Experimental quantum cloning of single photons

Although perfect copying of unknown quantum systems is forbidden by the laws of quantum mechanics, approximate cloning is possible. A natural way of realizing quantum cloning of photons is by stimulated emission. In this context, the fundamental quantum limit to the quality of the clones is imposed by the unavoidable presence of spontaneous emission. In our experiment, a single input photon stimulates the emission of additional photons from a source on the basis of parametric down-conversion. This leads to the production of quantum clones with near-optimal fidelity. We also demonstrate universality of the copying procedure by showing that the same fidelity is achieved for arbitrary input states.     

Deterministic generation of single photons from one atom trapped in a cavity

A single cesium atom trapped within the mode of an optical cavity is used to generate single photons on demand. The photon wave packets are emitted as a Gaussian beam with temporal profile and repetition rate controlled by external driving fields. Each generation attempt is inferred to succeed with a probability near unity, whereas the efficiency for creating an unpolarized photon in the total cavity output is 0.69 +/- 0.10, as limited by passive cavity losses. An average of 1.4 x 10(4) photons are produced by each trapped atom. These results constitute an important step in quantum information science, for example, toward the realization of distributed quantum networking.     

The atom-cavity microscope: single atoms bound in orbit by single photons

The motion of individual cesium atoms trapped inside an optical resonator is revealed with the atom-cavity microscope (ACM). A single atom moving within the resonator generates large variations in the transmission of a weak probe laser, which are recorded in real time. An inversion algorithm then allows individual atom trajectories to be reconstructed from the record of cavity transmission and reveals single atoms bound in orbit by the mechanical forces associated with single photons. In these initial experiments, the ACM yields 2-micrometer spatial resolution in a 10-microsecond time interval. Over the duration of the observation, the sensitivity is near the standard quantum limit for sensing the motion of a cesium atom.     

Observing behavior in a vigilance task

it has been suggested that level of performance in a vigilance task is accurately reflected by frequency of observing responses. By means of photography it has been demonstrated that under conditions where a decrement in vigilance performance does not occur, the frequency of nonobserving behaviorand general activity increases in time.     

Probing quantum commutation rules by addition and subtraction of single photons to/from a light field

The possibility of arbitrarily "adding" and "subtracting" single photons to and from a light field may give access to a complete engineering of quantum states and to fundamental quantum phenomena. We experimentally implemented simple alternated sequences of photon creation and annihilation on a thermal field and used quantum tomography to verify the peculiar character of the resulting light states. In particular, as the final states depend on the order in which the two actions are performed, we directly observed the noncommutativity of the creation and annihilation operators, one of the cardinal concepts of quantum mechanics, at the basis of the quantum behavior of light. These results represent a step toward the full quantum control of a field and may provide new resources for quantum information protocols.     

Quantum-well states as fabry-Perot modes in a thin-film electron interferometer

Angle-resolved photoemission from atomically uniform silver films on iron (100) shows quantum-well states for absolutely determined film thicknesses ranging from 1 to approximately 100 monolayers. These states can be understood in terms of Fabry-Perot modes in an electron interferometer. A quantitative line shape analysis over the entire two orders of magnitude of thickness range yields an accurate measurement of the band structure, quasiparticle lifetime, electron reflectivity, and phase shift. Effects of confinement energy gap, reflection loss, and surface scattering caused by controlled roughness are demonstrated.     

Observing others: multiple action representation in the frontal lobe

Observation of actions performed by others activates monkey ventral premotor cortex, where action meaning, but not object identity, is coded. In a functional MRI (fMRI) study, we investigated whether other monkey frontal areas respond to actions performed by others. Observation of a hand grasping objects activated four frontal areas: rostral F5 and areas 45B, 45A, and 46. Observation of an individual grasping an object also activated caudal F5, which indicates different degrees of action abstraction in F5. Observation of shapes activated area 45, but not premotor F5. Convergence of object and action information in area 45 may be important for full comprehension of actions.     

Atom interferometer measurement of the newtonian constant of gravity

We measured the Newtonian constant of gravity, G, using a gravity gradiometer based on atom interferometry. The gradiometer measures the differential acceleration of two samples of laser-cooled Cs atoms. The change in gravitational field along one dimension is measured when a well-characterized Pb mass is displaced. Here, we report a value of G = 6.693 x 10(-11) cubic meters per kilogram second squared, with a standard error of the mean of +/-0.027 x 10(-11) and a systematic error of +/-0.021 x 10(-11) cubic meters per kilogram second squared. The possibility that unknown systematic errors still exist in traditional measurements makes it important to measure G with independent methods.     

