An efficient photoelectric X-ray polarimeter for the study of black holes and neutron stars

The study of astronomical objects using electromagnetic radiation involves four basic observational approaches: imaging, spectroscopy, photometry (accurate counting of the photons received) and polarimetry (measurement of the polarizations of the observed photons). In contrast to observations at other wavelengths, a lack of sensitivity has prevented X-ray astronomy from making use of polarimetry. Yet such a technique could provide a direct picture of the state of matter in extreme magnetic and gravitational fields, and has the potential to resolve the internal structures of compact sources that would otherwise remain inaccessible, even to X-ray interferometry. In binary pulsars, for example, we could directly 'see' the rotation of the magnetic field and determine if the emission is in the form of a 'fan' or a 'pencil' beam. Also, observation of the characteristic twisting of the polarization angle in other compact sources would reveal the presence of a black hole. Here we report the development of an instrument that makes X-ray polarimetry possible. The factor of 100 improvement in sensitivity that we have achieved will allow direct exploration of the most dramatic objects of the X-ray sky.     

阻挡杂质带甚长波红外探测器

阻挡杂质带(Blocked Impurity Band,BIB)探测器是从杂质带光电导(Impurity Band Conduction,IBC)探测器发展而来,利用杂质能级上的电子跃迁,可以探测光子能量远小于半导体禁带宽度的低能光子.BIB探测器的探测波长主要由衬底和掺杂杂质决定,可以覆盖5–300μm波段范围.得益于探测波长长、探测率高和抗辐射性好等优点,BIB探测器自问世以来就一直被大量研究,广泛应用于各种大型天文基红外探测平台上.目前,Si基BIB探测器发展迅速,但是Ge基和GaAs基BIB探测器的发展相对缓慢.BIB探测器对于推动天文和相关科学的发展具有不可替代的作用,欧美等发达国家在BIB探测器上的科研投入巨大.随着我国经济和科技的发展,我国科研人员急切盼望国产BIB探测器可以早日投入使用.本文主要针对BIB探测器物理模型、制备方法、测试手段和国内外发展现状等方面展开讨论,探究BIB探测器的主要工作机制以及器件研发的关键技术,帮助相关研究人员快速了解BIB探测器.     

Observation of Hanbury Brown-Twiss anticorrelations for free electrons

Fluctuations in the counting rate of photons originating from uncorrelated point sources become, within the coherently illuminated area, slightly enhanced compared to a random sequence of classical particles. This phenomenon, known in astronomy as the Hanbury Brown-Twiss effect, is a consequence of quantum interference between two indistinguishable photons and Bose Einstein statistics. The latter require that the composite bosonic wavefunction is a symmetric superposition of the two possible paths. For fermions, the corresponding two-particle wavefunction is antisymmetric: this excludes overlapping wave trains, which are forbidden by the Pauli exclusion principle. Here we use an electron field emitter to coherently illuminate two detectors, and find anticorrelations in the arrival times of the free electrons. The particle beam has low degeneracy (about 10(-4) electrons per cell in phase space); as such, our experiment represents the fermionic twin of the Hanbury Brown-Twiss effect for photons.     

天文用阻挡杂质带红外探测器

低温目标红外辐射的灵敏探测和成像在航天和深空探测中具有重要的应用价值.300 K以下低温目标的辐射强度很弱,辐射特征波长位于中远红外区.阻挡杂质带红外探测器是中远红外探测器中重要的一员,具有覆盖波段宽、灵敏度高、暗电流低、抗辐射性能高等优点,能够胜任空间技术和天文探测在中远红外波段探测的苛刻要求.本文以红外天文探测对红外探测器的应用需求作为出发点,主要就国内外红外天文学、红外天文探测器发展概况,阻挡杂质带红外探测器发展历史及其材料、器件结构等方面做了简要综述,并简明扼要地介绍了阻挡杂质带探测器的器件物理模型.     

我国空间天文发展的现状和展望

本文包括以下内容：（1）国际和国内空间天文的现状和发展态势概要;（2）我国空间天文发展的战略目标;（3）我国空间天文的研究计划和项目简介;（4）对我国未来相关研究领域发展的策略和措施建议.本文基本上不涉及空间太阳物理和太阳系探索的有关研究计划和项目.     

