单个原子中的量子跳跃

量子力学试图描述一个激发态原子在退激发过程中发射光子的自发辐射的动态过程.通常,由于在测量发光时收集到的是许多原子的发光,这些引人注目的量子效应往往变得模糊了.但是,在测量单个原子的荧光时,可以观察到这种量子效应.在原子发生“量子跳跃”时,荧光的突然“开”和“关”就是最近观察到的这种效应的一个例子. 1985年,Cook和Kimble[1]发现.对于具有“V”型结构的三能级系统(即两个激发态可以退激发到同一个基态的情况),如果其中有一个激发态自发辐射寿命很短,而另一个激发态自发辐射寿命很长.则这样的系统非常适合用来研究原子的光…     

激光场缀饰的原子的自发辐射特性——谱的瞬时行为,Aulter-Townes双重线的选择性激发

本文从理论上研究了共振激光场缀饰的原子自发辐射谱随时间的演化.讨论了系统的初态“制备”及其对原子激发过程和自发辐射过程的影响,比较了全量子理论与半经典理论对激光场与原子共振相互作用的研究,当驱动场处于相干态时,它们之间可以建立等价关系,对自发辐射谱的时间特性及其观测和有关的一些物理概念进行了讨论.     

V形三能级原子的辐射压力和激光冷却

研究了压缩真空中V形三能级原子在单色行波场中的辐射压力和激光冷却. 从系统的哈密顿量出发,利用玻恩-马尔科夫近似,推导出了原子的光学布洛赫方程. 利用绝热近似,给出了平均辐射压力的表达式. 通过量子回归定理和爱因斯坦关系,给出了系统所达到的平衡温度的表达式. 讨论了量子相干强度、平均光子数和拉比频率对原子的自发辐射压力和冷却的最终温度的影响. 结果表明:低于多普勒极限的温度可以获得. 关键词： 压缩真空 自发辐射 量子干涉     

内禀退相干下薛定谔猫态光场与原子相互作用系统的场熵演化

通过求解系统的Milburn方程,研究了二能级原子与薛定谔猫态光场相互作用系统中的场熵演化特性.讨论了内禀退相干,光场强度,相干态间的相位角对场熵演化的影响.结果表明: 内禀退相干下,随着时间的演化,场熵振荡逐渐减弱,光场与原子的纠缠度逐渐趋于恒值.并且光场与原子的最大纠缠度值只取决光场强度和相干态间的相位角,与内禀退相干因子无关.光场强度较小时,奇相干态光场与原子的纠缠度最大;偶相干态光场与原子的纠缠度值为最小;Yurke-Stoler相干态光场与原子的纠缠度值介于两者之间. 当内禀退相干因子不变、光场强度较大时,分别处于Yurke-Stoler相干态、偶相干态和奇相干态的光场与原子的纠缠度值趋近于相同.     

内禀退相干下薛定谔猫态光场与原子相互作用系统的场熵演化

通过求解系统的Milburn方程,研究了二能级原子与薛定谔猫态光场相互作用系统中的场熵演化特性.讨论了内禀退相干,光场强度,相干态间的相位角对场熵演化的影响.结果表明: 内禀退相干下,随着时间的演化,场熵振荡逐渐减弱,光场与原子的纠缠度逐渐趋于恒值.并且光场与原子的最大纠缠度值只取决光场强度和相干态间的相位角,与内禀退相干因子无关.光场强度较小时,奇相干态光场与原子的纠缠度最大;偶相干态光场与原子的纠缠度值为最小;Yurke-Stoler相干态光场与原子的纠缠度值介于两者之间. 当内禀退相干因子不变、光场强度较大时,分别处于Yurke-Stoler相干态、偶相干态和奇相干态的光场与原子的纠缠度值趋近于相同.     

基于腔QED系统和绝热技术制备三粒子三维纠缠态

将三个原子分别囚禁于用光纤连接的三个光腔中,通过光学开关控制腔场间的相互作用,并利用绝热技术制备了三粒子三维纠缠态.在整个制备过程中,原子-光腔-光纤系统仅在暗态空间中演化,从而有效地抑制了原子的自发辐射、腔场的泄漏以及光纤损耗等消相干因素的影响.     

观察玻色-爱因斯坦凝聚体的晶化

对于一团密集的处于激发态的二能级原子气体,Robert Dicke在1954年预言了一个非凡的现象--超辐射[1]:当每个原子独立地自发辐射光子时,这团原子是一个非相干辐射源的聚集体;但在特定的条件下,原子可以通过它们所辐射的光子而相干地相互作用,从而自发地和集体地辐射出相干的并且高度偏振的光.     

低频强场作用下三维光子晶体中单个二能级原子的自发辐射

控制受激原子的自发辐射是当前量子光学领域的一个重要课题.现有的研究表明,原子不同跃迁通道之间的量子干涉导致了新的光学现象,如无粒子数反转光放大,光谱变化,自发辐射相消等.Evers和Keitel提出了一种非常有效的方法,相当大地降低了一个二能级系统中上能态布居数的衰减;在他们的方法中,一个相干的强的低频场作用于单个二能级原子,此场的频率低于原子跃迁的整个衰减宽度,产生了由上能级到下能级原子态的不同衰减通道,从而导致自发辐射的量子干涉,显著地抑制了自发辐射.另一方面:在光学和固体物理领域,光子晶体引起了人们很大的兴趣.     

