Transport and Anderson localization in disordered two-dimensional photonic lattices

One of the most interesting phenomena in solid-state physics is Anderson localization, which predicts that an electron may become immobile when placed in a disordered lattice. The origin of localization is interference between multiple scatterings of the electron by random defects in the potential, altering the eigenmodes from being extended (Bloch waves) to exponentially localized. As a result, the material is transformed from a conductor to an insulator. Anderson's work dates back to 1958, yet strong localization has never been observed in atomic crystals, because localization occurs only if the potential (the periodic lattice and the fluctuations superimposed on it) is time-independent. However, in atomic crystals important deviations from the Anderson model always occur, because of thermally excited phonons and electron-electron interactions. Realizing that Anderson localization is a wave phenomenon relying on interference, these concepts were extended to optics. Indeed, both weak and strong localization effects were experimentally demonstrated, traditionally by studying the transmission properties of randomly distributed optical scatterers (typically suspensions or powders of dielectric materials). However, in these studies the potential was fully random, rather than being 'frozen' fluctuations on a periodic potential, as the Anderson model assumes. Here we report the experimental observation of Anderson localization in a perturbed periodic potential: the transverse localization of light caused by random fluctuations on a two-dimensional photonic lattice. We demonstrate how ballistic transport becomes diffusive in the presence of disorder, and that crossover to Anderson localization occurs at a higher level of disorder. Finally, we study how nonlinearities affect Anderson localization. As Anderson localization is a universal phenomenon, the ideas presented here could also be implemented in other systems (for example, matter waves), thereby making it feasible to explore experimentally long-sought fundamental concepts, and bringing up a variety of intriguing questions related to the interplay between disorder and nonlinearity.     

Direct observation of Anderson localization of matter waves in a controlled disorder

In 1958, Anderson predicted the localization of electronic wavefunctions in disordered crystals and the resulting absence of diffusion. It is now recognized that Anderson localization is ubiquitous in wave physics because it originates from the interference between multiple scattering paths. Experimentally, localization has been reported for light waves, microwaves, sound waves and electron gases. However, there has been no direct observation of exponential spatial localization of matter waves of any type. Here we observe exponential localization of a Bose-Einstein condensate released into a one-dimensional waveguide in the presence of a controlled disorder created by laser speckle. We operate in a regime of pure Anderson localization, that is, with weak disorder-such that localization results from many quantum reflections of low amplitude-and an atomic density low enough to render interactions negligible. We directly image the atomic density profiles as a function of time, and find that weak disorder can stop the expansion and lead to the formation of a stationary, exponentially localized wavefunction-a direct signature of Anderson localization. We extract the localization length by fitting the exponential wings of the profiles, and compare it to theoretical calculations. The power spectrum of the one-dimensional speckle potentials has a high spatial frequency cutoff, causing exponential localization to occur only when the de Broglie wavelengths of the atoms in the expanding condensate are greater than an effective mobility edge corresponding to that cutoff. In the opposite case, we find that the density profiles decay algebraically, as predicted in ref. 13. The method presented here can be extended to localization of atomic quantum gases in higher dimensions, and with controlled interactions.     

光子的旋量波动方程

给出自由和非自由光子的旋量波动方程及光子自旋算符与自旋波函数.通过计算光子的螺旋度,证明存在左旋和右旋光子.由单光子自旋波函数得到两光子或多光子的自旋波函数,并给出多光子的自旋纠缠态.     

光速:不变,又变

提出了“波运动假说” ;运用“波运动守恒律”证明了物体质量对速度的依赖关系m =m0 / [1- (v2 /c2 ) ]1/2 ,并根据这个关系证明了任两个有静止质量的物体之间的相对速度必须小于作为常量的绝对速限c ;推论光子的静止质量并不为 0而为无穷小 ,光子的速度并不绝对等于c而是无限逼进c ,不同光子的速度并不完全相等 ;指出光子频率的红移或蓝移正是以另一种量纲 (超量纲 )的形式表现出的光子的速度的变化 ;认为相对论实质上是引入了波运动效应的牛顿力学     

Anderson localization of a non-interacting Bose-Einstein condensate

Anderson localization of waves in disordered media was originally predicted fifty years ago, in the context of transport of electrons in crystals. The phenomenon is much more general and has been observed in a variety of systems, including light waves. However, Anderson localization has not been observed directly for matter waves. Owing to the high degree of control over most of the system parameters (in particular the interaction strength), ultracold atoms offer opportunities for the study of disorder-induced localization. Here we use a non-interacting Bose-Einstein condensate to study Anderson localization. The experiment is performed with a one-dimensional quasi-periodic lattice-a system that features a crossover between extended and exponentially localized states, as in the case of purely random disorder in higher dimensions. Localization is clearly demonstrated through investigations of the transport properties and spatial and momentum distributions. We characterize the crossover, finding that the critical disorder strength scales with the tunnelling energy of the atoms in the lattice. This controllable system may be used to investigate the interplay of disorder and interaction (ref. 7 and references therein), and to explore exotic quantum phases.     

