Quantum storage of photonic entanglement in a crystal

Entanglement is the fundamental characteristic of quantum physics-much experimental effort is devoted to harnessing it between various physical systems. In particular, entanglement between light and material systems is interesting owing to their anticipated respective roles as 'flying' and stationary qubits in quantum information technologies (such as quantum repeaters and quantum networks). Here we report the demonstration of entanglement between a photon at a telecommunication wavelength (1,338?nm) and a single collective atomic excitation stored in a crystal. One photon from an energy-time entangled pair is mapped onto the crystal and then released into a well-defined spatial mode after a predetermined storage time. The other (telecommunication wavelength) photon is sent directly through a 50-metre fibre link to an analyser. Successful storage of entanglement in the crystal is proved by a violation of the Clauser-Horne-Shimony-Holt inequality by almost three standard deviations (S = 2.64?±?0.23). These results represent an important step towards quantum communication technologies based on solid-state devices. In particular, our resources pave the way for building multiplexed quantum repeaters for long-distance quantum networks.     

PICASSO——一个用于模糊决策系统的基于规则的程序设计语言

本论文介绍一种新的基于规则的程序设计语言,其名为 PICASSO,它为具体执行模糊决策支持系统(Fuzzy Decision SupportSystem)而设计。PICASSO 已在美国休斯顿大学计算中心的 AT&T 公司的3B2OS 小型计算机上部分运行。整个系统用 Franz Lisp 书写。PICASSO 具有以下特征。它使用正向链接(Forward chaining)推理手段。该语言有三种控制策略:面向宽度优先的系统控制模式,用户控制模式和元规则(meta-rule)~(*2)控制模式。PICASSO 支持匹配变量以及其他类型的变量,以便于使用以模式匹配为主的调用方式。在该语言中,不同规则之间的信息能够用信息传送和数据共享两种方法来进行通讯、交换。PICASSO 的规则工作在可永久保存的知识库上,这个知识库必须预先用称为符号图形(S—diagram)的一种强有力的数据模式来定义。PICASSO 的推理器(inference engine)实施知识库的一致性。知识库中可以保存模糊信息(fuzzy information),这些信息用区间模式的二值法来表示。区间模式允许分配一个概率给一个事实(断言),同时还要表示我们相信这种概率估计的程度,这一点对于模糊决策支持系统来说是重要的,因为这种系统常牵涉到不同程度的经验知识的判定规则。有了对某些事实(断言)的概率估计,又有专家对此概率的相信程度(凭经验),就可较全面地选择某种决策。对于区间模式来说,我们已经提供了一些特定的运算公式,用于对不确定性(uncertainty)知识进行自动推理。PICASSO 支持基于规则的程序设计语言和函数型的程序设计语言两者的组合并支持多个基于规则的程序设计语言和函数型的程序设计语言两者的组合,并支持多个基于规则的程序之间的协同操作:可以容易地把 PICASSO 程序综合成象一般的 LISP 函数那样的 LISP 程序,而且 PICASSO程序本身也可以任意地调用 LISP 函数。     

An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays

Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.     

Femtosecond time-delay X-ray holography

Extremely intense and ultrafast X-ray pulses from free-electron lasers offer unique opportunities to study fundamental aspects of complex transient phenomena in materials. Ultrafast time-resolved methods usually require highly synchronized pulses to initiate a transition and then probe it after a precisely defined time delay. In the X-ray regime, these methods are challenging because they require complex optical systems and diagnostics. Here we propose and apply a simple holographic measurement scheme, inspired by Newton's 'dusty mirror' experiment, to monitor the X-ray-induced explosion of microscopic objects. The sample is placed near an X-ray mirror; after the pulse traverses the sample, triggering the reaction, it is reflected back onto the sample by the mirror to probe this reaction. The delay is encoded in the resulting diffraction pattern to an accuracy of one femtosecond, and the structural change is holographically recorded with high resolution. We apply the technique to monitor the dynamics of polystyrene spheres in intense free-electron-laser pulses, and observe an explosion occurring well after the initial pulse. Our results support the notion that X-ray flash imaging can be used to achieve high resolution, beyond radiation damage limits for biological samples. With upcoming ultrafast X-ray sources we will be able to explore the three-dimensional dynamics of materials at the timescale of atomic motion.     

Postcranial evidence from early Homo from Dmanisi, Georgia

The Plio-Pleistocene site of Dmanisi, Georgia, has yielded a rich fossil and archaeological record documenting an early presence of the genus Homo outside Africa. Although the craniomandibular morphology of early Homo is well known as a result of finds from Dmanisi and African localities, data about its postcranial morphology are still relatively scarce. Here we describe newly excavated postcranial material from Dmanisi comprising a partial skeleton of an adolescent individual, associated with skull D2700/D2735, and the remains from three adult individuals. This material shows that the postcranial anatomy of the Dmanisi hominins has a surprising mosaic of primitive and derived features. The primitive features include a small body size, a low encephalization quotient and absence of humeral torsion; the derived features include modern-human-like body proportions and lower limb morphology indicative of the capability for long-distance travel. Thus, the earliest known hominins to have lived outside of Africa in the temperate zones of Eurasia did not yet display the full set of derived skeletal features.     

