The Operation of the Atomic Fountains Developed at VNIIFTRI

We have compared the frequency of a room-temperature cesium-fountain primary frequency standard with that of a cryogenic (∼80K) cesium fountain. This comparison yields a measurement of the blackbody frequency shift of the room-temperature fountain.

We demonstrate the efficient capture of cesium atoms from a low-velocity intense source (LVIS) of atoms into an optical molasses. The high load rates that are achievable with the technique can potentially improve the stability of and reduce collisional shifts in atomic fountain clocks. An LVIS is an atomic beam created from a small hole in one of the retroreflectors in an otherwise ordinary magnetooptical trap (MOT). Our typical LVIS flux was 10/sup 10/ atoms/s. The asymptotic value for the number of atoms captured in the optical molasses was 1.1(1) /spl times/ 10/sup 9/ atoms, and the fill time constant was /spl tau/ = 290(30) ms. The initial molasses fill rate was R/sub t=0/ = 3.8(5) /spl times/ 10/sup 6/ atoms/ms. At this rate, it would require 24 ms to capture and launch 10/sup 7/ state-selected atoms in an atomic fountain. The fill rate at short times indicates that approximately 40% of the LVIS atoms were being captured in the optical molasses.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text M. A. Kasevich, E. Riis, S. Chu, and R. G. DeVoe, "Atomic fountains and clocks," Optics News 15(12), 31-32 (1989) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

This paper focuses on the development of tools aiming to solve several problems related to the microwave interrogation signal in atomic fountains. We first consider the problem related to cycle synchronous phase transients caused by the sequential operation of the atomic fountain. To search for such systematic phase variations deeply buried in the microwave synthesizer phase noise, we have developed a novel triggered-phase transient analyzer capable of processing the microwave signal to extract the phase in a synchronous manner even in the presence of frequency modulation. With this device we check in vivo the LNE-SYRTE fountain's interrogation signals with a resolution approaching 1 microradian. In addition, using this device, we investigate an innovative approach to solve a second problem, namely, the shift caused by microwave leakage in the fountain. Our approach consists of switching off the fountain microwave interrogation signal when atoms are outside the microwave cavity. To do that, we have developed a switch that is almost free of phase transients and is thus able to eliminate the frequency shift caused by microwave leakage without inducing significant phase transients on the interrogation signal.

This book provides an in-depth and comprehensive survey of the current state-of-the-art of the international atomic timescale and its underpinning relationship to practical and legal timescales. The authors are two of the leading figures in French time metrology. Claude Audoin, formerly Director of the Laboratoire de l'Horloge Atomique at Orsay, has had a long and distinguished career in the development of atomic frequency standards and atomic clocks. Bernard Guinot, formerly Director of the Bureau International de l'Heure (now absorbed by the Bureau International des Poids et Mesures, BIPM), has a lifetime's experience in the development of timescales. The work reviewed here is a revised and updated translation of Les fondaments de la mesure du temps (Paris: Masson, 1998). It is written at a level appropriate for an experienced graduate student or postdoctoral scientist entering either the field of time and frequency metrology or that of precise astronomical observation, whilst having sufficient depth to be useful as a work of reference for the established researcher in these fields. The first third of the book discusses the physical foundations of time metrology and introduces key concepts in the use of physical oscillators as clocks. The historical account of the complex process by which mean solar time came to be replaced by Coordinated Universal Time (UTC) will be particularly informative for newcomers to the field. About half of the volume is devoted to atomic frequency standards, with an exposition of the physics of caesium beam clocks and caesium fountains and a review of other commonly used standards such as the hydrogen maser, rubidium cell and stored ion clock. Although thorough, this 127-page chapter does not replace Audoin's earlier work with Jacques Vanier, The Quantum Physics of Atomic Frequency Standards (Bristol: IOP Publishing, 1989, out of print), which will remain the Bible for physicists working on clock development. The present time is one of great excitement in the development of atomic frequency standards. At the time the French edition was published, the first caesium fountain frequency standard, at the BNM-LPTF in Paris, had recently demonstrated 1.5 × 10-15 relative frequency uncertainty. Two such standards, at PTB and NIST, are now contributing to International Atomic Time (TAI). Clocks based on optical transitions in ions or atoms are just beginning to become a reality, thanks to Ted Haensch's breakthrough with the femtosecond optical frequency comb generator, which arrived on the scene just in time to receive a mention in this English edition. In a rapidly evolving field, any general survey is bound to be overtaken by events. Primary caesium frequency standards are designed to realize the SI definition of the second. The relationships between the SI second, TAI and UTC, as well as the national approximations to UTC generated by national standards laboratories such as NPL, the timescale available from GPS and timescales used in astronomy, are carefully set out. Finally, the practical applications of these timescales and frequency standards are reviewed, ranging from ultraprecise determinations of fundamental physical constants to positioning and navigation. The problem of longitude is as pressing as in Harrison's day, but for the foreseeable future, clockmakers will continue to have the upper hand over astronomers! Despite betraying the occasional Gallicism, the English translation reads with the clarity of the French original. The reader is guided painlessly through the multiplicity of acronyms with which the topic abounds. Audoin and Guinot have produced a book that will stand the test of time. Stephen N Lea

We have observed that the collisional frequency shift in primary caesium fountain clocks varies with the clock state population composition and, in particular, is zero for a given fraction of the |F=4,mF=0) atoms, depending on the initial cloud parameters. We present a theoretical model explaining our observations. The possibility of the collisional shift cancellation implies an improvement in the performance of caesium fountain standards and a simplification in their operation.

