Optical frequency standard based on a single 40Ca+

Research on the development of the optical frequency standard based on trapped and cold 40 Ca+ with the 4s2S1/2-3d2D5/2 clock transition at 729nm is reported. A single calcium ion was trapped and laser cooled in the Paul trap and stay in trap for more than 15 days. The linewidth of a 729nm laser was reduced to less than 10Hz by locking to a cavity for longer than 50 hours uninterruptedly. The overall systematic uncertainty of the clock transition has been characterized to be better than 6.5×10 16 . The absolute frequency of the clock transition was measured at 10 15 level using an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the global positioning system (GPS). The frequency value was 411042129776393.0(1.6)Hz after the correction of the systematic shifts.     

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.     

Optical frequency metrology

Extremely narrow optical resonances in cold atoms or single trapped ions can be measured with high resolution. A laser locked to such a narrow optical resonance could serve as a highly stable oscillator for an all-optical atomic clock. However, until recently there was no reliable clockwork mechanism that could count optical frequencies of hundreds of terahertz. Techniques using femtosecond-laser frequency combs, developed within the past few years, have solved this problem. The ability to count optical oscillations of more than 1015 cycles per second facilitates high-precision optical spectroscopy, and has led to the construction of an all-optical atomic clock that is expected eventually to outperform today's state-of-the-art caesium clocks.     

A frequency comb in the extreme ultraviolet

Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency 'combs' (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated 'oscilloscope' for light waves. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.     

Molecular fingerprinting with the resolved modes of a femtosecond laser frequency comb

The control of the broadband frequency comb emitted from a mode-locked femtosecond laser has permitted a wide range of scientific and technological advances--ranging from the counting of optical cycles for next-generation atomic clocks to measurements of phase-sensitive high-field processes. A unique advantage of the stabilized frequency comb is that it provides, in a single laser beam, about a million optical modes with very narrow linewidths and absolute frequency positions known to better than one part in 10(15) (ref. 5). One important application of this vast array of highly coherent optical fields is precision spectroscopy, in which a large number of modes can be used to map internal atomic energy structure and dynamics. However, an efficient means of simultaneously identifying, addressing and measuring the amplitude or relative phase of individual modes has not existed. Here we use a high-resolution disperser to separate the individual modes of a stabilized frequency comb into a two-dimensional array in the image plane of the spectrometer. We illustrate the power of this technique for high-resolution spectral fingerprinting of molecular iodine vapour, acquiring in a few milliseconds absorption images covering over 6 THz of bandwidth with high frequency resolution. Our technique for direct and parallel accessing of stabilized frequency comb modes could find application in high-bandwidth spread-spectrum communications with increased security, high-resolution coherent quantum control, and arbitrary optical waveform synthesis with control at the optical radian level.     

Frequency Calibration and Stabilization of the Cooling Laser of Ytterbium Lattice Clock with Molecular Iodine Spectroscopy

In order to apply the frequency comparison among the different optical clocks, transportable optical clocks(TOCs) is suggested as a valuable tool. For transportable clocks, the key technique is how to lock the lasers involved in the whole system. In this paper, we carry out a new way to calibrate and lock the second stage cooling laser of the ytterbium lattice clock with molecular iodine spectroscopy. The locking spectral width of 556 nm laser can be narrowed to 195 kHz, which can be used for frequency control and the stabilization of the second stage cooling. So the new method will make the transportable optical clocks more compact and robust in the future.     