Photon blockade in an optical cavity with one trapped atom

At low temperatures, sufficiently small metallic and semiconductor devices exhibit the 'Coulomb blockade' effect, in which charge transport through the device occurs on an electron-by-electron basis. For example, a single electron on a metallic island can block the flow of another electron if the charging energy of the island greatly exceeds the thermal energy. The analogous effect of 'photon blockade' has been proposed for the transport of light through an optical system; this involves photon-photon interactions in a nonlinear optical cavity. Here we report observations of photon blockade for the light transmitted by an optical cavity containing one trapped atom, in the regime of strong atom-cavity coupling. Excitation of the atom-cavity system by a first photon blocks the transmission of a second photon, thereby converting an incident poissonian stream of photons into a sub-poissonian, anti-bunched stream. This is confirmed by measurements of the photon statistics of the transmitted field. Our observations of photon blockade represent an advance over traditional nonlinear optics and laser physics, into a regime with dynamical processes involving atoms and photons taken one-by-one.     

Observation of the radiative decay mode of the free neutron

The theory of quantum electrodynamics (QED) predicts that beta decay of the neutron into a proton, electron and antineutrino should be accompanied by a continuous spectrum of soft photons. While this inner bremsstrahlung branch has been previously measured in nuclear beta and electron capture decay, it has never been observed in free neutron decay. Recently, the photon energy spectrum and branching ratio for neutron radiative decay have been calculated using two approaches: a standard QED framework and heavy baryon chiral perturbation theory (an effective theory of hadrons based on the symmetries of quantum chromodynamics). The QED calculation treats the nucleons as point-like, whereas the latter approach includes the effect of nucleon structure in a systematic way. Here we observe the radiative decay mode of free neutrons, measuring photons in coincidence with both the emitted electron and proton. We determined a branching ratio of (3.13 +/- 0.34) x 10(-3) (68 per cent level of confidence) in the energy region between 15 and 340 keV, where the uncertainty is dominated by systematic effects. The value is consistent with the predictions of both theoretical approaches; the characteristic energy spectrum of the radiated photons, which differs from the uncorrelated background spectrum, is also consistent with the calculated spectrum. This result may provide opportunities for more detailed investigations of the weak interaction processes involved in neutron beta decay.     

光子的质量与静电场中的慢光子

先前的研究已经说明,根据至今在高频率范围内对光速的测量结果,既不能证明真空中光速没有色散,也不能够对光子有质量与否做出判断,因为光子质量引起光速的色散只会集中地发生在频率逼近于零的长波光子辐射的小频率范围之内.所以,为寻找光子质量引起的光速色散,本文关注于静电场的研究.按照波粒二象性,必须放弃"静电场的频率等于零"的观念;静电场应被视为长波光子的辐射场.由光子质量引起的光速色散就发生在静电场的小频率范围内,静电场的传播速度就是频率最低端的光速.计算表明,若光子有质量,则静电场中存在着可观数量的慢光子,它们的速度与极限速度c相比大约低3%到10%.为证明光子质量引起光速的色散,应该测量并分解静电场的传播速度.     

光超导波中正常态声子行为研究:声子对束缚态

考虑了光超导波理论中光声相互作用的高阶项，研究了声子与双光子的相互作用，利用推广的光子库柏对算符，在平均场近似下得出两动量相反的横光学声子可以通过交换虚光子库柏对产生一个有效的吸引力而形成声子对束缚态，其准粒子是由声子和库柏对组成的复合准粒子，讨论了这种情况下的光超导波基态能．     

一个基于腔衰减的四原子纠缠态制备方案

基于A型三能级原子与腔场及经典场的相互作用理论,利用单光子探测器对从光腔中泄漏出来的光子进行符合测量,提出了一个四原子纠缠态的制备方案,四个分别处于不同光腔中的原子将以一定的概率处于GHZ态。     

Quantum interference between two single photons emitted by independently trapped atoms

