2个原子与2个耗散腔场量子体系中的量子退相干

应用J-C模型与相互作绘景中的密度算符理论,研究了2个相互纠缠的理想腔体中2个二能级Rydberg原子与2个纠缠耗散腔场单光子共振相互作用过程中的量子退相干,得到了2个二能级原子的退相干因子.通过对数值计算,讨论了耗散系数和原子-光场相互作用耦合系数对原子态的量子相干性的演化特性的影响.结果表明,耗散系数和原子-光场相互作用强度不仅影响原子态的量子相干性的演化的振荡性,而且影响其演化的周期性.     

Orbital excitation blockade and algorithmic cooling in quantum gases

Interaction blockade occurs when strong interactions in a confined, few-body system prevent a particle from occupying an otherwise accessible quantum state. Blockade phenomena reveal the underlying granular nature of quantum systems and allow for the detection and manipulation of the constituent particles, be they electrons, spins, atoms or photons. Applications include single-electron transistors based on electronic Coulomb blockade and quantum logic gates in Rydberg atoms. Here we report a form of interaction blockade that occurs when transferring ultracold atoms between orbitals in an optical lattice. We call this orbital excitation blockade (OEB). In this system, atoms at the same lattice site undergo coherent collisions described by a contact interaction whose strength depends strongly on the orbital wavefunctions of the atoms. We induce coherent orbital excitations by modulating the lattice depth, and observe staircase-like excitation behaviour as we cross the interaction-split resonances by tuning the modulation frequency. As an application of OEB, we demonstrate algorithmic cooling of quantum gases: a sequence of reversible OEB-based quantum operations isolates the entropy in one part of the system and then an irreversible step removes the entropy from the gas. This technique may make it possible to cool quantum gases to have the ultralow entropies required for quantum simulation of strongly correlated electron systems. In addition, the close analogy between OEB and dipole blockade in Rydberg atoms provides a plan for the implementation of two-quantum-bit gates in a quantum computing architecture with natural scalability.     

超高斯啁啾脉冲中里德堡钾原子的相干激发

采用含时多态展开方法(TDML),对超高斯频率啁啾的激光场中的里德堡钾原子布居数相干迁移进行了计算研究,结果表明:在合适的激光参数下,采用超高斯啁啾脉冲可以实现布居数在量子态间的有效跃迁.通过对该体系动态Rabi频率的分析,得出:实现布居数囚禁的关键不在于具体Rabi频率的大小,而在于实现Rabi频率的适时跳转.     

蒙特卡罗波函数方法模拟Rydberg原子系统的集体跃迁

波函数蒙特卡罗(wave function monte carlo,WFMC)方法,是一种广泛应用于研究量子开放系统的新方法,它与标准的主方程是等价的。将详细介绍了WFMC算法的过程,以及其与主方程的等价性证明和该算法的不足与改进,并针对由Rydberg原子组成的1维(1D)量子系统,运用WFMC方法对该系统的动力学特性进行模拟。研究结果显示,1D Rydberg系统由于原子间dipole-dipole相互作用而会出现集体激发行为。     

Strong atom-field coupling for Bose-Einstein condensates in an optical cavity on a chip

An optical cavity enhances the interaction between atoms and light, and the rate of coherent atom-photon coupling can be made larger than all decoherence rates of the system. For single atoms, this 'strong coupling regime' of cavity quantum electrodynamics has been the subject of many experimental advances. Efforts have been made to control the coupling rate by trapping the atom and cooling it towards the motional ground state; the latter has been achieved in one dimension so far. For systems of many atoms, the three-dimensional ground state of motion is routinely achieved in atomic Bose-Einstein condensates (BECs). Although experiments combining BECs and optical cavities have been reported recently, coupling BECs to cavities that are in the strong-coupling regime for single atoms has remained an elusive goal. Here we report such an experiment, made possible by combining a fibre-based cavity with atom-chip technology. This enables single-atom cavity quantum electrodynamics experiments with a simplified set-up and realizes the situation of many atoms in a cavity, each of which is identically and strongly coupled to the cavity mode. Moreover, the BEC can be positioned deterministically anywhere within the cavity and localized entirely within a single antinode of the standing-wave cavity field; we demonstrate that this gives rise to a controlled, tunable coupling rate. We study the heating rate caused by a cavity transmission measurement as a function of the coupling rate and find no measurable heating for strongly coupled BECs. The spectrum of the coupled atoms-cavity system, which we map out over a wide range of atom numbers and cavity-atom detunings, shows vacuum Rabi splittings exceeding 20 gigahertz, as well as an unpredicted additional splitting, which we attribute to the atomic hyperfine structure. We anticipate that the system will be suitable as a light-matter quantum interface for quantum information.     

