Precision spectroscopy with a single ~(40)Ca~+ ion in a Paul trap

Precision measurement of the 4s2S1/2–3d2D5/2clock transition based on40Ca+ion at 729 nm is reported. A single40Ca+ion is trapped and laser-cooled in a ring Paul trap, and the storage time for the ion is more than one month. The linewidth of a 729 nm laser is reduced to about 1 Hz by locking to a super cavity for longer than one month uninterruptedly.The overall systematic uncertainty of the clock transition is evaluated to be better than 6.5×10-16. The absolute frequency of the clock transition is measured at the 10-15 level by using an optical frequency comb referenced to a hydrogen maser which is calibrated to the SI second through the global positioning system(GPS). The frequency value is 411 042 129776 393.0(1.6) Hz with the correction of the systematic shifts. In order to carry out the comparison of two40Ca+optical frequency standards, another similar40Ca+optical frequency standard is constructed. Two optical frequency standards exhibit stabilities of 1×10-14τ-1/2with 3 days of averaging. Moreover, two additional precision measurements based on the single trapped40Ca+ion are carried out. One is the 3d2D5/2state lifetime measurement, and our result of 1174(10) ms agrees well with the results reported in [Phys. Rev. A 62 032503(2000)] and [Phys. Rev. A 71 032504(2005)]. The other one is magic wavelengths for the 4s2S1/2–3d2D5/2clock transition; λ|m j|=1/2= 395.7992(7) nm and λ|m j|=3/2=395.7990(7) nm are reported, and it is the first time that two magic wavelengths for the40Ca+clock-transition have been reported.     

Secular Motion Frequencies of ~9Be~+ Ions and ~(40)Ca~+ Ions in Bi-component Coulomb Crystals

We obtain bi-component Coulomb crystals using laser-cooled ~(40)Ca~+ ions to sympathetically cool ~9Be~+ ions in a linear Paul trap. The shell structures of the bi-component Coulomb crystals are investigated. The secular motion frequencies of the two different ions are determined and compared with those in the single-component Coulomb crystals. In the radial direction, the resonant motion frequencies of the two ionic species shift toward each other due to the strong motion coupling in the ion trap. In the axial direction, the motion frequency of the laser-cooled~(40)Ca~+is impervious to the sympathetically cooled ~9Be~+ ions because the spatially separation of the two different ionic species leads to the weak motion coupling in the axial direction.     

Relationship measurement between ac-Stark shift of ~(40)Ca~+ clock transition and laser polarization direction

Ac-Stark shift of atom levels is caused by an ac-electromagnetic field. As an electromagnetic wave, laser light does induce ac-Stark shift. It is proved experimentally that if the light is linearly polarized, the dynamic polarizability changes with polarization direction. The polarization direction of the linearly-polarized laser is tuned by 720?, and the ac-Stark shifts of the 4S_(1/2),m=±1/2→3D_(5/2,m=±1/2) clock transitions in ~(40)Ca~+ are measured in steps of 10?. The frequency shifts change with laser polarization in a periodical manner and have values opposite to each other.     

Sympathetic Cooling of ~(40)Ca~(+_27)Al~+ Ion Pair Crystal in a Linear Paul Trap

The ~(27)Al~+ ion optical clock is one of the most attractive optical clocks due to its own advantages such as low black-body radiation shift at room temperature and insensitivity to the magnetic drift.However,it cannot be laser-cooled directly in the absence of 167 nm laser to date.This problem can be solved by sympathetic cooling.In this work,a linear Paul trap is used to trap both ~(40)Ca~+ and ~(27)Al~+ ions simultaneously,and a single Dopplercooled ~(40)Ca~+ ion is employed to sympathetically cool a single ~(27)Al~+ ion.Thus a 'bright-dark' two-ion crystal has been successfully synthesized.The temperature of the crystal has been estimated to be about 7mK by measuring the ratio of carrier and sideband spectral intensities.Finally,the dark ion is proved to be an ~(27)Al~+ ion by precise measuring of the ion crystal's secular motion frequency,which means that it is a great step for our ~(27)Al~+ quantum logic clock.     

