Inductive shielding of NMR phase noise

We report on a solution to the problem of phase noise in nuclear magnetic resonance (NMR) experiments. Phase noise refers to the variation in the phases of NMR signals from successive acquisitions due to an unstable applied field. Such a situation exists in high-field resistive Bitter magnets and, for sufficiently long timescales, can cause serious signal degradation upon signal averaging. An inductive shield, formed by a highly conducting metal tube placed around the sample and along the applied field, provides screening of the AC components of the applied field and thereby retains phase coherence over long periods. Although simple in principle there are technical difficulties for practical implementation of this method. We present demonstrations of the utility of this approach. In particular, we show a significant extension of the effective transverse coherence time of the ¹³C resonance in doubly ¹³C-labeled glycerol in a resistive Bitter magnet. This was accomplished through the use of a highly conducting aluminum shield, cooled to 4 K with liquid helium.     

Quantum dynamics of Gaudin magnets

Quantum dynamics of many-body systems is a fascinating and significant subject for both theory and experiment. The question of how an isolated many-body system evolves to its steady state after a sudden perturbation or quench still remains challenging. In this paper, using the Bethe ansatz wave function, we study the quantum dynamics of an inhomogeneous Gaudin magnet. We derive explicit analytical expressions for various local dynamic quantities with an arbitrary number of flipped bath spins, such as: the spin distribution function, the spin–spin correlation function, and the Loschmidt echo. We also numerically study the relaxation behavior of these dynamic properties, gaining considerable insight into coherence and entanglement between the central spin and the bath. In particular, we find that the spin–spin correlations relax to their steady value via a nearly logarithmic scaling, whereas the Loschmidt echo shows an exponential relaxation to its steady value. Our results advance the understanding of relaxation dynamics and quantum correlations of long-range interacting models of the Gaudin type.     

低相噪光学频率梳

光学频率梳是光钟的重要组成部分,相噪是光梳的基本特性。文章从光梳相噪的描述方法入手,对光学频率梳相噪产生的几种常见来源及相噪抑制技术进行了综述,详细介绍了业已实现的几种低相噪光学频率梳。文末展望了未来低相噪光学频率梳的发展新趋势。     

Magnon bands in twisted bilayer honeycomb quantum magnets

We study the magnon bands of twisted bilayer honeycomb quantum magnets using linear spin wave theory. Although the interlayer coupling can be ferromagnetic or antiferromagnetic, we keep the intralayer one ferromagnetic to avoid possible frustration. For the interlayer ferromagnetic case, we find the magnon bands have similar features with the corresponding electronic energy spectrums. Although the linear dispersions near the Dirac points are preserved in the magnon bands of twisted bilayer magnets, their slopes are reduced with the decrease of the twist angles. On the other hand, the interlayer antiferromagnetic couplings generate quite different magnon spectra. The two single-layered magnon spectra are usually decoupled due to the opposite orientations of the spins in the two layers. We also develop a low-energy continuous theory for very small twist angles, which has been verified to fit well with the exact tight-binding calculations. Our results may be experimentally observed due to the rapid progress in two-dimensional magnetic materials.     

Spin-dependent negative differential conductance in transport through single-molecule magnets

Transport properties are theoretically studied through an anisotropy single-molecule magnet symmetrically connected to two identical ferromagnetic leads. It is found that even though in parallel configuration of leads’ magnetizations, the total current still greatly depends on the spin polarization of leads at certain particular bias region, and thus for large polarization a prominent negative differential conductance (NDC) emerges. This originates from the joint effect of single-direction transitions and spin polarization, which removes the symmetry between spin-up and spin-down transitions. The present mechanism of NDC is remarkably different from the previously reported mechanisms. To clarify the physics of the NDC, we further monitored the shot noise spectroscopy and found that the appearance of the NDC is accompanied by the rapid decrease of Fano factor.     

