混合态下运动原子与相干态光场相互作用的量子纠缠

利用负熵方法,研究了混合态运动原子与相干态光场相互作用系统的量子纠缠特性,讨论了原子初态、场模结构参数、相干场平均光子数、失谐量、跃迁光子数等物理参量对系统纠缠度的影响。结果表明：考虑原子运动时,系统纠缠度在整个时域范围内出现了规则的周期振荡。原子初态趋于纯态时系统纠缠度较高。随着相干场平均光子数的增大,系统纠缠度的峰值逐渐变小,规则振荡的周期不变。随着跃迁光子数的增大,系统纠缠度的峰值逐渐变大,振荡变得越来越快。随着失谐量的增大,系统纠缠度的峰值逐渐变小。     

多光子跃迁下the Number-Chaotic态光场与运动二能级原子相互作用系统的保真度

利用全量子理论,研究了多光子跃迁下NCS(the number-chaotic state)态光场与运动二能级原子相互作用系统的保真度。讨论了原子的初始状态、数态光子数、热平均光子数、跃迁光子数、场模结构参数对系统保真度的影响。结果表明:场模结构参数影响系统量子态的振荡周期;热平均光子数影响系统保真度大小;原子初始状态、数态光子数及跃迁光子数选取不同,系统量子态的保真度表现出不同的规律。     

混合态下运动原子与相干态光场相互作用的量子纠缠

利用负熵方法,研究了混合态运动原子与相干态光场相互作用系统的量子纠缠特性,讨论了原子初态、场模结构参数、相干场平均光子数、失谐量、跃迁光子数等物理参量对系统纠缠度的影响。结果表明：考虑原子运动时,系统纠缠度在整个时域范围内出现了规则的周期振荡。原子初态趋于纯态时系统纠缠度较高。随着相干场平均光子数的增大,系统纠缠度的峰值逐渐变小,规则振荡的周期不变。随着跃迁光子数的增大,系统纠缠度的峰值逐渐变大,振荡变得越来越快。随着失谐量的增大,系统纠缠度的峰值逐渐变小。     

高斯型耦合多光子Tavis-Cummings模型中场与原子的纠缠特性

利用全量子理论,研究了多光子跃迁过程高斯型耦合Tavis-Cummings模型中场与原子的纠缠特性。讨论了多光子原子运动速度、光场初态的平均光子数及跃迁光子数对场与原子之间纠缠特性的影响。结果表明:原子运动速度的变化改变了场熵演化曲线的振荡时域;光场初态影响场熵演化曲线的周期性;跃迁光子数的增加使场熵演化曲线的振荡频率和场与原子之间的平均纠缠度变大。     

好腔中的Bell态原子与纠缠相干态光场相互作用系统的保真度

考虑原子间偶极相互作用,求出好腔中的Bell态原子与纠缠相干态光场相互作用系统的保真度.结果表明,对于理想腔,若原子初始时刻处于相干保持态,系统保真度始终等于1;若原子初始时刻处于其余Bell态之一.腔场初态的平均光子数很小,系统保真度在0～1之间作周期性振荡,随着腔场初态的平均光子数的增加,系统保真度的振荡频率增大,振幅减小.对于好腔,若原子初始时刻处于相干保持态,系统保真度呈指数单调衰减;若原子初始时刻处于其余Bell态之一,系统保真度呈指数振荡衰减,且随着腔场初态的平均光子数的增加,系统保真度的振荡频率增大,振幅减小.     

好腔中的Bell态原子与纠缠相干态光场相互作用系统的保真度

考虑原子间偶极相互作用,求出好腔中的Bell态原子与纠缠相干态光场相互作用系统的保真度。结果表明,对于理想腔,若原子初始时刻处于相干保持态,系统保真度始终等于1;若原子初始时刻处于其余Bell态之一,腔场初态的平均光子数很小,系统保真度在0～1之间作周期性振荡,随着腔场初态的平均光子数的增加,系统保真度的振荡频率增大,振幅减小。对于好腔,若原子初始时刻处于相干保持态,系统保真度呈指数单调衰减;若原子初始时刻处于其余Bell态之一,系统保真度呈指数振荡衰减,且随着腔场初态的平均光子数的增加,系统保真度的振荡频率增大,振幅减小。     