Single-spin addressing in an atomic Mott insulator

Ultracold atoms in optical lattices provide a versatile tool with which to investigate fundamental properties of quantum many-body systems. In particular, the high degree of control of experimental parameters has allowed the study of many interesting phenomena, such as quantum phase transitions and quantum spin dynamics. Here we demonstrate how such control can be implemented at the most fundamental level of a single spin at a specific site of an optical lattice. Using a tightly focused laser beam together with a microwave field, we were able to flip the spin of individual atoms in a Mott insulator with sub-diffraction-limited resolution, well below the lattice spacing. The Mott insulator provided us with a large two-dimensional array of perfectly arranged atoms, in which we created arbitrary spin patterns by sequentially addressing selected lattice sites after freezing out the atom distribution. We directly monitored the tunnelling quantum dynamics of single atoms in the lattice prepared along a single line, and observed that our addressing scheme leaves the atoms in the motional ground state. The results should enable studies of entropy transport and the quantum dynamics of spin impurities, the implementation of novel cooling schemes, and the engineering of quantum many-body phases and various quantum information processing applications.     

双光子J-C模型中原子与光场的纠缠及纠缠度

该文提出了在双光子J-C模型中利用二能级原子与单模场的相互作用，制备出原子与光场的纠缠态．随着时间的演化，原子与光场组成体系的纠缠度呈现出周期性变化规律．研究结果表明：在失谐量等于拉比频率时，该体系的纠缠度将长时间处于最大纠缠态。     

Quantum oscillations in a molecular magnet

The term 'molecular magnet' generally refers to a molecular entity containing several magnetic ions whose coupled spins generate a collective spin, S (ref. 1). Such complex multi-spin systems provide attractive targets for the study of quantum effects at the mesoscopic scale. In these molecules, the large energy barriers between collective spin states can be crossed by thermal activation or quantum tunnelling, depending on the temperature or an applied magnetic field. There is the hope that these mesoscopic spin states can be harnessed for the realization of quantum bits--'qubits', the basic building blocks of a quantum computer--based on molecular magnets. But strong decoherence must be overcome if the envisaged applications are to become practical. Here we report the observation and analysis of Rabi oscillations (quantum oscillations resulting from the coherent absorption and emission of photons driven by an electromagnetic wave) of a molecular magnet in a hybrid system, in which discrete and well-separated magnetic clusters are embedded in a self-organized non-magnetic environment. Each cluster contains 15 antiferromagnetically coupled S = 1/2 spins, leading to an S = 1/2 collective ground state. When this system is placed into a resonant cavity, the microwave field induces oscillatory transitions between the ground and excited collective spin states, indicative of long-lived quantum coherence. The present observation of quantum oscillations suggests that low-dimension self-organized qubit networks having coherence times of the order of 100 micros (at liquid helium temperatures) are a realistic prospect.     

Experimental long-lived entanglement of two macroscopic objects

Entanglement is considered to be one of the most profound features of quantum mechanics. An entangled state of a system consisting of two subsystems cannot be described as a product of the quantum states of the two subsystems. In this sense, the entangled system is considered inseparable and non-local. It is generally believed that entanglement is usually manifest in systems consisting of a small number of microscopic particles. Here we demonstrate experimentally the entanglement of two macroscopic objects, each consisting of a caesium gas sample containing about 1012 atoms. Entanglement is generated via interaction of the samples with a pulse of light, which performs a non-local Bell measurement on the collective spins of the samples. The entangled spin-state can be maintained for 0.5 milliseconds. Besides being of fundamental interest, we expect the robust and long-lived entanglement of material objects demonstrated here to be useful in quantum information processing, including teleportation of quantum states of matter and quantum memory.     

Experimental demonstration of quantum memory for light

The information carrier of today's communications, a weak pulse of light, is an intrinsically quantum object. As a consequence, complete information about the pulse cannot be perfectly recorded in a classical memory, even in principle. In the field of quantum information, this has led to the long-standing challenge of how to achieve a high-fidelity transfer of an independently prepared quantum state of light onto an atomic quantum state. Here we propose and experimentally demonstrate a protocol for such a quantum memory based on atomic ensembles. Recording of an externally provided quantum state of light onto the atomic quantum memory is achieved with 70 per cent fidelity, significantly higher than the limit for classical recording. Quantum storage of light is achieved in three steps: first, interaction of the input pulse and an entangling field with spin-polarized caesium atoms; second, subsequent measurement of the transmitted light; and third, feedback onto the atoms using a radio-frequency magnetic pulse conditioned on the measurement result. The density of recorded states is 33 per cent higher than the best classical recording of light onto atoms, with a quantum memory lifetime of up to 4 milliseconds.     

非简并双光子Jaynes—Cummings模型的量子塌陷与复苏

根据辐射的量子统计理论,讨论了理想非简并双光子Jaynes-Cummings模型的原子算子动力学行为,发现了原子布居数反转在时间演化过程中Rabi振荡的量子塌陷与复苏现象。该现象的窄波包形式、低对称性及短周期,是低简并度多光子转变条件下原子与辐射场作用的突出特征。     

Quantum simulation of ferrocytochrome c

The dramatic progress in the understanding of the dynamics of biomolecules has been largely fuelled by computer simulations based on the law of classical mechanics. However in some respects biomolecules are at the borders of the domain of applicability of classical mechanics. The role of quantum mechanical effects in biomolecular structure and function is therefore worth investigating. Here we present preliminary results from a quantum simulation of a protein and contrast them with results from full classical simulations. The most significant differences are found in motions of high frequency, such as bond stretching or the torsional oscillation of groups that bear hydrogen atoms. The amplitudes of such motions are significantly increased by the penetration of atoms into classically forbidden regions. These differences will directly influence the rates of such processes as proton and electron transfer.     