激光的相位噪声特性以及抑制方法研究

回顾了到目前为止激光相位噪声的研究成果,阐述了由经典模型和弛豫振荡导致的在线宽展宽影响下的激光线宽。提出了自发辐射放大会对激光谱线产生进一步影响的问题。在噪声的抑制方面,分析了电子介入系统锁相方案的锁相效果。通过比较已锁相激光和未锁相激光与理想激光之间的互相关函数,结果表明,成功的锁相会使锁相激光的相干长度扩展到无限长。     

远程干涉型光纤传感系统的非线性相位噪声分析

以远程干涉型光纤传感系统为背景, 研究了系统非线性相位噪声构成, 对各构成要素的具体影响进行了详细分析和综合评价, 简要讨论了噪声抑制方案. 研究表明, 系统相位噪声主要包括强度噪声转化而来的相位噪声、非线性效应引起激光 线宽展宽导致的相位噪声以及自相位调制和交叉相位调制引入的相位噪声. 受激布里渊散射和四波混频可引入强度噪声并转化为相位噪声, 对于探测带宽较窄的光纤传感系统, 四波混频引入的该部分噪声往往可以忽略. 受激布里渊散射、四波混频和调制不稳定性都可引起激光线宽展宽从而造成相位噪声的增大. 当系统信道数目较多时, 交叉相位调制对相位噪声的贡献不可忽略. 所得结论对远程干涉型光纤传感系统的实际应用具有重要的指导意义.     

Stimulated Brillouin scattering-induced phase noise in an interferometric fiber sensing system

Stimulated Brillouin scattering-induced phase noise is harmful to interferometric fiber sensing systems. The localized fluctuating model is used to study the intensity noise caused by the stimulated Brillouin scattering in a single-mode fiber. The phase noise structure is analyzed for an interferometric fiber sensing system, and an unbalanced Michelson interferometer with an optical path difference of 1 m, as well as the phase-generated carrier technique, is used to measure the phase noise. It is found that the phase noise is small when the input power is below the stimulated Brillouin scattering threshold, increases dramatically at first and then gradually becomes flat when the input power is above the threshold, which is similar to the variation in relative intensity noise. It can be inferred that the increase in phase noise is mainly due to the broadening of the laser linewidth caused by stimulated Brillouin scattering, which is verified through linewidth measurements in the absence and presence of the stimulated Brillouin scattering.     

Stimulated Brillouin scattering-induced phase noise in an interferometric fiber sensing system

Stimulated Brillouin scattering-induced phase noise is harmful to interferometric fiber sensing systems.The localized fluctuating model is used to study the intensity noise caused by the stimulated Brillouin scattering in a single-mode fiber.The phase noise structure is analyzed for an interferometric fiber sensing system,and an unbalanced Michelson interferometer with an optical path difference of 1 m,as well as the phase-generated carrier technique,is used to measure the phase noise.It is found that the phase noise is small when the input power is below the stimulated Brillouin scattering threshold,increases dramatically at first and then gradually becomes flat when the input power is above the threshold,which is similar to the variation in relative intensity noise.It can be inferred that the increase in phase noise is mainly due to the broadening of the laser linewidth caused by stimulated Brillouin scattering,which is verified through linewidth measurements in the absence and presence of the stimulated Brillouin scattering.     

微米气室铯原子自旋噪声谱

利用自旋噪声谱技术研究了无缓冲气体133Cs原子气室的自旋动力学和展宽机制.在宏观原子气室中,自旋弛豫速率失谐频率谱的线型为高斯分布;在空间局域较强的微米气室中,自旋弛豫速率失谐频率谱的线型为洛伦兹分布.实验测量得到的自旋弛豫速率失谐频率谱的展宽约4 GHz,明显大于宏观原子气室中约度强烈依赖于激光相对于原子共振跃迁的频率失谐;在微米气室中,由于较强的均匀展宽,总噪声的失谐频率谱中心处出现明显的凹陷.通过建立简化的物理模型来计算微米气室的展宽机制,在实验与理论中解释了原子的均匀展宽特性.     

On the role of shot noise in carrier‐envelope phase stabilization

This paper represents a systematic investigation of detection shot noise in carrier‐envelope phase (CEP) stabilization. Numerical simulations are conducted to calculate the influence of shot noise in laser oscillators. These results are compared with experimental results for Ti:sapphire lasers. It is found that shot noise imposes a limitation for obtaining sub‐100 mrad CEP jitters. Careful interferometer design is necessary to push this limit toward 10 mrad. In contrast to oscillator stabilization, shot noise appears to play a much more restrictive role in amplifier stabilization. Using spectral interferometry together with spectral broadening in sapphire, it already appears practically challenging to reach sub‐100 mrad jitters. Adaption of the optical nonlinearity in the broadening step appears key to further improvements of the CEP jitter of amplified systems. We believe that these improvements open a perspective for currently unfeasible applications of CEP stabilized pulses. Moreover, our considerations can be easily adapted to CEP stabilization of other laser types beyond Ti:sapphire.     