Pólya态光场与Bell态双原子相互作用系统的量子纠缠

利用全量子理论,研究了Bell态双原子与Pólya态光场相互作用系统的量子纠缠特性,分别讨论了原子初态、最大光子数M、光场参量p和r对系统场熵的影响。结果表明:当最大光子数M足够大时,随着光场参量p和r的逐渐增大,场熵振幅逐渐缩小,场熵演化曲线趋于平稳;与原子初态为|φA(0)〉的情况相比,原子初态为|ФA(0)〉时,系统的纠缠状态更容易受到光场参量p和r的影响。     

运动纠缠双原子与二项式光场相互作用的纠缠特性

本文主要利用量子熵理论,讨论运动纠缠双原子与二项式光场的量子纠缠特性随时间的演化规律,结果表明:光场与原子纠缠度依赖于场模结构参数、光场的调节参量以及光子数.场模结构参数改变,影响纠缠周期性的变化且纠缠度随其增大而降低;当场调节参数处于中间值时,场熵呈明显周期性变化,系统纠缠度最大,退纠缠持续时间最短;光子数的改变不影响场熵演化的周期性,但是会影响周期内振荡频率和纠缠度的最大值.     

运动纠缠双原子与二项式光场相互作用的纠缠特性

本文主要利用量子熵理论,讨论运动纠缠双原子与二项式光场的量子纠缠特性随时间的演化规律,结果表明:光场与原子纠缠度依赖于场模结构参数、光场的调节参量以及光子数.场模结构参数改变,影响纠缠周期性的变化且纠缠度随其增大而降低;当场调节参数处于中间值时,场熵呈明显周期性变化,系统纠缠度最大,退纠缠持续时间最短;光子数的改变不影响场熵演化的周期性,但是会影响周期内振荡频率和纠缠度的最大值.     

运动纠缠双原子与压缩相干态光场相互作用系统的场熵演化

利用全量子理论,研究了运动纠缠双原子与压缩相干态光场相互作用系统的场熵演化特性,讨论了相干态振幅参量、光场压缩参量、原子的运动和场模结构参数对系统场熵的影响.结果表明:(1)原子运动导致场熵演化具有周期性.(2)随着相干态振幅参量的增大,原子与光场的纠缠度逐渐减小;而随着压缩参量的增大,原子与光场的纠缠度逐渐增大.(3)随着原子运动速度的增大或场模结构参数的增大,场熵的演化周期缩短,并且场与原子的纠缠度逐渐减小.     

Detection of Incompatible Properties in a Double-Slit Experiment

In this work we show that, within the framework of double-slit experiment, it is possible to ascertain simultaneously more incompatible properties together with the measurement of the position of the final impact-point. A wide family of solutions is concretely found and an ideal experiment realizing such a detection is designed, relatively to the detection of two incompatible properties. In the case of three incompatible properties, general conditions for the existence of solutions are singled out and a particular family of solutions is provided.     

Quantum packet switching generation using correlated photons in a microring resonator system

We propose a new system of a packet of quantum bits generation using a soliton pulse within a microring resonator. A quantum gate can be formed by using a polarization control unit incorporating into the system. The random signal and idler pairs can be formed within the photon correlation bandwidth, which can be generated, and randomly form the packet quantum bits, i.e. quantum packet switching. Each random code (logic) can be performed by combining the signal and idler of each entangled photon pair via the quantum gate. Results obtained have shown that the packet of quantum logic bits can be generated using the entangled photon pairs generated by the proposed system.     

Nanoparticle in a quantum crystal with a narrow vacancy band

Vacancy-assisted diffusion of a neutral probe nanoparticle with a radius R_p of a few lattice constants in a quantum crystal with a narrow vacancy band is considered. The diffusion coefficient of the probe D_p(T) in such a crystal should fall exponentially near T_melt, and it can go through a maximum at temperatures T_tr, where the transition from thermally activated hopping of localized vacancies to a proper band motion of delocalized vacancions takes place, under the condition that the mean free path of the vacancions l_v(T) at T_tr is less than R_p and increases with lowering the temperature quicker than the inverse value of the relative concentration of vacancies X_v(T). Below T_tr, where l_v is much longer than the probe diameter, the value of D_p should fall proportionally to X_v(T).     