光波干涉采样示波器

光频梳技术是本世纪初随着飞秒激光技术、非线性光学的飞速发展应运而起的一项重要的精密频率测量技术.它跨越了射频和光频电磁波谱之间长期存在的技术鸿沟,通过频率和相位的相干关系将二者巧妙的衔接,在时间频率标准、精密光谱计量以及基础物理学常数的高精度测量等领域具有举足轻重的应用价值.正因为The-odor W.H?nsch(本文第一作者)和John L.Hall以光频梳技术为代表的激光精密测量技术领域的杰出贡献,二人同Roy J.Glauber(光相干量子理论的贡献)共同分享了2005年诺贝尔物理学奖.从光频梳的基本原理出发,解释了光频梳技术基本概念,稳定工作的光频梳以及作为高精密的频率测量技术,光频梳主要的应用领域等.同时回顾了作者研发光频梳技术的初衷,即用于研究氢原子的精密光谱,验证量子电动力学理论,提高里德堡常数和质子电荷半径等物理学基本常数体系的测量精度.介绍了通过双光频梳实现高精密的光谱直接测量技术,包括该技术的原理以及在气体分子精密光谱测量方面的应用,举例说明了基于双光频梳的光谱直接测量技术在频率分辨率和灵敏度方面相对于传统光频梳表现出来的显著优势.实验验证了单光子水平的光频梳光谱测量可行性,指出其在极紫外或甚至在软x射线这些产生少量光子的应用领域的应用前景.最后介绍了微小区域实现厘米级尺寸的光学腔的脉冲激光的激发、片上稳定的双光频梳实现等微型光频梳和片上光谱系统集成等关键技术和解决方案,并分析了片上光频梳光谱测量系统的发展和应用前景.     

囚禁在光格中的波色-爱因斯坦凝聚体孤子

利用数值模拟解Gross-Pitaevskii方程来研究囚禁在光格中的波色-爱因斯坦凝聚体明孤子，发现周期性光格可被用作为可控的物质波分裂器，其分裂效果与孤子在光格中的初始位置、势阱中的原子数、以及光格电势的幅度、波矢有关。     

Optical frequency comb generation from a monolithic microresonator

Optical frequency combs provide equidistant frequency markers in the infrared, visible and ultraviolet, and can be used to link an unknown optical frequency to a radio or microwave frequency reference. Since their inception, frequency combs have triggered substantial advances in optical frequency metrology and precision measurements and in applications such as broadband laser-based gas sensing and molecular fingerprinting. Early work generated frequency combs by intra-cavity phase modulation; subsequently, frequency combs have been generated using the comb-like mode structure of mode-locked lasers, whose repetition rate and carrier envelope phase can be stabilized. Here we report a substantially different approach to comb generation, in which equally spaced frequency markers are produced by the interaction between a continuous-wave pump laser of a known frequency with the modes of a monolithic ultra-high-Q microresonator via the Kerr nonlinearity. The intrinsically broadband nature of parametric gain makes it possible to generate discrete comb modes over a 500-nm-wide span (approximately 70 THz) around 1,550 nm without relying on any external spectral broadening. Optical-heterodyne-based measurements reveal that cascaded parametric interactions give rise to an optical frequency comb, overcoming passive cavity dispersion. The uniformity of the mode spacing has been verified to within a relative experimental precision of 7.3 x 10(-18). In contrast to femtosecond mode-locked lasers, this work represents a step towards a monolithic optical frequency comb generator, allowing considerable reduction in size, complexity and power consumption. Moreover, the approach can operate at previously unattainable repetition rates, exceeding 100 GHz, which are useful in applications where access to individual comb modes is required, such as optical waveform synthesis, high capacity telecommunications or astrophysical spectrometer calibration.     

非对称囚禁二维玻色-爱因斯坦凝聚孤子的演化

利用变分法解G ross-P itaevsk ii方程,研究了囚禁在各向异性势阱中的二维饼状玻色-爱因斯坦凝聚体(BEC)孤子的演化规律,发现通过在圆柱形对称的磁阱中的某一方向引入光格电势,不仅使BEC孤子在该方向趋向稳定,而且通过相互耦合作用也能影响其它方向从而使孤子的膨胀变慢,使BEC孤子的稳定性增加。其效果与势阱中的原子数、势阱系数和光格参数有关。     

Quantum information and many body physics with cold atoms

We review our recent theoretical advances in quantum information and many body physics with cold atoms in various external potential, such as harmonic potential, kagome optical lattice, triangular optical lattice, and honeycomb lattice. The many body physics of cold atom in harmonic potential is investigated in the frame of mean-field Gross-Pitaevskii equation. Then the quantum phase transition and strongly correlated effect of cold atoms in triangular optical lattice, and the interacting Dirac fermions on honeycomb lattice, are investigated by using cluster dynamical mean-field theory and continuous time quantum Monte Carlo method. We also study the quantum spin Hall effect in the kagome optical lattice.     

