A semiconductor source of triggered entangled photon pairs

Entangled photon pairs are an important resource in quantum optics, and are essential for quantum information applications such as quantum key distribution and controlled quantum logic operations. The radiative decay of biexcitons-that is, states consisting of two bound electron-hole pairs-in a quantum dot has been proposed as a source of triggered polarization-entangled photon pairs. To date, however, experiments have indicated that a splitting of the intermediate exciton energy yields only classically correlated emission. Here we demonstrate triggered photon pair emission from single quantum dots suggestive of polarization entanglement. We achieve this by tuning the splitting to zero, through either application of an in-plane magnetic field or careful control of growth conditions. Entangled photon pairs generated 'on demand' have significant fundamental advantages over other schemes, which can suffer from multiple pair emission, or require post-selection techniques or the use of photon-number discriminating detectors. Furthermore, control over the pair generation time is essential for scaling many quantum information schemes beyond a few gates. Our results suggest that a triggered entangled photon pair source could be implemented by a simple semiconductor light-emitting diode.     

Retroviral invasion of the koala genome

Endogenous retroviruses are a common ancestral feature of mammalian genomes with most having been inactivated over time through mutation and deletion. A group of more intact endogenous retroviruses are considered to have entered the genomes of some species more recently, through infection by exogenous viruses, but this event has never been directly proved. We have previously reported koala retrovirus (KoRV) to be a functional virus that is associated with neoplasia. Here we show that KoRV also shows features of a recently inserted endogenous retrovirus that is vertically transmitted. The finding that some isolated koala populations have not yet incorporated KoRV into their genomes, combined with its high level of activity and variability in individual koalas, suggests that KoRV is a virus in transition between an exogenous and endogenous element. This ongoing dynamic interaction with a wild species provides an exciting opportunity to study the process and consequences of retroviral endogenization in action, and is an attractive model for studying the evolutionary event in which a retrovirus invades a mammalian genome.     

Discovery of two new satellites of Pluto

Pluto's first known satellite, Charon, was discovered in 1978. It has a diameter (approximately 1,200 km) about half that of Pluto, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful, but they were not sensitive to objects less, similar150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1's diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of approximately 38 days (P1) and approximately 25 days (P2).     

SOVA解码的有效中止判据

Turbo解码的迭代解码的循环次数越多,误码性能就越能得到改善.但是SOVA解码的循环次数有一个限度,超过这一限度,改善程度有限,只会增加计算量和解码迟滞.为了减少不必要的计算量和解码迟滞,需要更有效率的迭代解码中止判据.提出为了SOVA(Soft output Viterbi algorithm)解码的高效率的迭代解码中止判据,通过模拟,比较、分析了其性能.     

An exceptional Devonian fish from Australia sheds light on tetrapod origins

The transition from fishes to tetrapods was one of the most dramatic events in the evolution of vertebrates, but many pivotal fossils are incomplete, resulting in gaps in the data that are used for phylogenetic reconstruction. Here we present new observations from the most complete, acid-prepared Devonian tetrapodomorph fish yet discovered, Gogonasus, which was previously placed just crownward of Kenichthys and rhizodontids, the most primitive taxa on the tetrapod lineage. Unexpectedly, Gogonasus shows a mosaic of plesiomorphic and derived tetrapod-like features. Whereas the braincase and dermal cranial skeleton exhibit generalized morphologies with respect to Eusthenopteron or Panderichthys, taxa that are traditionally considered to be phyletically close to tetrapods, the presence of a deeply invaginated, wide spiracle, advanced internal spiracular architecture and near-horizontal hyomandibula are specialized features that are absent from Eusthenopteron. Furthermore, the pectoral fin skeleton of Gogonasus shares several features with that of Tiktaalik, the most tetrapod-like fish. A new phylogenetic analysis places Gogonasus crownward of Eusthenopteron as the sister taxon to the Elpistostegalia. Aspects of the basic tetrapod limb skeleton and middle ear architecture can now be traced further back within the tetrapodomorph radiation.     

国际间节能减碳相关产业的发展状况

由于温室效应对于全球气候变化的影响已经相当显著,因此许多国家与产业都将投入大量资源以努力降低全球气候变化的影响,其中当然隐现着未来全球节能减碳领域的庞大新商机,各项节能减碳相关创新技术因而相继发展.笔者指出,节能减碳产业是藉由材料、设备、制程以及产品之改善,以达成节能减碳目的之产业,其产业范畴可略分为节能材料、节能设备及产品、系统能源整合、节能减碳认验证、碳资产管理等.综述了国际间节能减碳相关产业之发展现状及发展趋势.指出,面对未来之挑战,应确实评估节能减碳相关产业之发展潜力,确定发展对象,擘划出适合节能减碳之产业发展策略及措施.     

大鹏湾沉积物重金属污染的初步研究

用电感耦合等离子焰光谱技术测定大鹏湾４个采样站位沉积物岩芯中Ｐｂ，Ｍｎ，Ｎｉ，Ｃｒ，Ｃｄ和Ｆｅ，Ｍｇ，Ｚｎ，Ｃｕ，Ｃｏ共１０种重金属元素的浓度－深度关系，并根据相应的沉积速率分析了该海域的污染史．     

Chromosomal landscape of nucleosome-dependent gene expression and silencing in yeast

Eukaryotic genomes are packaged into nucleosomes, which are thought to repress gene expression generally. Repression is particularly evident at yeast telomeres, where genes within the telomeric heterochromatin appear to be silenced by the histone-binding silent information regulator (SIR) complex (Sir2, Sir3, Sir4) and Rap1 (refs 4-10). Here, to investigate how nucleosomes and silencing factors influence global gene expression, we use high-density arrays to study the effects of depleting nucleosomal histones and silencing factors in yeast. Reducing nucleosome content by depleting histone H4 caused increased expression of 15% of genes and reduced expression of 10% of genes, but it had little effect on expression of the majority (75%) of yeast genes. Telomere-proximal genes were found to be de-repressed over regions extending 20 kilobases from the telomeres, well beyond the extent of Sir protein binding and the effects of loss of Sir function. These results indicate that histones make Sir-independent contributions to telomeric silencing, and that the role of histones located elsewhere in chromosomes is gene specific rather than generally repressive.