A correlation between the cosmic microwave background and large-scale structure in the Universe

Observations of distant supernovae and the fluctuations in the cosmic microwave background (CMB) indicate that the expansion of the Universe may be accelerating under the action of a 'cosmological constant' or some other form of 'dark energy'. This dark energy now appears to dominate the Universe and not only alters its expansion rate, but also affects the evolution of fluctuations in the density of matter, slowing down the gravitational collapse of material (into, for example, clusters of galaxies) in recent times. Additional fluctuations in the temperature of CMB photons are induced as they pass through large-scale structures and these fluctuations are necessarily correlated with the distribution of relatively nearby matter. Here we report the detection of correlations between recent CMB data and two probes of large-scale structure: the X-ray background and the distribution of radio galaxies. These correlations are consistent with those predicted by dark energy, indicating that we are seeing the imprint of dark energy on the growth of structure in the Universe.     

用Monte Carlo方法模拟HPGe单能光子能量响应函数

为了弥补用于HPGe探测器低能相对探测效率刻度、单能光子在HPGe探测系统上的能量全响应函数拟合实验数据不足的缺点,采用Monte Carlo方法模拟了一系列单能光子在该探测器上能量响应曲线。比较分析这些能量响应曲线,研究该探测器能量响应曲线随入射能量变化特征,并获得了能量响应曲线特征数据,为探测器低能区域相对探测效率刻度、单能光子在HPGe探测系统上的能量响应函数拟合,提供了可靠的研究数据。     

宇宙背景光子对Blazar高能伽马射线的吸收过程

Blazar高能伽马射线在宇宙传播过程中,将会和宇宙背景光子发生相互作用,从而能量会有一个降低的过程。文章将在理论上计算和分析宇宙微波背景（CMB）和河外背景光（EBL）对高能伽马射线的吸收过程,并讨论二者对不同红移情况下的高能伽马射线的吸收过程。     

用X射线衍射研究不同直径镍纳米线阵列的生长方向

大面积有序的不同直径的镍纳米线阵列在多孔氧化铝模板中通过直流电化学沉积制备,利用X光衍射研究了它们的生长方向。充分利用X光的特点对镍纳米线阵列的正反两面进行了测量,结果发现随着纳米线直径的增加,镍纳米线方向向由[110]方向开始向[111]方向转变。并且,利用高分辨透射电镜进行观测,结果发现和XRD得到的结果是一致的。这进一步说明了利用X光来研究纳米线的生长方向既简单又准确．该方法将在以后的纳米器件的研究中起到重要的作用,     

Experimental demonstration of a BDCZ quantum repeater node

Quantum communication is a method that offers efficient and secure ways for the exchange of information in a network. Large-scale quantum communication (of the order of 100 km) has been achieved; however, serious problems occur beyond this distance scale, mainly due to inevitable photon loss in the transmission channel. Quantum communication eventually fails when the probability of a dark count in the photon detectors becomes comparable to the probability that a photon is correctly detected. To overcome this problem, Briegel, Dür, Cirac and Zoller (BDCZ) introduced the concept of quantum repeaters, combining entanglement swapping and quantum memory to efficiently extend the achievable distances. Although entanglement swapping has been experimentally demonstrated, the implementation of BDCZ quantum repeaters has proved challenging owing to the difficulty of integrating a quantum memory. Here we realize entanglement swapping with storage and retrieval of light, a building block of the BDCZ quantum repeater. We follow a scheme that incorporates the strategy of BDCZ with atomic quantum memories. Two atomic ensembles, each originally entangled with a single emitted photon, are projected into an entangled state by performing a joint Bell state measurement on the two single photons after they have passed through a 300-m fibre-based communication channel. The entanglement is stored in the atomic ensembles and later verified by converting the atomic excitations into photons. Our method is intrinsically phase insensitive and establishes the essential element needed to realize quantum repeaters with stationary atomic qubits as quantum memories and flying photonic qubits as quantum messengers.     

关于我国未来天文大设备的一点战略思考

本文回顾了近年来国际上天文大设备的现状和发展态势,简要叙述了国内目前天文观测设备的现状,并指出一批重要观测设备的建成标志着我国初步形成了天文学研究的实测基础.一个突出的例子是由我国天文学家自主创新的郭守敬望远镜LAMOST.这是一架新型光谱巡天型望远镜,它的建成标志着我国大型天文光学望远镜技术的突破.正在建设的500m口径球面射电望远镜(FAST)是目前世界上最大的单天线望远镜,它的建成将使我国射电天文研究走到世界前列.本文还介绍了一些已经提出的天文地面和空间大设备计划,并对我国未来天文大设备的发展进行了一点战略思考,提出了一些个人的建议.     