Cs法拉第反常色散光学滤波应用于边缘滤波器特性分析

提出将Cs法拉第反常色散光学滤波应用于边缘滤波器,以弥补利用127I2和129I2分子的共振跃迁实现分子边缘滤波器的不足.对Cs 6S1/27P1/2法拉第反常色散谱的滤波特性、频率调谐特性进行了理论分析.结果表明:Cs原子的法拉第反常色散谱对称地存在2个陡的线翼透射峰,通过调节气室温度和外加磁场强度,线翼透射峰中心频率可以被调节到适当的位置,以满足边缘滤波器的实际需要.     

A steady-state superradiant laser with less than one intracavity photon

The spectral purity of an oscillator is central to many applications, such as detecting gravity waves, defining the second, ground-state cooling and quantum manipulation of nanomechanical objects, and quantum computation. Recent proposals suggest that laser oscillators which use very narrow optical transitions in atoms can be orders of magnitude more spectrally pure than present lasers. Lasers of this high spectral purity are predicted to operate deep in the 'bad-cavity', or superradiant, regime, where the bare atomic linewidth is much less than the cavity linewidth. Here we demonstrate a Raman superradiant laser source in which spontaneous synchronization of more than one million rubidium-87 atomic dipoles is continuously sustained by less than 0.2 photons on average inside the optical cavity. By operating at low intracavity photon number, we demonstrate isolation of the collective atomic dipole from the environment by a factor of more than ten thousand, as characterized by cavity frequency pulling measurements. The emitted light has a frequency linewidth, measured relative to the Raman dressing laser, that is less than that of single-particle decoherence linewidths and more than ten thousand times less than the quantum linewidth limit typically applied to 'good-cavity' optical lasers, for which the cavity linewidth is much less than the atomic linewidth. These results demonstrate several key predictions for future superradiant lasers, which could be used to improve the stability of passive atomic clocks and which may lead to new searches for physics beyond the standard model.     

用于北斗三号卫星导航系统的星载铷原子钟特性

本文介绍用于北斗三号卫星导航系统的星载铷原子钟主要设计特点和性能指标.铷原子钟频率稳定度主要取决于原子跃迁信号信噪比、电路噪声和原子体系的物理环境效应.为提高原子信号信噪比,物理系统采用了微波场方向因子高于0.9的开槽管微波腔和Xe气启辉的铷光谱灯,并采用了光学滤光和同位素滤光双重滤光方案.电路系统采用了低相噪微波链路,交互调制噪声对铷原子钟稳定度的影响被控制在4.9×10^-13/τ^1/2水平.通过工作参数优化,将物理环境效应对天频率稳定度的影响降低到3×10^-15以下.研制了高精度和甚高精度两型号星载铷原子钟.高精度铷原子钟典型指标为短稳1.5×10^-12/τ^1/2,万秒稳1.3×10^?14,天稳9.4×10^-15;甚高精度铷原子钟典型指标为短稳6.1×10^-13/τ^1/2,万秒稳7.1×10^-15,天稳3.9×10^-15.本文还分析了铷原子钟最新研究进展,预期铷原子钟的性能还可以进一步提升.     