A single-particle Green's function (propagator) is introduced to study the detection of laser-cooled Cesium atoms in an atomic fountain due to RF field gradients in the Ramsey TE011 cavity. The detection results in a state-dependent loss of atoms at apertures in the physics package, resulting in a frequency bias. A model accounting only for motion in one dimension transverse to the symmetry axis of the fountain is discussed in detail and then generalized to two transverse dimensions. Results for fractional frequency shifts due to transverse field gradients are computed for NIST F-1 and F-2 Cesium fountains. The shifts are found to be negligible except in cases of higher RF power applied to the cavities.

The Unit of Time in the International Systems of Units is realized at the relative frequency accuracy level of few parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-16</sup> by laser cooled Cesium fountains [1]; since 2003 INRIM has operated a laser cooled Cesium fountain with relative frequency accuracy at the level of 5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-16</sup> and has contributed with this frequency standard to the generation of the International Atomic Timescale of Bureau International des Poids et des Mesures (BIPM). Since 2009 a cryogenic Cesium fountain is under operation and its metrological characterization is in progress, with a target accuracy of 1×10{-16}.

The cesium fountain under development at the NIST Time and Frequency Division contains two identical microwave cavities, one used for state selection and the other for Ramsey excitation. These cavities, designed to hold distributed cavity phase shift to a minimum are described and data relevant to the designs of such cavities presented.

Two cesium fountain frequency standards are now in operation at BNM-LPTF, FO1 and the transportable PHARAO fountain. We present a first accuracy evaluation of the PHARAO fountain, a new evaluation of FO1, and a frequency comparison between both clocks. They are found to agree to better than 2/spl times/10/sup -15/.

Development of compact transportable frequency standards is an important goal of the modern applied physics. Despite of the great progress in the field of optical frequency standards with their ultimate accuracy and stability, they remain bulky, complex and sensitive systems. At the other hand, the best microwave frequency standards like cesium fountains are also bulky and sensitive to adjustments. Combination of techniques developed for optical and microwave standards can result in compact and robust transportable frequency standard. Here we suggest the microwave transition in 25Mg+ laser cooled ion cloud for realization of highly accurate microwave frequency reference. We describe its design and experimental sequence, estimate its expected accuracy and stability.

We have designed and constructed a cesium fountain as our new generation primary frequency standard. Serial performance measurements of components and assemblies are finished. A cold atomic cloud with diameter 1.5 mm and temperature /spl les/150 /spl mu/K has been observed in a magneto-optical trap set-up. It is used to obtain the optical molasses designed for the fountain clock.

This work deals with the determination of the spatial phase distribution in a cesium fountain microwave cavity, taking into account functional requirements such as beam apertures and feeding holes, and the effect of imperfect metallic walls.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

SI-traceable measurements of optical frequencies using International Atomic Time (TAI) do not require a local primary frequency reference, but suffer from an uncertainty in tracing to the SI second. For the measurement of the 87Sr lattice clock transition, we have reduced this uncertainty to the low 10-16 level by averaging three sets of ten-day intermittent measurements, in which we operated the lattice clock for 104 s on each day. Moreover, a combined oscillator of two hydrogen masers was employed as a local flywheel oscillator (LFO) in order to mitigate the impact of sporadic excursion of LFO frequency. The resultant absolute frequency with fractional uncertainty of 4.3 × 10-16 agrees with other measurements based on local state-of-the-art cesium fountains.

For almost two decades, caesium fountain primary frequency standards (PFS) have represented the best realization of the definition of the second in the International System of units. Their accuracy has progressively improved with time, reaching a few parts in 1016. In this paper, we present the accuracy evaluation of ITCsF2, the new Cs fountain PFS developed at the Italian National Metrological Institute, designed to be operated at cryogenic temperature to reduce the blackbody radiation shift. The short-term stability of the ITCsF2 fountain is 2 × 10−13τ−1/2 when operated at high atomic density, and the relative inaccuracy reaches 2.3 × 10−16. We also report four calibrations of International Atomic Time with a relative frequency agreement of (−1.7 ± 3.2) × 10−16, between ITCsF2 and the average of the other fountains operated in the world during the reference periods.