State Preparation in a Cold Atom Clock by Optical Pumping

We implement optical pumping to prepare cold atoms in our prototype of the ~(87)Rb space cold atom clock, which operates in the one-way mode. Several modifications are made on our previous physical and optical system. The effective atomic signal in the top detection zone is increased to 2.5 times with 87% pumping efficiency. The temperature of the cold atom cloud is increased by 1.4 μK. We study the dependences of the effective signal gain and pumping efficiency on the pumping laser intensity and detuning. The effects of σ,transition are discussed.This technique may be used in the future space cold atom clocks.     

高双折射光纤环镜改善40 GHz光时钟消光比的研究

时钟信号的好坏在同步、解复用和光判决中起着决定性的作用,双折射光纤环镜具有灵活可调的滤波特性,光时钟信号通过双折射环形镜可以使其消光比得到改善,从而提高时钟信号的质量。为此提出了利用高双折射光纤环形镜提高光时钟信号消光比的方案,数值上模拟了环形镜透射谱随双折射器件偏振延时量(Δt)、偏振控制器快慢轴角度以及偏振控制器所引入光程差的变化而变化的情况,并理论分析了消光比不理想的时钟信号经过环形镜后的光谱形式。用法布里-珀罗滤波器提取的40 GHz时钟信号进行实验,时钟经过环形镜后消光比改善了13 dB,实验结果和理论模拟得到了很好的匹配。     

Entrance-channel dynamics in the reaction ~(40)Ca+~(208)Pb

The entrance-channel dynamics including capture, fusion, and quasifission processes for the reaction ~(40)Ca+~(208)Pb is investigated in the fully microscopic time-dependent Hartree-Fock(TDHF) theory. The calculations are performed in three-dimensional Cartesian coordinate without any symmetry restrictions, in which the full Skyrme energy functional SLy4d and SLy5 are adopted.We study the energy dependence of capture cross sections, and find that the experimental data are well reproduced by the TDHF calculations. Both fusion and quasifission events are observed in the reaction ~(40)Ca+~(208)Pb. The contact time, mass and charge of quasifission fragments show a wide distribution in SLy4d compared with SLy5, implying that more nucleons are transferred in the SLy4d calculations. We find that the total kinetic energy of quasifission fragments in the TDHF calculations is distributed around Viola systematics, indicating that most of the relative kinetic energy is dissipated in quasifission dynamics.     

Realization of Closed-Loop Operation of Optical Lattice Clock Based on ~(171)Yb

We report the realization of closed-loop operation of an optical lattice clock based on ~(171)Yb atoms. We interrogate the ~1 S_0→~3 P_0 clock transition using single Rabi pulses of 578 nm laser light. The two π-transitions from mF=±1/2 ground states are alternatively interrogated,and the clock laser frequency is locked to the center of the two resonances. The in-loop error signal stability of the clock reaches 3×10~(-17) for an average time of 3500 s. We also perform interleaved operations of the clock with two independent servo loops, and the fractional frequency difference averages down to 2×10~(-16) in 7200 s.     

Hertz-Level Clock Spectroscopy of ~(171)Yb Atoms in a One-Dimensional Optical Lattice

An ultra-narrow spectroscopy of clock transition with high signal-to-noise ratio is crucial for a high-performance atomic optical clock.We present a detailed study about how to obtain a Hertz-level clock transition spectrum of~(171)Yb atoms.About 4 × 10~4 atoms are loaded into a one-dimensional optical lattice with a magic wavelength of 759 nm,and a long lifetime of 3 s is realized with the lattice power of 1 W.Through normalized shelving detection and spin polarization,~(171)Yb clock spectroscopy with a Fourier-limited linewidth of 5.9 Hz is obtained.Our work represents a key step toward an ytterbium optical clock with high frequency stability.     