Controlled exchange interaction between pairs of neutral atoms in an optical lattice

Ultracold atoms trapped by light offer robust quantum coherence and controllability, providing an attractive system for quantum information processing and for the simulation of complex problems in condensed matter physics. Many quantum information processing schemes require the manipulation and deterministic entanglement of individual qubits; this would typically be accomplished using controlled, state-dependent, coherent interactions among qubits. Recent experiments have made progress towards this goal by demonstrating entanglement among an ensemble of atoms confined in an optical lattice. Until now, however, there has been no demonstration of a key operation: controlled entanglement between atoms in isolated pairs. Here we use an optical lattice of double-well potentials to isolate and manipulate arrays of paired (87)Rb atoms, inducing controlled entangling interactions within each pair. Our experiment realizes proposals to use controlled exchange coupling in a system of neutral atoms. Although 87Rb atoms have nearly state-independent interactions, when we force two atoms into the same physical location, the wavefunction exchange symmetry of these identical bosons leads to state-dependent dynamics. We observe repeated interchange of spin between atoms occupying different vibrational levels, with a coherence time of more than ten milliseconds. This observation demonstrates the essential component of a neutral atom quantum SWAP gate (which interchanges the state of two qubits). Its 'half-implementation', the root SWAP gate, is entangling, and together with single-qubit rotations it forms a set of universal gates for quantum computation.     

GPS卫星钟差分析、建模及仿真

卫星钟差是卫星导航系统中的一项重要误差源,本文根据IGS观测数据分析GPS卫星铷钟和铯钟的噪声特性,并对星钟误差进行建模和仿真.首先采用修正Allan方差表征和分析了GPS卫星上不同种类原子钟随机噪声的时域特性;然后基于最小二乘分段拟合确定了卫星钟差确定性分量的时差、频差、线性频漂参数值;对于多项式拟合后的残差部分,利用随机噪声幂律谱模型建立了随机性分量误差模型;最后利用修正Allan方差反演的方法模拟产生了各噪声分量的独立随机序列并进行合成;对合成序列与原始数据进行了对比分析以验证本文所提出建模与仿真方法的正确性.     

Controlled collisions for multi-particle entanglement of optically trapped atoms

Entanglement lies at the heart of quantum mechanics, and in recent years has been identified as an essential resource for quantum information processing and computation. The experimentally challenging production of highly entangled multi-particle states is therefore important for investigating both fundamental physics and practical applications. Here we report the creation of highly entangled states of neutral atoms trapped in the periodic potential of an optical lattice. Controlled collisions between individual neighbouring atoms are used to realize an array of quantum gates, with massively parallel operation. We observe a coherent entangling-disentangling evolution in the many-body system, depending on the phase shift acquired during the collision between neighbouring atoms. Such dynamics are indicative of highly entangled many-body states; moreover, these are formed in a single operational step, independent of the size of the system.     

基于第一性原理计算OsB_2的点阵动力学

利用基于密度泛函理论平面波赝势法的第一性原理计算,研究过渡金属化合物OsB2的点阵动力学.结果表明所有的带心光学声子模对应的原子都沿c轴方向振动,低频拉曼活性模式由重原子Os运动产生,重原子Os对电声耦合作用的贡献最大,而总的电声耦合作用较弱,说明OsB2是弱的电声耦合超导体.     

Evidence for superfluidity of ultracold fermions in an optical lattice

The study of superfluid fermion pairs in a periodic potential has important ramifications for understanding superconductivity in crystalline materials. By using cold atomic gases, various models of condensed matter can be studied in a highly controllable environment. Weakly repulsive fermions in an optical lattice could undergo d-wave pairing at low temperatures, a possible mechanism for high temperature superconductivity in the copper oxides. The lattice potential could also strongly increase the critical temperature for s-wave superfluidity. Recent experimental advances in bulk atomic gases include the observation of fermion-pair condensates and high-temperature superfluidity. Experiments with fermions and bosonic bound pairs in optical lattices have been reported but have not yet addressed superfluid behaviour. Here we report the observation of distinct interference peaks when a condensate of fermionic atom pairs is released from an optical lattice, implying long-range order (a property of a superfluid). Conceptually, this means that s-wave pairing and coherence of fermion pairs have now been established in a lattice potential, in which the transport of atoms occurs by quantum mechanical tunnelling and not by simple propagation. These observations were made for interactions on both sides of a Feshbach resonance. For larger lattice depths, the coherence was lost in a reversible manner, possibly as a result of a transition from superfluid to insulator. Such strongly interacting fermions in an optical lattice can be used to study a new class of hamiltonians with interband and atom-molecule couplings.