C波段低相位噪声高温超导介质振荡器

高温超导技术在现代军用、民用电子系统中的应用日益广泛,文中的设计应用超导技术制作了一个振荡器,该振荡器具有极低的相位噪声,在77K温区,该振荡器工作频率5.012GHz,偏离载频1kHz处相位噪声达-129.8dBc/Hz.     

小波变换重建超短脉冲光谱相位的误差分析

分析了噪声对小波变换重建超短脉冲光谱相位的影响. 在理想情况和附加噪声、倍增噪声， 以及干涉条纹采样量化干扰的情况下，分析了小波变换对几种光谱相位的重建精度. 结果显 示，在理想情况和噪声干扰的情况下，小波变换的相位重建精度都在003rad范围以内. 这 说明小波变换是一种精确可信的光谱相位重建方法，具有很强的抗噪声干扰能力. 另外，将 小波变换的轨迹图类比SHG FROG的轨迹图，可以直接定性判断脉冲的啁啾特征. 关键词： 超短脉冲 相位重建 小波变换 噪声     

强泵浦条件下光纤放大器相位噪声的测量

利用多抖动法对强泵浦条件下光纤放大器引入的相位噪声进行了测量。利用功率谱、积分谱对具体泵浦条件下的相位噪声特性进行定性分析,结果表明,泵浦功率越大,高频成分占的比例越大。利用结构函数定量分析相位噪声的变化特性,得出相位噪声控制系统的带宽需求指标。实验测得在71,181和230 W泵浦条件下,所需相位控制系统带宽最小值分别为830 Hz,3.1 kHz和10 kHz。     

Single molecular shuttle-junction: Shot noise and decoherence

Single molecular shuttle-junction is one kind of nanoscale electromechanical tunneling system. In this junction, a molecular island oscillates depending on its charge occupation, and this charge dependent oscillation leads to modulation of electron tunneling through the molecular island. This paper reviews recent development on the study of current, shot noise and decoherence of electrons in the single molecular shuttle-junction. We will give detailed discussion on this topic using the typical system model, the theory of fully quantum master equation and the Aharonov–Bohm interferometer.     

高场核磁共振波谱仪频率合成器设计

提出了一种基于锁相环(Phase-locked Loop, PLL)与直接数字合成(Direct Digital Synthesis, DSS)等技术相结合的高场核磁共振波谱仪频率合成器设计方案．该系统以单片机为控制器完成算法运行、参数配置和CAN 总线通信功能,运用PLL 技术和DDS 技术相结合的频率合成方案,通过两次混频,使频率粗调和细调灵活可控,实现宽带低噪声频率输出．将该频率合成器用于自主研制开发的核磁共振波谱仪上进行实验验证,测试得到的线形和灵敏度均达到指标要求,结果证明该设计方案具有可行性．     

300 MHz核磁共振频率源的研制

介绍了基于Analog Device公司AD9954设计的数字化频率源,可以实现宽带高频（5 MHz~125 MHz、280 MHz~301 MHz）、高稳定度（1×10^-9）、低相位噪声（301 MHz,≤-127 dBc/Hz@250 kHz offset）频率输出以及频率、相位和幅度的高速切换. 该数字化频率源可应用于300 MHz核磁共振谱仪和磁共振成像仪,可部分取代昂贵的进口频率源. 本文最后给出核磁实验结果.     

开设核磁共振实验的探讨

针对目前高校理科物理专业近代物理实验中核磁共振实验开放现状的不足之处，提出了在核磁共振基本实验中增加脉冲核磁共振实验内容的设想，并在分析了该设想可行性的基础上设计了实验方案。     

Artifacts introduced by zero order phase correction in proton NMR spectroscopy and a method of elimination by phase filtering

In in-vivo applications of proton NMR spectroscopic imaging, an oscillatory "ringing" artifact has been observed in some of the spectra. The source of this artifact was found to be the presence of a harmonic "beating" effect in the amplitude of the water reference free induction decay (FID) which was used for zero order phase correction for B0 inhomogeneity and eddy current compensation. The source of the beats was found to be the presence of distinct populations of spins resonating at slightly different frequencies. When the common method of zero order phase correction was implemented using such an FID, the resulting phase-corrected, water-suppressed spectra displayed ringing. Examination of the unwrapped phase correction angle revealed unexpected jumps in phase at points in time corresponding to nodes in the amplitude of the FID. Low-pass filtering of the phase correction angle of the reference FID was found to smooth out these unanticipated phase jumps. When used as a reference for phase correction, the filtered phase information gave a phase-corrected, water-suppressed spectrum free from ringing.