Atomic ion frequency standards

The history and status of trapped-ion frequency standards are reviewed. In a trapped-ion frequency standard, the frequency of an oscillator is servoed to a resonance which corresponds to a transition between two energy levels of an atomic ion. The ions are suspended in space by a combination of electric and magnetic fields. In a conventional rubidium cell, the atoms are surrounded by a buffer gas having a pressure of about 10³ Pa (approximately 10 torr). In an ion trap, the ions are held either in a vacuum or in a low-pressure buffer gas (less than 10^-3 Pa). In an atomic beam, the atoms also move through a vacuum, without collisions. However, the time available for interaction with the electromagnetic field is limited to their flight time through the apparatus, usually about 10 ms or less. Trapped ions can be observed for much longer periods. Consequently clocks based on ions trapped in electromagnetic fields portend orders-of-magnitude improvement in the development of new frequency standards. Prospects for future standards are discussed     

Quartz frequency standards

This paper reviews progress made over the past few years in the field of quartz crystal frequency standards. Several accomplishments which are thought to be of special importance are discussed in detail. These subjects include, among others, quartz vibrator enclosures, modes of motion including the "trapped energy" concept, and studies of quartz material. It has been demonstrated that the Q of synthetic quartz can be improved to a value equivalent to natural quartz. Progress made in the field of long-term stability of precision crystal units is described and the various reasons for aging of such units is discussed. The characteristics of precision crystal oscillators are reviewed, including temperature compensation and the influences of severe environment. The properties of high-precision crystal frequency standards are described with specific emphasis on aging, and the problem of short-term stability of such standards is discussed. Precision crystal standards are available today with a daily drift rate as low as a few parts in 10¹¹and a short-term stability better than a few parts in 10¹⁰for a period of one millisecond.     

A historical review of atomic frequency standards

An attempt is made to trace the historical development of the leading contenders in the atomic frequency standards field--cesium and thallium atomic beam devices, rubidium gas cell standard, and the hydrogen maser. Many of the important experiments leading to the development of techniques basic to the various types of standards, such as the magnetic resonance method, optical pumping, buffer gases and wall coatings, and maser techniques are briefly described. Finally, the application of these basic techniques to the development of the specific types of atomic standards is discussed.     

Laser-pumped rubidium frequency standards: new analysis andprogress

We have achieved a stability of 3·10^-13 τ ^-1/2 for 3<τ<30 s with a laser-pumped rubidium gas-cell frequency standard by reducing the effects due to noise in the microwave and laser sources. This result is one order of magnitude better than the best present performance of lamp-pumped devices     

A survey of research and standards in high-speed networks

This paper surveys research and standards in high-speed networks. It first classifies high-speed networks according to the area of coverage. Secondly, it discusses the switches. Then, it examines network management protocols: media access control, routeing, flow control and error control. Finally, it investigates current hig-speed network standards: DQDB, FDDI, ATM, and SONET.     

Optical frequency standards and measurements

We describe the performance characteristics and frequency measurements of two high-accuracy high-stability laser-cooled atomic frequency standards. One is a 657-nm (456-THz) reference using magneto-optically trapped Ca atoms, and the other is a 282-nm (1064-THz) reference based on a single Hg⁺ ion confined in an RF-Paul trap. A femtosecond mode-locked laser combined with a nonlinear microstructure fiber produces a broad and stable comb of optical modes that is used to measure the frequencies of the reference lasers locked to the atomic standards. The measurement system is referenced to the primary frequency standard NIST F-1, a Cs atomic fountain clock. Both optical standards demonstrate exceptional short-term instability (≈5×10^-15 at 1 s), as well as excellent reproducibility over time. In light of our expectations for the future of optical frequency standards, we consider the present performance of the femtosecond optical frequency comb, along with its limitations and future requirements     

An introduction to frequency standards

Quartz oscillators and rubidium, cesium, and hydrogen atomic frequency standards are described in terms of the physics of operation. Sources of frequency instability and inaccuracy are given. The author's intention is to provide a basis for characterization of the more practical aspects of commercially available frequency standards, and to present a comparison of such standards which may be useful in selecting devices for various applications. Emphasis is laid on stability performance of precision frequency standards, but some discussion is also given of other physical characteristics and cost considerations     