Tissue factor and PAR1 promote microbiota-induced intestinal vascular remodelling

The gut microbiota is a complex ecosystem that has coevolved with host physiology. Colonization of germ-free (GF) mice with a microbiota promotes increased vessel density in the small intestine, but little is known about the mechanisms involved. Tissue factor (TF) is the membrane receptor that initiates the extrinsic coagulation pathway, and it promotes developmental and tumour angiogenesis. Here we show that the gut microbiota promotes TF glycosylation associated with localization of TF on the cell surface, the activation of coagulation proteases, and phosphorylation of the TF cytoplasmic domain in the small intestine. Anti-TF treatment of colonized GF mice decreased microbiota-induced vascular remodelling and expression of the proangiogenic factor angiopoietin-1 (Ang-1) in the small intestine. Mice with a genetic deletion of the TF cytoplasmic domain or with hypomorphic TF (F3) alleles had a decreased intestinal vessel density. Coagulation proteases downstream of TF activate protease-activated receptor (PAR) signalling implicated in angiogenesis. Vessel density and phosphorylation of the cytoplasmic domain of TF were decreased in small intestine from PAR1-deficient (F2r(-/-)) but not PAR2-deficient (F2rl1(-/-)) mice, and inhibition of thrombin showed that thrombin-PAR1 signalling was upstream of TF phosphorylation. Thus, the microbiota-induced extravascular TF-PAR1 signalling loop is a novel pathway that may be modulated to influence vascular remodelling in the small intestine.     

Adenylylation control by intra- or intermolecular active-site obstruction in Fic proteins

Fic proteins that are defined by the ubiquitous FIC (filamentation induced by cyclic AMP) domain are known to catalyse adenylylation (also called AMPylation); that is, the transfer of AMP onto a target protein. In mammalian cells, adenylylation of small GTPases through Fic proteins injected by pathogenic bacteria can cause collapse of the actin cytoskeleton and cell death. It is unknown how this potentially deleterious adenylylation activity is regulated in the widespread Fic proteins that are found in all domains of life and that are thought to have critical roles in intrinsic signalling processes. Here we show that FIC-domain-mediated adenylylation is controlled by a conserved mechanism of ATP-binding-site obstruction that involves an inhibitory α-helix (α(inh)) with a conserved (S/T)XXXE(G/N) motif, and that in this mechanism the invariable glutamate competes with ATP γ-phosphate binding. Consistent with this, FIC-domain-mediated growth arrest of bacteria by the VbhT toxin of Bartonella schoenbuchensis is intermolecularly repressed by the VbhA antitoxin through tight binding of its α(inh) to the FIC domain of VbhT, as shown by structure and function analysis. Furthermore, structural comparisons with other bacterial Fic proteins, such as Fic of Neisseria meningitidis and of Shewanella oneidensis, show that α(inh) frequently constitutes an amino-terminal or carboxy-terminal extension to the FIC domain, respectively, partially obstructing the ATP binding site in an intramolecular manner. After mutation of the inhibitory motif in various Fic proteins, including the human homologue FICD (also known as HYPE), adenylylation activity is considerably boosted, consistent with the anticipated relief of inhibition. Structural homology modelling of all annotated Fic proteins indicates that inhibition by α(inh) is universal and conserved through evolution, as the inhibitory motif is present in ～90% of all putatively adenylylation-active FIC domains, including examples from all domains of life and from viruses. Future studies should reveal how intrinsic or extrinsic factors modulate adenylylation activity by weakening the interaction of α(inh) with the FIC active site.     

Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser

Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength. The introduction of X-ray free-electron lasers makes it possible to pump new atomic X-ray lasers with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.     

Bottom-up effects of plant diversity on multitrophic interactions in a biodiversity experiment

Biodiversity is rapidly declining, and this may negatively affect ecosystem processes, including economically important ecosystem services. Previous studies have shown that biodiversity has positive effects on organisms and processes across trophic levels. However, only a few studies have so far incorporated an explicit food-web perspective. In an eight-year biodiversity experiment, we studied an unprecedented range of above- and below-ground organisms and multitrophic interactions. A multitrophic data set originating from a single long-term experiment allows mechanistic insights that would not be gained from meta-analysis of different experiments. Here we show that plant diversity effects dampen with increasing trophic level and degree of omnivory. This was true both for abundance and species richness of organisms. Furthermore, we present comprehensive above-ground/below-ground biodiversity food webs. Both above ground and below ground, herbivores responded more strongly to changes in plant diversity than did carnivores or omnivores. Density and richness of carnivorous taxa was independent of vegetation structure. Below-ground responses to plant diversity were consistently weaker than above-ground responses. Responses to increasing plant diversity were generally positive, but were negative for biological invasion, pathogen infestation and hyperparasitism. Our results suggest that plant diversity has strong bottom-up effects on multitrophic interaction networks, with particularly strong effects on lower trophic levels. Effects on higher trophic levels are indirectly mediated through bottom-up trophic cascades.