基于腔QED系统的量子相位门设计

提出一个基于腔QED系统实现量子相位门的设计方案.结果表明:在两个全同的二能级原子与光场相互作用过程中,借助经典场对原子实施Rabi翻转,控制原子射入腔场的速度v,选择原子与场相互作用参数l0,n和κ,可实现量子相位门.     

基于原子系综系统实现Controlled-Hadamard 门和制备多量子比特的W-type态

提出了利用Rydberg阻滞机制,实现基于原子系综系统的控制Hadamard门和制备n个量子比特W-type态的方案.在该方案中,选取Rydberg态为激发态,其是1个辅助态,2个基态作为编码态. 由于控制Hadamard 门实现的操作时间比激发态的寿命小得多,因此,激发态的衰减效应可以忽略不计. 该方案可以得到高保真度的n个量子比特W-type态,且该方案在量子信息处理方面具有可扩展性.     

Quantum jumps of light recording the birth and death of a photon in a cavity

A microscopic quantum system under continuous observation exhibits at random times sudden jumps between its states. The detection of this quantum feature requires a quantum non-demolition (QND) measurement repeated many times during the system's evolution. Whereas quantum jumps of trapped massive particles (electrons, ions or molecules) have been observed, this has proved more challenging for light quanta. Standard photodetectors absorb light and are thus unable to detect the same photon twice. It is therefore necessary to use a transparent counter that can 'see' photons without destroying them. Moreover, the light needs to be stored for durations much longer than the QND detection time. Here we report an experiment in which we fulfil these challenging conditions and observe quantum jumps in the photon number. Microwave photons are stored in a superconducting cavity for times up to half a second, and are repeatedly probed by a stream of non-absorbing atoms. An atom interferometer measures the atomic dipole phase shift induced by the non-resonant cavity field, so that the final atom state reveals directly the presence of a single photon in the cavity. Sequences of hundreds of atoms, highly correlated in the same state, are interrupted by sudden state switchings. These telegraphic signals record the birth, life and death of individual photons. Applying a similar QND procedure to mesoscopic fields with tens of photons should open new perspectives for the exploration of the quantum-to-classical boundary.     

飞秒激光参数对三态梯度型分子态布居数的影响

 利用含时波包法研究并且首次量化了强飞秒泵浦-探测激光场中泵浦场强、泵浦波长和脉宽对三态梯度型K₂分子态布居数的影响。泵浦场强、泵浦波长和脉宽影响Rabi振荡,而Rabi振荡的变化又导致了基态和激发态布居数周期性变化。量化这3个激光场参数对激发态布居数的影响验证了此周期性变化规律,表明变化频率随着泵浦场强和脉宽的增大而变化。研究表明调节激光场参数可实现对态布居数的选择性分布,可以为实验上实现分子的光控制提供重要参考。     

一价铝离子3sns(J=0)系列的高激发态结构

本文利用本征通道量子亏损理论方法计算了一价铝离子3sns(J=0)里德伯系列的激发态结构,给出了高激发态结构的具体数值结果,并讨论了干状态的影响.     

Stationary entanglement of two atoms in a common reservoir

We study the dynamics of two entangled atoms interacting with a common structured reservoir. By means of the exact solution of atomic dynamics, we show a novel quantum interference controlled by the relative phase of initial entangled state of the atoms. The quantum interference has a great influence on trapped excited-state population and stationary entanglement of the atoms. In particular, we construct an explicit condition under which atomic stationary entanglement can grow over their initial value.     

基于系综金刚石氮空位色心量子调控系统设计

为了实现量子的自旋调控和精密测量,将金刚石作为自旋载体材料,设计了基于系综金刚石氮空位（Nitrogen-vacancy, NV）色心量子调控系统。通过利用金刚石独特的自旋三重态容易被初始化,操控和读出,基于LabVIEW软件,设计编写了脉冲序列发生模块并搭建了共聚焦系统,调控了系综NV色心的自旋量子态。结果表明：该系统可以调控系综NV色心的自旋量子态;实现了NV色心拉比振荡实验的测量;拉比振荡周期为100ns。可见该系统结构设计简单,为下一步延长退相干时间,提高系统灵敏度打下基础。     

和频过程中的Rabi振荡与Landau-Zener遂穿过程

依据宏观现象与量子物理现象之间的相类似之处,利用干涉原理解释了和频场强度随着传输距离发生类似于原子系统中的Rabi振荡的物理机理,从Landau—Zener遂穿（LZT）N角度揭示了光学参量转换过程中的能量转移机理,以渐进解、解析解的形式详细展示出和频产生过程中的能量转移过程．