Laser-cooled neutral atom frequency standards

The authors review the prospects for a frequency standard based on what is called the atomic fountain. Atomic fountain frequency standards have better signal-to-noise ratios than ion standards, but suffer from larger systematic effects. An analysis of a Cs microwave fountain predicts potential stability and accuracy improvements of a factor of 100 over other laboratory standards, with an ultimate accuracy near 10 ^-16. An optical frequency standard based on a two-photon transition in xenon is analyzed and shown to have potential stability 5 orders of magnitude better than conventional Cs standards, and a possible accuracy of 10^-18. Neutral atom frequency standards will have much better signal-to-noise ratios than ion standards, but suffer from larger systematic effects, making the two technologies complementary in many ways     

Limitations of atomic beam frequency standards

Atomic beam frequency standards may be placed into two categories: field standards and laboratory standards. While this distinction is somewhat artificial, because the two types of standards are interdependent, each category does have different requirements of accuracy, size, and cost. Despite this separation, generally the developments which produce the best laboratory standards eventually give rise to improved field standards. Existing field standards are limited in long term fractional frequency stability to σ_y (τ) 3 x 10^-13, for τ 6 months. A laboratory standard such as NBS-6, the U.S. primary cesium standard, is limited in inaccuracy to Δ_y 8 x 10^-14. Proposed new cesium field standards are expected to yield long term stabilities of σ_y(τ) 1 x 10^-14 (τ = 6 months). Stored ion standards, prime candidates for new laboratory frequency standards, are expected to have better than Δy = 1 x 10^-15 inaccuracy. As other approaches to atomic beam frequency standards are considered, they should attempt to compete favorably with these emerging technologies.     

从米到秒:中国计量院的微波-光学频率基标准研究

文中介绍中国计量科学研究院(NIM)在微波-光学频率计量基标准研究的新进展:用NIM4激光冷却-铯原子喷泉钟复现国际单位制(SI)时间单位秒(s),用飞秒(fs)光学频率梳间接复现长度单位米(m)并标定稳频激光波长实际实施米定义.NIM4铯原子喷泉钟的不确定度达到5E-15;fm光梳锁定到NIM4钟控制的氢钟后,其频率不确定度为2.2E-14.在此基础上讨论铯原子喷泉钟,稳频激光,fm光梳的作用,意义和相互关系.最后报道2006年起NIM立题研制锶原子存储光钟.     

A Kind of Magnetron Cavity used in Rubidium Atomic Frequency Standards Yang Shiyu(杨世宇), Cui Jingzhong(崔敬忠), Tu Jianhui(涂建辉), Liang Yaoting(梁耀廷)

研究了一种应用于铷原子频标的磁控管腔,其中包括磁控管腔的主要特征和参数计算,测试结果与计算理论吻合较好,所设计的磁控管腔能够满足铷原子频标的设计要求。     

A kind of magnetron cavity used in rubidium atomic frequency standards

Research on the magnetron cavity used in the rubidium atomic frequency standards is developed, through which the main characteristic parameters of the magnetron cavity are studied,mainly including the resonant frequency,quality factor and oscillation mode.The resonant frequency and quality factor of the magnetron cavity were calculated,and the test results of the resonant frequency agreed well with the calculation theory.The test results also show that the resonant frequency of the magnetron cavity can be attenuated to 6.835 GHz,which is the resonant frequency of the rubidium atoms,and the Q-factor can be attenuated to 500-1000.The oscillation mode is a typical TE₀₁₁ mode and is the correct mode needed for the rubidium atomic frequency standard.Therefore these derivative magnetron cavities meet the requirements of the rubidium atomic frequency standards well.     

Long-baseline radio interferometry with independent frequency standards

The present stage of development of long-baseline interferometry using independent atomic frequency standards is reviewed. The technique is being applied to precision geodesy and astrometry and shows great promise as a means for the intercomparison of national time scales at the 1-ns error level. This paper discusses the development of radio interferometry and, in particular, the theory and practice of interferometry over a very wide (effective) bandwidth. The superb phase stability of the hydrogen maser frequency standard in the range of 100 s to several hours makes it the ideal local oscillator for long-baseline Interferometry.     