Magic Wavelength Measurement of the ~(87)Sr Optical Lattice Clock at NIM

We report on the magic wavelength measurement of our optical lattice clock based on fermion strontium atoms at the National Institute of Metrology(NIM).A Ti:sapphire solid state laser locked to a reference cavity inside a temperature-stabilized vacuum chamber is employed to generate the optical lattice.The laser frequency is measured by an erbium fiber frequency comb.The trap depth is modulated by varying the lattice laser power via an acousto-optic modulator.We obtain the frequency shift coefficient at this lattice wavelength by measuring the differential frequency shift of the clock transition of the strontium atoms at different trap depths,and the frequency shift coefficient at this lattice wavelength is obtained.We measure the frequency shift coefficients at different lattice frequencies around the magic wavelength and linearly fit the measurement data,and the magic wavelength is calculated to be 368554672(44) MHz.     

A Common Optical Potential for ~4He+~(12)C at Intermediate Energies

A common optical potential for ~4He+~(12)C at intermediate bombarding energies,which is essential in analyzing exotic nuclei with ~4He clusters,is obtained based on the Sao Paulo potential.Among systematic optical potentials for ~4He+~(12)C,this potential has the merit of using a fixed imaginary part of the Woods-Saxon form.By opticalmodel calculations,this potential reproduces the experimental elastic scattering angular distributions of ~4He+~(12)C well within the energy range of 26A-60 A MeV.It is also applied successfully in calculations of the breakup reactions of ~6Li+~(12)C and ~6He+~(12)C with a three-body continuum discretized coupled-channel method.     

25MeV/u 40)Ar+209Bi裂变反应研究

实验对25MeV/u ⁴⁰Ar+²⁰⁹Bi体系的裂变反应,利用线性动量转移的分窗选择不同的激发能,研究裂变动能分布和质量分布与热核初始激发能的关系.实验证实激发能小于380MeV时裂变总动能分布与低激发能复合核相似.激发能大于380MeV时,最可几动能呈现出随激发增加而增加,并出现高能非对称性,而且质量分布宽度随激发能增加而迅速增大.     

Significantly Improving the Escape Time of a Single ~(40)Ca~+ Ion in a Linear Paul Trap by Fast Switching of the Endcap Voltage

Sympathetic cooling is a method used to lower the kinetic energy of ions with complicated energy-level structures,via Coulomb interactions with laser-cooled ions in an ion trap.The ion to be sympathetically cooled is sometimes prepared outside of the trap,and it is critical to introduce this ion into the trap by temporarily lowering the potential of one endcap without allowing the coolant ion to escape.We study the time required for a laser-cooled ion to escape from a linear Paul trap when the v...     

30MeV/u 40Ar+natAg反应中中等质量碎片的发射源

在6°─110°范围内测量了30MeV/u的⁴⁰Ar轰击^natAg靶产生的中等质量碎片(3≤Z≤16)能谱,利用运动源模型拟合能谱,并详细讨论了类弹、类靶和中速这三种源的特性及其随角度和中等质量碎片电荷的变化规律.从前角区中等质量碎片的符合测量中得到关联测得的两个中等质量碎片分别来自于两个发射源,即一个来自于类弹源,另一个来自于中速源的几率占绝大多数.     

30MeV/u 40Ar+58,64Ni、115In反应中IMF的元素分布

在5°—140°范围内测量了30MeV/u40Ar束流轰击不同靶核(58Ni、64Ni、115In)时出射的中等质量碎片的元素分布.在20°—80°之间,观察到了原子序数大于弹核原子库数的出射碎片(Z=3—24),发现中等质量碎片的微分截面在其原子库数从小于弹核原子库数向大于弹核原子库数变化过程中是连续变化的,该角区出就碎片的元素分布与弹核原子库数关系不大.对中能区核反应的过渡性质做了讨论,指出了探测阈对所测得的元素分布的影响,并讨论了元素分布的幂指数规律.     