多光子过程中相位退相干时两原子布局数反转

利用全量子理论的方法,研究了存在相位退相干时多光子T-C模型中两个二能级原子与二项式光场相互作用系统中两原子的布居数反转。讨论了相位退相干系数、二项式光场系数、最大光子数、跃迁光子数对原子布居数反转的影响。结果表明：相位退相干减少了原子布居数反转的振幅、破坏了原子的量子特性。改变跃迁光子数,可以改变原子间布居数反转演化周期及演化强度。当二项式光场的最大光子数增大时,原子布居差的崩塌-回复现象就会逐渐消失。相位退相干因子不变时, 二项式光场从相干态过渡到数态过程中,原子布居的振荡频率由大变小,周期性的崩塌与回复现象逐渐消失。     

多光子过程中相位退相干时两原子布局数反转

利用全量子理论的方法,研究了存在相位退相干时多光子T-C模型中两个二能级原子与二项式光场相互作用系统中两原子的布居数反转。讨论了相位退相干系数、二项式光场系数、最大光子数、跃迁光子数对原子布居数反转的影响。结果表明：相位退相干减少了原子布居数反转的振幅、破坏了原子的量子特性。改变跃迁光子数,可以改变原子间布居数反转演化周期及演化强度。当二项式光场的最大光子数增大时,原子布居差的崩塌-回复现象就会逐渐消失。相位退相干因子不变时, 二项式光场从相干态过渡到数态过程中,原子布居的振荡频率由大变小,周期性的崩塌与回复现象逐渐消失。     

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.     

基于绝热捷径的两原子相位门设计

基于绝热捷径技术,我们提出了一个通过设计共振脉冲,仅用一个操作步骤便实现两原子相位门的理论方案。我们讨论了原子自发辐射和腔中光子泄漏对方案的影响,数值结果表明当前方案对上面的两种耗散效应和试验参数的误差都具有鲁棒性。此外,本方案所需的演化时间非常短并且在整个演化过程中都不需要额外引入复杂的脉冲。因此,这个方案在当前实验条件下是比较容易实现的。     

Distributed quantum computing in decoherence-free subspace via adiabatic passage

A scheme is proposed to implement distributed quantum computation in decoherence-free subspaces (DFSs) via adiabatic passage. The logical single-qubit is encoded in two atoms trapped in a single-mode cavity and the cavities are connected by an optical fiber. Our scheme is immune from the decoherence due to dephasing in virtue of encoding scheme and the decoherence due to spontaneous emission from excited states as the system in our scheme evolves along a dark state. Furthermore, the decoherence due to photon decay is greatly suppressed since the fiber mode remains in a vacuum state and the populations of the cavities’ modes being excited can be negligible under certain condition. It is shown that the minimum fidelity of the resultant gate operation for an arbitrary input state could be over 0.97.     

Scheme for Controlled Dense Coding via Cavity Decay

A scheme for controlled dense coding via cavity decay is proposed. In the scheme, two degenerate ground states of six-level atoms are used as the storage qubits and the leaky photons act as flying qubits. The system is robust against atomic spontaneous emissions and decoherence of cavity field. And the successful probability is nearly 1 with quantum nondemolition parity detectors and photon detectors, The scheme may be realized based on current technologies.     

Scheme for Controlled Dense Coding via Cavity Decay

A scheme for controlled dense coding via cavity decay is proposed. In the scheme, two degenerate ground states of six-level atoms are used as the storage qubits and the leaky photons act as flying qubits. The system is robust against atomic spontaneous emissions and decoherence of cavity field. And the successful probability is nearly 1 with quantum nondemolition parity detectors and photon detectors. The scheme may be realized based on current technologies.     

Conditional quantum CNOT gate between two four-level atoms

We propose a scheme to implement a quantum controlled-NOT (CNOT) gate between two four-level atoms inside the detuned optical cavity. The system state is evolved inside the decoherence-free (DF) subspace through stimulated Raman processes, which yields the desired unitary evolution operation for the CNOT.Our scheme is immune to decoherence due to dissipation of cavity excitation and spontaneous emission from the excited atomic level.     

Preparation of W State with Superconducting Quantum-Interference Devices in a Cavity via Adiabatic Passage

We propose an alternative scheme to prepare W state by using superconducting quantum-interference devices （SQUIDs） coupled to a largely-detuned cavity. The present scheme is based on evolution by adiabatic passage, where only by tuning adiabatically the Rabi frequencies of the classical microwave pulses we can obtain the standard W state without measurement or any auxiliary SQUIDs. Thus the procedure is simplified and the scheme can be achieved with very high success probability since the errors in dynamical or geometric ways can be avoided. In addition, the SQUID system and the cavity have no probability of being excited state. Thus decoherence caused by the excited-level spontaneous emission or the cavity decay is suppressed.     

在腔QED系统中变换W态到超单态

提出了一步变换三原子W态为三原子超单态的方案。在方案中,三个五能级原子同时与双模腔发生离散相互作用。方案的优点是可以有效抵御原子自发辐射和腔衰变引起的消相干的影响。     

Implementation of Controlled‐NOT Gate by Lyapunov Control

A scheme to implement the controlled‐NOT (CNOT) gate for quantum systems is proposed, which is based on Lyapunov control. The scheme does not require precise control of the interaction time since the system is stable when the control fields vanish. In particular, the control fields can be easily obtained by most initial states. As an example, the CNOT gate is realized for two atoms trapped in an optical cavity by exploiting two disturbance cases. Compared to continuous disturbance, the fidelity of the CNOT gate is higher under impulsive disturbance, however, interaction times are much longer. Numerical simulations indicate that the scheme is robust against variations of control parameters and decoherence caused by atomic spontaneous emission and cavity decay. Therefore, the scheme may provide useful applications in quantum computation.