Comparison of the Hanbury Brown-Twiss effect for bosons and fermions

Fifty years ago, Hanbury Brown and Twiss (HBT) discovered photon bunching in light emitted by a chaotic source, highlighting the importance of two-photon correlations and stimulating the development of modern quantum optics. The quantum interpretation of bunching relies on the constructive interference between amplitudes involving two indistinguishable photons, and its additive character is intimately linked to the Bose nature of photons. Advances in atom cooling and detection have led to the observation and full characterization of the atomic analogue of the HBT effect with bosonic atoms. By contrast, fermions should reveal an antibunching effect (a tendency to avoid each other). Antibunching of fermions is associated with destructive two-particle interference, and is related to the Pauli principle forbidding more than one identical fermion to occupy the same quantum state. Here we report an experimental comparison of the fermionic and bosonic HBT effects in the same apparatus, using two different isotopes of helium: (3)He (a fermion) and 4He (a boson). Ordinary attractive or repulsive interactions between atoms are negligible; therefore, the contrasting bunching and antibunching behaviour that we observe can be fully attributed to the different quantum statistics of each atomic species. Our results show how atom-atom correlation measurements can be used to reveal details in the spatial density or momentum correlations in an atomic ensemble. They also enable the direct observation of phase effects linked to the quantum statistics of a many-body system, which may facilitate the study of more exotic situations.     

利用动力学去耦合脉冲增大和调节量子相干性和三体纠缠

本文研究了动力学去耦合脉冲对腔量子电动力学系统中量子相干性, 量子失谐和量子纠缠的影响, 发现动力学去耦合脉冲不仅能够增大系统中两原子之间的量子相干性, 同时也能增大它们之间非经典关联（量子失谐和量子纠缠）. 同时, 凭借迹距离的方法, 探讨了动力学去耦合脉冲增大两原子之间量子相干性的原因, 通过探究可以看出动力学去耦合脉冲能够控制和加速量子信息从其他子系统回流到两个原子中去, 并减少两原子子系统和其他子系统之间的量子信息流动, 从而增加两原子间的量子相干性和非经典关联. 最后, 利用保真度的方法研究了系统中三体纠缠出现的情况, 结果显示在不同的时间, 系统中会出现三体纠缠, 特别值得指出的是, 可以通过动力学去耦合脉冲来调节和增加系统中三体纠缠出现的时间.     

A candidate redshift z ≈ 10 galaxy and rapid changes in that population at an age of 500 Myr

Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900-2,000?million years (Myr) after the Big Bang (redshifts 6?>?z?>?3; ref. 1). The Hubble Ultra Deep Field (HUDF09) data have yielded the first reliable detections of z?≈?8 galaxies that, together with reports of a γ-ray burst at z?≈?8.2 (refs 10, 11), constitute the earliest objects reliably reported to date. Observations of z?≈?7-8 galaxies suggest substantial star formation at z?>?9-10 (refs 12, 13). Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z?≈?10 galaxies in the heart of the reionization epoch, only 500?Myr after the Big Bang. Not only do we find one possible z?≈?10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (～10%) at this time than it is just ～200?Myr later at z?≈?8. This demonstrates how rapid galaxy build-up was at z?≈?10, as galaxies increased in both luminosity density and volume density from z?≈?10 to z?≈?8. The 100-200?Myr before z?≈?10 is clearly a crucial phase in the assembly of the earliest galaxies.     

耗散腔场中双光子Tavies-Cummings模型量子纠缠分析

用共生纠缠度度量的方法来研究两原子在耗散和非耗散腔场中的量子纠缠.研究结果表明：通过控制相互作用时间能够制备出最大纠缠态;且由于原子的双光子跃迁,导致双光子Tavies-Cummings模型是单光子Tavies-Cummings模型在制备最大纠缠态上所需要的时间的.且原子的自发辐射和腔场的损耗使得两原子的最大相干随时间呈周期性减小.     