Multiple ionization of atom clusters by intense soft X-rays from a free-electron laser

Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.     

非对称偏振三维纠缠态的预警制备

在量子信息处理过程中,量子纠缠态扮演着极为重要的角色,其特殊的物理性质,使得量子信息具有经典信息所没有的许多新的特征,为信息传输和信息处理提供了新的物理资源.针对非对称偏振三维纠缠态的制备,基于交叉相位调制技术,以纠缠光子对和两个单光子比特作为初态,通过单光子与相干光的相互作用以及双光子干涉来实现.如果通过三个非计数单光子探测器来预警制备三维最大纠缠态,其概率为3/64.而如果采用特殊的分段式光子探测器,其概率可以提高到3/8,达到理论极限值.该方案在理论上是可行的,效率相对较高,而且预警式的制备为其后续在量子信息过程中的使用提供了很大的灵活性.     

Plasmon-assisted transmission of entangled photons

The state of a two-particle system is said to be entangled when its quantum-mechanical wavefunction cannot be factorized into two single-particle wavefunctions. This leads to one of the strongest counter-intuitive features of quantum mechanics, namely non-locality. Experimental realization of quantum entanglement is relatively easy for photons; a starting photon can spontaneously split into a pair of entangled photons inside a nonlinear crystal. Here we investigate the effects of nanostructured metal optical elements on the properties of entangled photons. To this end, we place optically thick metal films perforated with a periodic array of subwavelength holes in the paths of the two entangled photons. Such arrays convert photons into surface-plasmon waves--optically excited compressive charge density waves--which tunnel through the holes before reradiating as photons at the far side. We address the question of whether the entanglement survives such a conversion process. Our coincidence counting measurements show that it does, so demonstrating that the surface plasmons have a true quantum nature. Focusing one of the photon beams on its array reduces the quality of the entanglement. The propagation of the surface plasmons makes the array effectively act as a 'which way' detector.     

2021年空间科学热点回眸

 回顾了2021年全球空间科学重要进展、美欧俄中等国家和地区组织最新空间科学重大发展战略和规划、新发射的空间科学任务平台和新选定的未来任务。盘点了“帕克太阳探测器”首次飞越日冕，“洞察号”揭示火星内部结构，阿联酋、中国、美国探测器齐聚火星，美国绘制天文学和天体物理学发展路线图，欧洲确定2035-2050年大型空间科学任务科学主题，中俄发布国际月球科研站路线图，“詹姆斯·韦伯空间望远镜”成功发射，中国成功发射太阳探测科学技术试验卫星和可持续发展科学卫星，美欧批准新的金星探测任务等重大成果和事件。     

2019年深空探测热点回眸

 深空探测是空间科学探索与发现的重要方式之一。2019年全球在轨工作的深空探测任务有29个，皆为非载人航天任务，主要开展了月球、火星和小行星等的探测、深空太阳观测及日-地L1点空间天气监测和L2点空间天文观测。在纪念人类登月50周年之际，美国加快推动重返月球的阿尔忒弥斯计划，中国嫦娥四号任务着陆器和玉兔二号月球车对月球背面的就位探测和巡视勘察陆续产出一批国际关注的科学成果。美国新视野号任务飞掠柯伊伯带小天体“2014 MU69”的首个科学成果揭示了该“接触双星系统”的发展、地质与构成。日本隼鸟二号任务完成对“龙宫”小行星的两次采样探测、启程返回地球。除了“史诗级”的太阳探测帕克号任务，被“遗忘”在L1点的多个超期服役的空间天气监测科学任务如SOHO、WIND等表现依然稳定和出色。2020年，人类将迎来探测火星的新发射窗口，中国、美国和欧洲等即将实施的多个火星任务引人注目。     

Astronomy (communication arising): black holes, fleas and microlithography

Fresnel lenses allow almost perfect imaging in widely different circumstances, but their focus is perfect only for a single wavelength. Wang et al. have shown how the effective bandpass may be widened for X-ray microscopy by using a compound diffractive/refractive lens near to an absorption edge. A compound lens has also been proposed for high-energy astronomy, working well above all absorption edges. Although the scale is very different, we point out here that the principle is the same. Ever since Galileo constructed an astronomical telescope that he was able to reconfigure to study fleas and gnats, astronomy and microscopy have relied on optics that are closely related, but different in detail.     