30MeV/u 40Ar+58,64Ni和115In反应中中等质量碎片的角分布

在5°—140°范围内测量了30MeV/u ⁴⁰Ar束流轰击不同靶核(⁵⁸Ni、⁶⁴Ni、¹¹⁵In)后出射的中等质量碎片(IMF,Z=3—19)的角分布.用指数分布函数dσ/dΩ=N·exp(–θ/α)对出射碎片角分布分角区做了拟合.对3个反应系统分别提取出与相互作用时间有关的衰减因子α和与发射源强度有关的物理量N,讨论了α与N在不同角区与反应系统和出射IMF电荷数的关系.考察了角分布与同位旋自由度和反应系统对称性的关系.     

Predictions for cross sections of light proton-rich isotopes in the ~(40)Ca + ~9Be reaction

The cross sections for Z=10–19 with isotopes T_z=-3/2 to-5 in the 140A Me V~(40)Ca+~9Be projectile fragmentation reaction have been predicted.An empirical formula based on the correlation between the cross section and average binding energy of an isotope has been adopted to predict the cross section.The binding energies in the AME16,WS4,and the theoretical prediction by the spherical relativistic continuum Hartree-Bogoliubov theory have been used.Meanwhile,the fracs parametrization and the modified statistical abrasion-ablation model are also used to predict the cross sections for the proton-rich isotopes.The predicted cross sections for the T_z=-3 isotopes are close to 10~(-10)mb,which hopefully can be studied experimentally.In addition,based on the predicted cross sections,Z=14 is suggested to be a new magic number in the light proton-rich nuclei with T_z-3/2,for which the phenomenon is much more evident than it is from the average binding energy per nucleon.     

Simulation of the fission dynamics of the excited compound nuclei ~(206)Po and ~(168)Yb produced in the reactions ~(12)C+~(194)Pt and ~(18)O+~(150)Sm

A two-dimensional dynamical model based on the Langevin equation was used to study the fission dynamics of the compound nuclei ~(206)Po and ~(168)Yb produced in the reactions ~(12)C+~(194)Pt and ~(18)O+~(150)Sm,respectively.The fission cross section and average pre-scission neutron multiplicity were calculated for the compound nuclei ~(206)Po and ~(168)Yb,and results of the calculations compared with the experimental data.The elongation coordinate was used as the first dimension and the projection of the total spin of the compound nucleus onto the symmetry axis,K,considered as the second dimension in the Langevin dynamical calculations.In the two-dimensional calculations,a constant dissipation coefficient of K and a non-constant dissipation coefficient have been used to reproduce the abovementioned experimental data.It is shown that the two-dimensional Langevin equation can satisfactorily reproduce the fission cross section and average pre-scission neutron multiplicity for the compound nuclei ~(206)Po and ~(168)Yb by using constant values of the dissipation coefficient of K equal to γκ=0.18(MeV zs)~(-1/2) and γκ= 0.20(MeV zs)~(-1/2)for the compound nuclei ~(206)Po and ~(168)Yb,respectively.     

An Investigation of ~4He+~(12)C and ~4He+~(16)O Reactions Using the Cluster Model

The α-target semimicroscopic single folding potentials have been derived by folding a composite (repul-sive and attractive) effective α-α interaction with the α-cluster distribution density in the target nuclei. The obtained potentials are considered as the real part of the nuclear optical model potentials, while the imaginary parts are phe-nomenologicaly expressed using the Woods-Saxon form. Nine sets of measured experimental data of the 4He+12C and 4 He+16O elastic rainbow scattering over the energy range 80-240 MeV are analyzed using the obtained potentials. The data are successfully reproduced using the extracted potentials. The resulted reaction cross sections are also investigated and compared with the available corresponding data.     

p+~(58)Ni反应中集体态及转移机制的影响

在p+58Ni反应的理论计算中考虑了集体态和转移反应机制,利用大型程序MEND计算了Ein≤200MeV能区6种轻粒子(n,p,α,d,t及3He)出射的能谱和双微分截面。计算结果与实验数据基本一致;与以前的工作相比,解释了能谱和双微分截面中出现的尖峰,并减小了单粒子态密度修正因子ccg1(v)。