When two indistinguishable single photons are fed into the two input ports of a beam splitter, the photons will coalesce and leave together from the same output port. This is a quantum interference effect, which occurs because two possible paths-in which the photons leave by different output ports-interfere destructively. This effect was first observed in parametric downconversion (in which a nonlinear crystal splits a single photon into two photons of lower energy), then from two separate downconversion crystals, as well as with single photons produced one after the other by the same quantum emitter. With the recent developments in quantum information research, much attention has been devoted to this interference effect as a resource for quantum data processing using linear optics techniques. To ensure the scalability of schemes based on these ideas, it is crucial that indistinguishable photons are emitted by a collection of synchronized, but otherwise independent sources. Here we demonstrate the quantum interference of two single photons emitted by two independently trapped single atoms, bridging the gap towards the simultaneous emission of many indistinguishable single photons by different emitters. Our data analysis shows that the observed coalescence is mainly limited by wavefront matching of the light emitted by the two atoms, and to a lesser extent by the motion of each atom in its own trap.     

使自由空间中光速变慢的研究进展

1905年Einstein发表了著名的狭义相对论( SR)文章,其中说光在真空中总以不变的速度传播。然而这个光速不变性原理一直缺乏可靠的实验证明,近年来的一些研究成果倒像是证伪了这一原理。例如,美国Maryland大学的物理学家James Franson于2014年6月发表论文引起物理界的广泛关注,文章宣称已可证明光速比过去所认为的值要慢。他的论据来源于对1987年超新星SN1987A的观测,当时在地球上检测到由爆发而来的光子和中微子,而光子比中微子晚到4.7h,过去对此现象人们只作了模糊的解释。 Franson认为这可能是由光子的真空极化造成的———光子分开为一个正电子和一个电子,在很短时间内又重组为光子。在引力势作用下,重组时粒子能量有微小改变,使速度变慢。粒子在飞经168000光年的过程中(从SN1987A到地球),这种不断发生的分合将造成光子晚到4.7h。另一个例子是,2015年1月英国Glasgow大学的Padgett研究组做到了使光的运行比真空中光速( c)要慢。他们使光子经过一个专用的散射结构物,波形被改变,从而速度变慢。令人惊奇的是,光子出来后(即回到自由空间)仍以减慢了的速度行进。Franson理论和Padgett小组实验损坏了真空中光速的恒值性,使c成为“不恒定的常数”,或“不常的常数”。这种情况妨害了SR理论及现行米定义的理论基础。另外,本文比较了1993年的SKC实验和2015年的空间结构化光子实验,前者以量子隧穿和消失波为基础,后者则改变光束的横向空间结构。二者都使用相关光子对,结果都显示光子群速的变化。如今或许可以终结关于光速不变原理的讨论。……在现代物理学中波粒二象性仍是难题,但新近研究对此有新的理解,与互补原理不同;故更宜于用波理论解释粒子实验。在Padgett小组实验中,无论Bessel波束或Gauss波束光子群速均减小,表现为在1m的实验距离上迟到若干微米。……最后,本文指出近年来的光速研究为波科学带来丰富体验,为理论思维造成新机会。     

Continuous generation of single photons with controlled waveform in an ion-trap cavity system

The controlled production of single photons is of fundamental and practical interest; they represent the lowest excited quantum states of the radiation field, and have applications in quantum cryptography and quantum information processing. Common approaches use the fluorescence of single ions, single molecules, colour centres and semiconductor quantum dots. However, the lack of control over such irreversible emission processes precludes the use of these sources in applications (such as quantum networks) that require coherent exchange of quantum states between atoms and photons. The necessary control may be achieved in principle in cavity quantum electrodynamics. Although this approach has been used for the production of single photons from atoms, such experiments are compromised by limited trapping times, fluctuating atom-field coupling and multi-atom effects. Here we demonstrate a single-photon source based on a strongly localized single ion in an optical cavity. The ion is optimally coupled to a well-defined field mode, resulting in the generation of single-photon pulses with precisely defined shape and timing. We have confirmed the suppression of two-photon events up to the limit imposed by fluctuations in the rate of detector dark counts. The stream of emitted photons is uninterrupted over the storage time of the ion, as demonstrated by a measurement of photon correlations over 90 min.     