关于平均值的几个定理

作者首先引入了几个正数的x次幂的平均值定义，进而得到在x变化过程中几个正数的x次幂的平均值的两个结果，另外，作者从关于函数平均值的引理入手，叙述了关于函数平均值的一系列性质，这些性质均以不等式的形式出现。     

Capture of a single molecule in a nanocavity

We describe a heptameric protein pore that has been engineered to accommodate two different cyclodextrin adapters simultaneously within the lumen of a transmembrane beta barrel. The volume between the adapters is a cavity of approximately 4400 cubic angstroms. Analysis of single-channel recordings reveals that individual charged organic molecules can be pulled into the cavity by an electrical potential. Once trapped, an organic molecule shuttles back and forth between the adapters for hundreds of milliseconds. Such self-assembling nanostructures are of interest for the fabrication of multianalyte sensors and could provide a means to control chemical reactions.     

从刑事诉讼的目的看刑事诉讼中的“阳光交易”

契约是以独立、自主的个人或个体为前提的,契约反映的是个人或个体的理性、愿望和要求,他们之间的结合只能通过他们的协议来实现。在刑事诉讼中,允许被告人、被害人或者被告人与公诉机关在被害人同意的基础上进行"阳光交易",对惩治腐败、打击犯罪具有促进作用。     

Observing the multiexciton state in singlet fission and ensuing ultrafast multielectron transfer

Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state.     

Probing the chiroptical response of a single molecule

Chirally sensitive measurement techniques have generally been restricted to bulk samples. Here, we report the observation of fluorescence-detected circular dichroism (FDCD) from single (bridgedtriarylamine) helicene molecules by using an excitation wavelength (457 nanometers) in the vicinity of an electronic transition that shows circular dichroism in bulk samples. The distributions of dissymmetry (g) parameters by analysis of signals from pure M- and P-type diastereomers are almost perfect mirror images of one another, each spanning a range of both positive and negative values. In addition, we observe a well-defined structure in the histogram of dissymmetry parameters suggestive of specific molecular orientations at the polymer interface. These single-molecule results highlight strong intrinsic circular dichroism responses that can be obscured by cancellation effects in ensemble measurements of a randomly oriented bulk sample.     

Global trajectories of the long-term decline of coral reef ecosystems

Degradation of coral reef ecosystems began centuries ago, but there is no global summary of the magnitude of change. We compiled records, extending back thousands of years, of the status and trends of seven major guilds of carnivores, herbivores, and architectural species from 14 regions. Large animals declined before small animals and architectural species, and Atlantic reefs declined before reefs in the Red Sea and Australia, but the trajectories of decline were markedly similar worldwide. All reefs were substantially degraded long before outbreaks of coral disease and bleaching. Regardless of these new threats, reefs will not survive without immediate protection from human exploitation over large spatial scales.     

A proposed framework for accelerating technology trajectories in agriculture: a case study in China

Precision agriculture (PA) technologies have great potential for promoting sustainable intensification of food production, ensuring targeted delivery of agricultural inputs, and hence food security and environmental protection. The benefits of PA technologies are applicable across a broad range of agronomic, environmental and rural socio-economic contexts globally. However, farmer and land-manager adoption in low to middle income countries has typically been slower than that observed in more affluent countries. China is currently engaged in the process of agricultural modernisation to ensure food security for its 1.4 billion population and has developed a portfolio of policies designed to improve food security, while simultaneously promoting environmental protection. Particular attention has been paid to the reduction of agricultural inputs such as fertilisers and pesticides. The widespread adoption of PA technologies across the Chinese agricultural landscape is central to the success of these policies. However, socio-economic and cultural barriers, farm scale, (in particular the prevalence of smaller family farms) and demographic changes in the rural population, (for example, the movement of younger people to the cities) represent barriers to PA adoption across China. A framework for ensuring an acceptable and accelerated PA technology trajectory is proposed which combines systematic understanding of farmer and end-user priorities and preferences for technology design throughout the technology development process, and subsequent end-user requirements for implementation (including demonstration of economic and agronomic benefits, and knowledge transfer). Future research will validate the framework against qualitative and quantitative socio-economic, cultural and agronomic indicators of successful, or otherwise, PA implementation. The results will provide the evidence upon which to develop further policies regarding how to secure sustainable food production and how best to implement PA in China, as well as practical recommendations for optimising end-user uptake.     

平均指标在统计分析中的应用

平均指标应用广泛,在统计工作中具有非常重要的作用。熟练掌握平均指标的计算与应用,有利于提高统计分析质量及统计工作效率。