A single-photon detector in the far-infrared range

The far-infrared region (wavelengths in the range 10 microm-1 mm) is one of the richest areas of spectroscopic research, encompassing the rotational spectra of molecules and vibrational spectra of solids, liquids and gases. But studies in this spectral region are hampered by the absence of sensitive detectors--despite recent efforts to improve superconducting bolometers, attainable sensitivities are currently far below the level of single-photon detection. This is in marked contrast to the visible and near-infrared regions (wavelengths shorter than about 1.5 microm), in which single-photon counting is possible using photomultiplier tubes. Here we report the detection of single far-infrared photons in the wavelength range 175-210 microm (6.0-7.1 meV), using a single-electron transistor consisting of a semiconductor quantum dot in high magnetic field. We detect, with a time resolution of a millisecond, an incident flux of 0.1 photons per second on an effective detector area of 0.1 mm2--a sensitivity that exceeds previously reported values by a factor of more than 10(4). The sensitivity is a consequence of the unconventional detection mechanism, in which one absorbed photon leads to a current of 10(6)-10(12) electrons through the quantum dot. By contrast, mechanisms of conventional detectors or photon assisted tunnelling in single-electron transistors produce only a few electrons per incident photon.     

Radiation-pressure cooling and optomechanical instability of a micromirror

Recent table-top optical interferometry experiments and advances in gravitational-wave detectors have demonstrated the capability of optical interferometry to detect displacements with high sensitivity. Operation at higher powers will be crucial for further sensitivity enhancement, but dynamical effects caused by radiation pressure on the interferometer mirrors must be taken into account, and the appearance of optomechanical instabilities may jeopardize the stable operation of the next generation of interferometers. These instabilities are the result of a nonlinear coupling between the motion of the mirrors and the optical field, which modifies the effective dynamics of the mirror. Such 'optical spring' effects have already been demonstrated for the mechanical damping of an electromagnetic waveguide with a moving wall, the resonance frequency of a specially designed flexure oscillator, and the optomechanical instability of a silica microtoroidal resonator. Here we present an experiment where a micromechanical resonator is used as a mirror in a very high-finesse optical cavity, and its displacements are monitored with unprecedented sensitivity. By detuning the laser frequency with respect to the cavity resonance, we have observed a drastic cooling of the microresonator by intracavity radiation pressure, down to an effective temperature of 10 kelvin. For opposite detuning, efficient heating is observed, as well as a radiation-pressure-induced instability of the resonator. Further experimental progress and cryogenic operation may lead to the experimental observation of the quantum ground state of a micromechanical resonator, either by passive or active cooling techniques.     

Thermal and electrical conductivity of iron at Earth's core conditions

The Earth acts as a gigantic heat engine driven by the decay of radiogenic isotopes and slow cooling, which gives rise to plate tectonics, volcanoes and mountain building. Another key product is the geomagnetic field, generated in the liquid iron core by a dynamo running on heat released by cooling and freezing (as the solid inner core grows), and on chemical convection (due to light elements expelled from the liquid on freezing). The power supplied to the geodynamo, measured by the heat flux across the core-mantle boundary (CMB), places constraints on Earth's evolution. Estimates of CMB heat flux depend on properties of iron mixtures under the extreme pressure and temperature conditions in the core, most critically on the thermal and electrical conductivities. These quantities remain poorly known because of inherent experimental and theoretical difficulties. Here we use density functional theory to compute these conductivities in liquid iron mixtures at core conditions from first principles--unlike previous estimates, which relied on extrapolations. The mixtures of iron, oxygen, sulphur and silicon are taken from earlier work and fit the seismologically determined core density and inner-core boundary density jump. We find both conductivities to be two to three times higher than estimates in current use. The changes are so large that core thermal histories and power requirements need to be reassessed. New estimates indicate that the adiabatic heat flux is 15 to 16 terawatts at the CMB, higher than present estimates of CMB heat flux based on mantle convection; the top of the core must be thermally stratified and any convection in the upper core must be driven by chemical convection against the adverse thermal buoyancy or lateral variations in CMB heat flow. Power for the geodynamo is greatly restricted, and future models of mantle evolution will need to incorporate a high CMB heat flux and explain the recent formation of the inner core.