The relative merits of atomic frequency standards

The relative merits of atomic frequency standards based upon resonances in hydrogen, rubidium, or cesium depend upon the particular application and the specific requirement for each of several performance factors combined with physical characteristics. While the properties of an ideal atomic frequency standard may be established, practical instruments depart from the ideal as the result of compromises in the apparatus design and construction. The resonance line sharpness is an important factor which is dependent upon the apparatus; however, others my have a greater influence upon the essential characteristics. These include instrumental offsets due to atomic collisions with neighboring atoms or walls and magnetic fields. The intensity of the resonance signal is also essential in the determination of merit. These factors are discussed in relationship to hydrogen maser rubidium gas cell and cesium beam atomic frequency standards and the merits of each are compared. The possible merits of frequency standards based upon thallium beams are also discussed; however, a lack of extensive operating experience limits the knowledge in this case.     

Short-term stability of passive atomic frequency standards

The instability of passive atomic frequency standards caused by shot noise introduced by the atomic reference is calculated. This instability can be characterized by a figure of merit for the atomic reference, and the asymptotic functions for the rms frequency fluctuation for long and short averaging times expressed in terms of this quantity. Measurements of the rms frequency fluctuation of cesium atomic beam and rubidium vapor frequency standards are compared with theory, and the predicted performances of various existing and proposed atomic standards are tabulated. It is feasible to build atomic beam frequency standards whose stability for averaging times less than one second is limited by noise in the crystal oscillator rather than by shot noise, a criterion generally met by rubidium vapor frequency standards. For long averaging times, shot noise will always be the fundamental limiting factor in atomic frequency standard stability.     

Results of a survey on calibration standards requirements

This paper presents the results of a mail survey on the calibration and standards requirements of the millimeter submillimeter wave and far infrared research community. Brief accounts of the work in this area that is currently in progress at NBS and NPL are included.     

Some aspects of the theory and measurement of frequency fluctuations in frequency standards

Precision quartz oscillators have three main sources of noise contributing to frequency fluctuations: thermal noise in the oscillator, additive noise contributed by auxiliary circuitry such as AGC, etc., and fluctuations in the quartz frequency itself as well as in the reactive elements associated with the crystal, leading to an f^-1type of power spectral density in frequency fluctuations. Masers are influenced by the first two types of noise, and probably also by the third. The influence of these sources of noise on frequency fluctuation vs. averaging time measurements is discussed. The f^-1-spectral density leads to results that depend on the length of time over which the measurements are made. An analysis of the effects of finite observation time is given. The characteristics of both passive and active atomic standards using a servo-controlled oscillator are discussed. The choice of servo time constant influences the frequency fluctuations observed as a function of averaging time and should be chosen for best performance with a given quartz oscillator and atomic reference. The conventional methods of handling random signals, i.e., variances, autocorrelation, and spectral densities, are applied to the special case of frequency and phase fluctuations in oscillators, in order to obtain meaningful criteria for specifying oscillator frequency stability. The interrelations between these specifications are developed in the course of the paper.     

用于铷原子频标的磁控管腔研究

研究了一种应用于铷原子频标的磁控管腔，对用于磁控管腔的主要特征和参数计算进行了研究，主要包括磁控管腔的谐振频率、Q值、微波场模式。研究结果表明磁控管腔的谐振频率可以调谐至6.835GHz，Q值都能够调至600～1000之间，其微波场谐振模式是典型的TE011模式，频率温度系数较小（32.5KHz/℃－35.0 KHz/℃），所设计的磁控管腔能够满足铷原子钟物理部分的设计要求。     

ATM local area networks: a survey of requirements, architectures,and standards

There is increasing interest in deploying ATM technology in local or campus networks. ATM is an ideal technology to overcome many of the limitations of today's LAN technologies. This article focuses on the application of ATM in the LAN environment to interconnect high-end host computers, and on the interworking of ATM-based LANs with legacy LANs. The authors introduce ATM LAN requirements, followed by a discussion of possible ATM LAN architectures to support these requirements. The article then covers current standards and their relation to the possible architectures, and concludes with a discussion of current ATM LAN issues and directions