Nonlinear optics in the extreme ultraviolet

Nonlinear responses to an optical field are universal in nature but have been difficult to observe in the extreme ultraviolet (XUV) and soft X-ray regions owing to a lack of coherent intense light sources. High harmonic generation is a well-known nonlinear optical phenomenon and is now drawing much attention in attosecond pulse generation. For the application of high harmonics to nonlinear optics in the XUV and soft X-ray regime, optical pulses should have both large pulse energy and short pulse duration to achieve a high optical electric field. Here we show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically. The XUV pulses with pulse durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon above-threshold ionization of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of XUV pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics at shorter wavelengths.     

Hidden symmetries in the energy levels of excitonic 'artificial atoms'

Quantum dots or 'artificial atoms' are of fundamental and technological interest--for example, quantum dots may form the basis of new generations of lasers. The emission in quantum-dot lasers originates from the recombination of excitonic complexes, so it is important to understand the dot's internal electronic structure (and of fundamental interest to compare this to real atomic structure). Here we investigate artificial electronic structure by injecting optically a controlled number of electrons and holes into an isolated single quantum dot. The charge carriers form complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms. We observe that electrons and holes occupy the confined electronic shells in characteristic numbers according to the Pauli exclusion principle. In each degenerate shell, collective condensation of the electrons and holes into coherent many-exciton ground states takes place; this phenomenon results from hidden symmetries (the analogue of Hund's rules for real atoms) in the energy function that describes the multi-particle system. Breaking of the hidden symmetries leads to unusual quantum interferences in emission involving excited states.     

Li+/Be2+与DNA碱基和水相互作用的量子化学研究

采用量子化学方法 B3LYP/6-311+G(d,p)对气相中的Li+/Be2+与4种DNA碱基和水的相互作用进行研究,优化Li+/Be2+与碱基及水形成的复合物的几何结构,计算其结合能和电荷分布等.结果表明,Be2+与配体碱基及水的距离要比与Li+的距离小0.3左右;Be2+与其配体的结合能要远比与Li+的配合能大.随着水数目的增加,Li+/Be2+到配位原子的平均距离越来越大,但变化幅度较小.在复合物中,金属离子的正电荷主要转移到水中.     

Gas accretion as the origin of chemical abundance gradients in distant galaxies

It has recently been suggested that galaxies in the early Universe could have grown through the accretion of cold gas, and that this may have been the main driver of star formation and stellar mass growth. Because the cold gas is essentially primordial, it has a very low abundance of elements heavier than helium (referred to as metallicity). If funnelled to the centre of a galaxy, it will result in the central gas having an overall lower metallicity than gas further from the centre, because the gas further out has been enriched by supernovae and stellar winds, and not diluted by the primordial gas. Here we report chemical abundances across three rotationally supported star-forming galaxies at redshift z?≈?3, only 2?Gyr after the Big Bang. We find 'inverse' gradients, with the central, star-forming regions having lower metallicities than less active ones, which is opposite to what is seen in local galaxies. We conclude that the central gas has been diluted by the accretion of primordial gas, as predicted by 'cold flow' models.     

非简并双光子Jaynes-Commings模型中纠缠度对光场的影响

研究两纠缠原子之一在光腔中与两个非简并双光子的相互作用,通过针对性选择测量,使得系统场被约化为不同的态矢,得到纠缠交换的目的;分析原子的纠缠度对约化后光场性质的影响,结果表明纠缠度强烈影响光场的平均光子数分布和二阶量子相干性.     

