理论,研究了热场动力学理论中量子纯态与相应混合态的Husimi分布函数及Wehrl熵的一致性问题.结果表明,热相干态与相应混合态的Husimi分布函数及Wehrl熵完全相同,支持了热场动力学理论.且热相干态的Wehrl熵与平移因子无关,故在热相干态中,量子系统的可观测量的量子涨落及不确定关系也与平移因子无关.     

类氢原子核质量对电子状态的影响

在孤立的两体复合系统中,讨论其中一体的变化如何影响另一体的状态,有助于了解单粒子混合态与纯态的关系.本文讨论5个孤立的一维类氢原子模型系统,原子核的质量互不相同.这5个两体(电子与原子核)复合系统的相对运动状态都处于纠缠态,其中电子状态都用约化密度矩阵表示的混合态描述.在原子核质量趋近无穷大的一维氢原子模型中,电子处于纯态.为比较这里的纯态和混合态,在位置表象中计算了这些混合态的纯度、它们分别与纯态的保真度、以及所有这些态的相干性.研究表明,原子核的质量越大,纯度和保真度越接近1,混合态的相干性与纯态的也越接近.这样的纯态及其相干性可以是这种混合态及其相干性的近似,并与原子核及库仑相互作用有关.     

双模纠缠相干场控制下的双光子过程中有效二能级原子的信息熵压缩

根据量子信息理论,研究了与双模纠缠相干场相互作用双光子过程中有效二能级原子的信息熵压缩。讨论了场的初态对信息熵压缩的因影响,并且比较了分别从基于信息熵测不准关系和海森堡测不准关系出发得出的结果。结果表明：信息熵压缩的稳定周期性、压缩方向分别由场的平均光子数和方向角控制. 在强场条件下, 场的纠缠度对信息熵压缩不产生影响。当原子反转为零时,基于海森堡测不准关系的方差压缩定义不再有效, 而信息熵压缩实现了对原子压缩效应的高灵敏量度.     

基于双模压缩信道的双模高斯态量子隐形传态

研究了双模高斯态的两个模经由两个双模压缩量子信道的隐形传态.结果表明，当输入态有纠缠时，若要输出态的纠缠不为零，量子信道的纠缠必须大于一确定值，其大小在输入态为纯态时依赖于输入态的纠缠度；在输入态为混合态时不仅与输入态的纠缠有关，还依赖于输入态的整体纯度. 关键词： 量子隐形传态 量子纠缠 双模高斯态     

Quantum uncertainty relations of two generalized quantum relative entropies of coherence

In this paper, we consider the quantum uncertainty relations of two generalized relative entropies of coherence based on two measurement bases. First, we give quantum uncertainty relations for pure states in a d-dimensional quantum system by making use of the majorization technique; these uncertainty relations are then generalized to mixed states. We find that the lower bounds are always nonnegative for pure states but may be negative for some mixed states. Second, the quantum uncertainty relations for single qubit states are obtained by the analytical method. We show that the lower bounds obtained by this technique are always positive for single qubit states. Third, the lower bounds obtained by the two methods described above are compared for single qubit states.     

Gaussian states as minimum uncertainty states

In bosonic fields, Gaussian states, which consist of a rather wide family of states including coherent states, squeezed states, thermal states, etc., have many classical-like features, and are usually defined from the mathematical perspective in terms of characteristic functions. It is well known that some special Gaussian states, such as coherent states, are minimum uncertainty states for the conventional Heisenberg uncertainty relation involving canonical pair of position and momentum observables. A natural question arises as whether all Gaussian states can be characterized as minimum uncertainty states. In this work, we show that indeed Gaussian states coincide with minimum uncertainty states for an information-theoretic refinement of the conventional uncertainty relation established in Luo (2005) [40]. This characterization puts Gaussian states on a novel basis of physical significance.     

Uncertainty relation for the discrete Fourier transform

We derive an uncertainty relation for two unitary operators which obey a commutation relation of the form UV=e(i phi) VU. Its most important application is to constrain how much a quantum state can be localized simultaneously in two mutually unbiased bases related by a discrete fourier transform. It provides an uncertainty relation which smoothly interpolates between the well-known cases of the Pauli operators in two dimensions and the continuous variables position and momentum. This work also provides an uncertainty relation for modular variables, and could find applications in signal processing. In the finite dimensional case the minimum uncertainty states, discrete analogues of coherent and squeezed states, are minimum energy solutions of Harper's equation, a discrete version of the harmonic oscillator equation.     