Computing with a single qubit faster than the computation quantum speed limit

The possibility to save and process information in fundamentally indistinguishable states is the quantum mechanical resource that is not encountered in classical computing. I demonstrate that, if energy constraints are imposed, this resource can be used to accelerate information-processing without relying on entanglement or any other type of quantum correlations. In fact, there are computational problems that can be solved much faster, in comparison to currently used classical schemes, by saving intermediate information in nonorthogonal states of just a single qubit. There are also error correction strategies that protect such computations.     

Performance comparison between classical and quantum control for a simple quantum system

Brańczyk et al. pointed out that the quantum control scheme is superior to the classical control scheme for a simple quantum system using simulation [A.M. Brańczyk, P.E.M.F. Mendonca, A. Gilchrist, A.C. Doherty, S.D. Barlett, Quantum control theory of a single qubit, Physical Review A 75 (2007) 012329 or arXiv e-print quant-ph/0608037]. Here we rigorously prove the result. Furthermore we will show that any quantum operation does not universally “correct” the dephasing noise.     

Towards a definition of the quantum ergodic hierarchy: Ergodicity and mixing

In a previous paper we have given a general framework for addressing the definition of quantum chaos by identifying the conditions that a quantum system must satisfy to lead to non-integrability in its classical limit. In this paper we will generalize those results, with the purpose of defining the two lower levels of the quantum ergodic hierarchy: ergodicity and mixing. We will also argue for the physical relevance of this approach by considering a particular example where our formalism has been successfully applied.     

Coherent control of quantum entropy via quantum interference in a four-level N-type atomic system

The time evaluation of quantum entropy in a four-level N-type atomic system is theoretically investigated. Quantum entanglement of the atom and its spontaneous emission fields is then discussed via quantum entropy. It is found that the degree of entanglement can be increased by the quantum interference induced by spontaneous emission. The phase dependence of the atom-field entanglement is also presented.     

Diagonalizations on a correlated basis

Unsymmetrical quantum-dot systems are generally difficult to study using wave-function techniques, like quantum Monte Carlo (QMC) or exact diagonalization (ED) methods. The initial trial wave function for Monte Carlo methods is difficult to find, and the exact diagonalization method can only handle very few particles.In this article a two-dimensional semiconductor quantum dot containing a non-centered impurity ion is studied, using a new exact wave-function method. Results are analyzed and compared to density-functional-theory calculations. The computational method allows one to relax the commonly used lowest-Landau level (LLL) approximation, and it's effects are studied, e.g., on the charge and current density profiles.The method, which is a combination of QMC and ED methods, is described. It combines the scalability of Monte Carlo methods with the benefits of exact diagonalization, and allows one to accurately obtain the wave function for unsymmetrical quantum dots up to more than ten electrons. Also, excited states are accessible and are analyzed in this article.     

Cryptanalysis and improvement of a secure quantum sealed-bid auction

The security of a secure quantum sealed-bid auction protocol using quantum secure direct communication [Mosayeb Naseri, Opt. Commun. 282 (2009) 1939] is analyzed. It is shown that this protocol is unfair as a malicious bidder can obtain others’ bids without being found, and then he can optimize his bid to win the auction. Finally, a simple improvement to resist this attack is proposed.     

Entanglement and the process of measuring the position of a quantum particle

We explore the entanglement-related features exhibited by the dynamics of a composite quantum system consisting of a particle and an apparatus (here referred to as the “pointer”) that measures the position of the particle. We consider measurements of finite duration, and also the limit case of instantaneous measurements. We investigate the time evolution of the quantum entanglement between the particle and the pointer, with special emphasis on the final entanglement associated with the limit case of an impulsive interaction. We consider entanglement indicators based on the expectation values of an appropriate family of observables, and also an entanglement measure computed on particular exact analytical solutions of the particle–pointer Schrödinger equation. The general behavior exhibited by the entanglement indicators is consistent with that shown by the entanglement measure evaluated on particular analytical solutions of the Schrödinger equation. In the limit of instantaneous measurements the system’s entanglement dynamics corresponds to that of an ideal quantum measurement process. On the contrary, we show that the entanglement evolution corresponding to measurements of finite duration departs in important ways from the behavior associated with ideal measurements. In particular, highly localized initial states of the particle lead to highly entangled final states of the particle–pointer system. This indicates that the above mentioned initial states, in spite of having an arbitrarily small position uncertainty, are not left unchanged by a finite-duration position measurement process.