Helium solubility in olivine and implications for high 3He/4He in ocean island basalts

High 3He/4He ratios found in ocean island basalts are the main evidence for the existence of an undegassed mantle reservoir. However, models of helium isotope evolution depend critically on the chemical behaviour of helium during mantle melting. It is generally assumed that helium is strongly enriched in mantle melts relative to uranium and thorium, yet estimates of helium partitioning in mantle minerals have produced conflicting results. Here we present experimental measurements of helium solubility in olivine at atmospheric pressure. Natural and synthetic olivines were equilibrated with a 50% helium atmosphere and analysed by crushing in vacuo followed by melting, and yield a minimum olivine-melt partition coefficient of 0.0025 +/- 0.0005 (s.d.) and a maximum of 0.0060 +/- 0.0007 (s.d.). The results indicate that helium might be more compatible than uranium and thorium during mantle melting and that high 3He/4He ratios can be preserved in depleted residues of melting. A depleted source for high 3He/4He ocean island basalts would resolve the apparent discrepancy in the relative helium concentrations of ocean island and mid-ocean-ridge basalts.     

Significant effects of branches and bromine substitution of near-infrared two-photon photosensitizers on the generation of singlet oxygen

This letter reports the influence of branches and bromine substitution of the photosensitizers on one-and two-photon absorption generation of singlet oxygen.Near-infrared femtosecond Ti:squassier laser was utilized to determine two-photon properties of the photosensitizers tuning wavelength from 700 to 880 nm at intervals of 20 nm.One-and two-photon optical and photophysical properties of the photosensitizers show significant dependence on the branches and substituted bromine atoms.     

渤海湾盆地黄骅拗陷天然气中幔源氦

黄骅拗陷天然气中存在强烈的幔源氦入侵，氦同位素比值３Ｈｅ／４Ｈｅ０．１２Ｒａ至３．６２Ｒａ，表明天然气中氦组分有１％至４５％来自幔源。３Ｈｅ／４Ｈｅ频率分布图呈双峰式特点，低值峰０．１２Ｒａ至１．１９Ｒａ，位于新生代中后期构造、岩浆活动不发育和远离断裂交汇带地区，其仍远高于壳源流体特征０．０２Ｒａ，表明拗陷区总体上存在较强的幔源氦混染。高值峰１．２８Ｒａ至２．８０Ｒａ，均位于长期活动断裂，特别是断裂交汇带和新生代火山活动区。幔源氦自断裂交汇带释放．沿ＮＥ—ＮＥＥ向伸展断裂带运移、聚集，并向四周扩散。早第三纪伸展活动与幔源岩浆作用期是幔源氦主要释放期；晚第三纪至第四纪ＮＷ向深断裂活动和深源岩浆作用对区内幔源氦聚集亦具重要贡献。     

Quantum information processing with atoms and photons

Quantum information processors exploit the quantum features of superposition and entanglement for applications not possible in classical devices, offering the potential for significant improvements in the communication and processing of information. Experimental realization of large-scale quantum information processors remains a long-term vision, as the required nearly pure quantum behaviour is observed only in exotic hardware such as individual laser-cooled atoms and isolated photons. But recent theoretical and experimental advances suggest that cold atoms and individual photons may lead the way towards bigger and better quantum information processors, effectively building mesoscopic versions of 'Schr?dinger's cat' from the bottom up.     

水合锌（Ⅱ）团簇Zn2+（H2O）n（n=10~12）的理论化学研究

应用高水平量子化学方法 MP2/6-311++G(2d,2p)//M062X/6-311+G(d,p)对气相中的Zn2+(H2O)n(n=10~12)团簇进行优化,获得其多种稳定结构,计算并比较不同Zn2+的配位数下的结合能及电荷分布.结果表明:当n=10~12时,Zn2+的配位数可以为4、5或6,其中最稳定结构的Zn2+配位数为5或6.随着水分子数的增加,体系最稳定结构的氢键数和离子与内层水分子中氧的距离RZn-O都在不断增加.     