Entanglement evolution of three-qubit mixed states in multipartite cavity—reservoir systems

We analyze the multipartite entanglement evolution of three-qubit mixed states composed of a GHZ state and a W state.For a composite system consisting of three cavities interacting with independent reservoirs,it is shown that the entanglement evolution is restricted by a set of monogamy relations.Furthermore,as quantified by the negativity,the entanglement dynamical property of the mixed entangled state of cavity photons is investigated.It is found that the three cavity photons can exhibit the phenomenon of entanglement sudden death(ESD).However,compared with the evolution of a generalized three-qubit GHZ state which has the equal initial entanglement,the ESD time of mixed states is later than that of the pure state.Finally,we discuss the entanglement distribution in the multipartite system,and point out the intrinsic relation between the ESD of cavity photons and the entanglement sudden birth of reservoirs.     

Is quantum gravity deterministic and/or time symmetric?

S. W. Hawking suggests that quantum gravity introduces a new level of uncertainty into physics by turning pure states into mixed states. Although the evidence for this information loss is based upon a semiclassical approximation and hence is not conclusive, it is interesting to examine the implications. As originally formulated, Hawking's proposal violates CPT invariance by singling out one direction of time in which pure states turn into mixed states. An alternate hypothesis is suggested whereby the theory could be time symmetric and yet allow a loss of information. In this model the universe as a whole would be an open system, and even its density matrix would not have a deterministic evolution. The question remains of how much uncertainty there actually is in quantum gravity.     

Dynamical correlations of negativity and entropy for pure and mixed states in two coupled quartic oscillators

Various measures of entanglement have triggered considerable interest in the relationship between entanglement measures and other well-known quantities. As a demonstration, the dynamical correlation of negativity and entropy is studied in two coupled quartic oscillators for initial pure and mixed states that are respectively taken to be the products and mixed density matrices of coherent states and squeezed states on each oscillator. The correlation with energy is also considered. It is shown that for the initial pure states with a small magnitude, two negativities are positively correlated with the von Neumann entropy while they are anti-correlated with the energy of each oscillator in the weak coupling regime. For mixed states with a small magnitude the two negativities and the mutual entropy exhibit dominantly positive correlation, whereas those three quantities are dominantly anti-correlated with the sum of energies of two oscillators in the case of weak interactions. Such correlation behaviors in the mixed state with small magnitudes are most striking at the same step in maximal and minimal values and in oscillation. The differences in entropies and negativities between coherent states and squeezed states are discussed. These are useful for quantum entanglement and quantum information processing.     

A rigorous uncertainty relation (Cauchy Inequality) for an electromagnetic field in quantum superpositions of coherent states and in a two-photon coherent state

It is shown that the Heisenberg uncertainty relation (or soft uncertainty relation) determined by the commutation properties of operators of electromagnetic field quadratures differs significantly from the Robertson–Schrödinger uncertainty relation (or rigorous uncertainty relation) determined by the quantum correlation properties of field quadratures. In the case of field quantum states, for which mutually noncommuting field operators are quantum-statistically independent or their quantum central correlation moment is zero, the rigorous uncertainty relation makes it possible to measure simultaneously and exactly the observables corresponding to both operators or measure exactly the observable of one of the operators at a finite measurement uncertainty for the other observable. The significant difference between the rigorous and soft uncertainty relations for quantum superpositions of coherent states and the two-photon coherent state of electromagnetic field (which is a state with minimum uncertainty, according to the rigorous uncertainty relation) is analyzed.     

Generalized measures of quantum correlations for mixed states

The exponential speedup achieved in certain quantum algorithms based on mixed states with negligible entanglement has renewed the interest on alternative measures of quantum correlations. Here we discuss a general measure of quantum correlations for composite systems based on generalized entropic functions, defined as the minimum information loss due to a local measurement. For pure states, the present measure becomes an entanglement entropy, i.e., it reduces to the generalized entropy of the reduced state. However, for mixed states it can be nonzero in separable states, vanishing just for states diagonal in a general product basis, like the quantum discord. Quadratic measures of quantum correlations can be derived as particular cases of the present formalism. The minimum information loss due to a joint local measurement is also considered. The evaluation of these measures in a simple yet relevant case is also discussed.