论单光子研究

对单光子研究的历史情况和近年来的新成就作了介绍,指出对光子的正确认识必须以经典物理学及量子力学为基础.简述了单光子的产生与检测技术,讨论了与单光子有关的新实验.例如由于量子纠缠,一个光子可以瞬时地影响另一光子的特性.不久前的D.Salart等人的实验说明了这个"瞬时地"究竟有多快--这是一种超光速现象.     

Non-classical light generated by quantum-noise-driven cavity optomechanics

Optomechanical systems, in which light drives and is affected by the motion of a massive object, will comprise a new framework for nonlinear quantum optics, with applications ranging from the storage and transduction of quantum information to enhanced detection sensitivity in gravitational wave detectors. However, quantum optical effects in optomechanical systems have remained obscure, because their detection requires the object’s motion to be dominated by vacuum fluctuations in the optical radiation pressure; so far, direct observations have been stymied by technical and thermal noise. Here we report an implementation of cavity optomechanics using ultracold atoms in which the collective atomic motion is dominantly driven by quantum fluctuations in radiation pressure. The back-action of this motion onto the cavity light field produces ponderomotive squeezing. We detect this quantum phenomenon by measuring sub-shot-noise optical squeezing. Furthermore, the system acts as a low-power, high-gain, nonlinear parametric amplifier for optical fluctuations, demonstrating a gain of 20?dB with a pump corresponding to an average of only seven intracavity photons. These findings may pave the way for low-power quantum optical devices, surpassing quantum limits on position and force sensing, and the control and measurement of motion in quantum gases.     

Generation of ultraviolet entangled photons in a semiconductor

Entanglement is one of the key features of quantum information and communications technology. The method that has been used most frequently to generate highly entangled pairs of photons is parametric down-conversion. Short-wavelength entangled photons are desirable for generating further entanglement between three or four photons, but it is difficult to use parametric down-conversion to generate suitably energetic entangled photon pairs. One method that is expected to be applicable for the generation of such photons is resonant hyper-parametric scattering (RHPS): a pair of entangled photons is generated in a semiconductor via an electronically resonant third-order nonlinear optical process. Semiconductor-based sources of entangled photons would also be advantageous for practical quantum technologies, but attempts to generate entangled photons in semiconductors have not yet been successful. Here we report experimental evidence for the generation of ultraviolet entangled photon pairs by means of biexciton resonant RHPS in a single crystal of the semiconductor CuCl. We anticipate that our results will open the way to the generation of entangled photons by current injection, analogous to current-driven single photon sources.     

Quantum nonlinear optics with single photons enabled by strongly interacting atoms

The realization of strong nonlinear interactions between individual light quanta (photons) is a long-standing goal in optical science and engineering, being of both fundamental and technological significance. In conventional optical materials, the nonlinearity at light powers corresponding to single photons is negligibly weak. Here we demonstrate a medium that is nonlinear at the level of individual quanta, exhibiting strong absorption of photon pairs while remaining transparent to single photons. The quantum nonlinearity is obtained by coherently coupling slowly propagating photons to strongly interacting atomic Rydberg states in a cold, dense atomic gas. Our approach paves the way for quantum-by-quantum control of light fields, including single-photon switching, all-optical deterministic quantum logic and the realization of strongly correlated many-body states of light.     

生物光子与藏药七十味珍珠丸的药理机制

采用高度敏感的生物光子检测(成像)系统(主要由电子倍增CCD和体视显微镜组成),研究了藏药七十味珍珠丸(RNSP)灌胃处理对健康成年雌性昆明小鼠脑片、肝和肾组织的生物光子活动调控的影响.实验结果表明:脑片、肝和肾均具有自发光现象(生物光子).在光刺激下,脑片、肝和肾具有不同程度的诱发光现象.RNSP处理后的脑组织诱发光的最大值高于对照组,并且衰减较慢.研究结果提示RNSP可能通过影响效应组织如脑组织的生物光子的活动来发挥其药理作用.其作用机制可能是通过影响生物光子信号传递对组织细胞功能产生调节作用,确切的作用机制值得进一步研究.     

等离激元微腔中的量子阱太赫兹光吸收

利用Zubarev’s格林函数方法研究了强太赫兹电场作用下微腔中的双量子阱激子光吸收特性.将腔场和太赫兹场处理为光子,根据哈密顿量得到运动方程.研究考虑了失谐、太赫兹场耦合常数以及电子数的衰减率.研究发现,在强太赫兹场作用时,光谱呈现丰富的非线性效应,如出现复制峰和Autler-Townes分裂.该研究为微腔-量子阱系统量子光学响应提供了一个简单、方便的全量子理论处理方法.