超薄样室(ETC)中受限原子的高分辨率光谱技术

基于ETC的高分辨率激光光谱测量技术,探索获得更高分辨率光谱的方法。由于受限原子瞬态行为和慢原子/效应增强,以及双光子光谱的消多普勒增宽配置,使得双光子情形下基于ETC的AS、SR、FWM光谱具有极高的分辨率。基于ETC高分辨率光谱可用于同位素含量、食物中的有害成分、原子/分子的化学反应过程、生物领域大分子的活动等的监测,以及各类薄膜的制作过程的监测和控制等。     

Direct frequency comb spectroscopy in the extreme ultraviolet

The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40?nanometres, by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82?nanometres and a neon transition at 63?nanometres, thus confirming the combs' coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100?nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium, the development of next-generation 'nuclear' clocks and searches for variation of fundamental constants using the enhanced sensitivity of highly charged ions.     

M（CH3 OH）1~4（M=Be2+/Mg2+/Ca2+）团簇的量子化学研究

应用量子化学方法 B3LYP/6-311+G(d,p),对气相团簇M(CH3OH)1~4(M=Be2+/Mg2+/Ca2+)的结构、能量进行计算,采用HF/STO-3G方法对电荷分布进行研究.结果表明,甲醇分子通过O原子配位于金属离子形成最稳定结构,金属与CH3OH中的O的距离由大到小为RCa-ORMg-ORBe-O,而结合能由大到小依次为Be2+Mg2+Ca2+.随着甲醇分子数n的增加,金属离子与O的距离RM-O逐渐增大,总结合能逐渐增大,连续结合能逐渐减小,金属离子所带的正电荷在逐渐减小,对于相同配位数n,Be2+的电荷转移量最大,Ca2+的电荷转移量最小;甲醇分子中各原子所得正电荷的大小顺序为H(-HO)H(-H3C)CO.     

Allowed and forbidden transitions in artificial hydrogen and helium atoms

The strength of radiative transitions in atoms is governed by selection rules that depend on the occupation of atomic orbitals with electrons. Experiments have shown similar electron occupation of the quantized energy levels in semiconductor quantum dots--often described as artificial atoms. But unlike real atoms, the confinement potential of quantum dots is anisotropic, and the electrons can easily couple with phonons of the material. Here we report electrical pump-and-probe experiments that probe the allowed and 'forbidden' transitions between energy levels under phonon emission in quantum dots with one or two electrons (artificial hydrogen and helium atoms). The forbidden transitions are in fact allowed by higher-order processes where electrons flip their spin. We find that the relaxation time is about 200 micro s for forbidden transitions, 4 to 5 orders of magnitude longer than for allowed transitions. This indicates that the spin degree of freedom is well separated from the orbital degree of freedom, and that the total spin in the quantum dots is an excellent quantum number. This is an encouraging result for potential applications of quantum dots as basic entities for spin-based quantum information storage.     

Evidence for Efimov quantum states in an ultracold gas of caesium atoms

Systems of three interacting particles are notorious for their complex physical behaviour. A landmark theoretical result in few-body quantum physics is Efimov's prediction of a universal set of bound trimer states appearing for three identical bosons with a resonant two-body interaction. Counterintuitively, these states even exist in the absence of a corresponding two-body bound state. Since the formulation of Efimov's problem in the context of nuclear physics 35 years ago, it has attracted great interest in many areas of physics. However, the observation of Efimov quantum states has remained an elusive goal. Here we report the observation of an Efimov resonance in an ultracold gas of caesium atoms. The resonance occurs in the range of large negative two-body scattering lengths, arising from the coupling of three free atoms to an Efimov trimer. Experimentally, we observe its signature as a giant three-body recombination loss when the strength of the two-body interaction is varied. We also detect a minimum in the recombination loss for positive scattering lengths, indicating destructive interference of decay pathways. Our results confirm central theoretical predictions of Efimov physics and represent a starting point with which to explore the universal properties of resonantly interacting few-body systems. While Feshbach resonances have provided the key to control quantum-mechanical interactions on the two-body level, Efimov resonances connect ultracold matter to the world of few-body quantum phenomena.