Cesium Clock Research Articles

Introduction: Keeping Time

Introduction:Keeping Time Jocelyn Holland (bio) and Wolf Kittler (bio) Already in what they call their “pre-history,” humans kept, measured, and calculated time by looking at the revolutions of the sun, the planets, and the stars in the sky. Within the long history of time, one could define modernity by two technological innovations that shift timekeeping from the macrocosm to the microcosm. The first innovation, an initial step away from astronomical time, is the invention of the pendulum clock. The legend of Galileo’s epiphany, as recorded by his biographer Vincenzo Viviani, defines a decisive moment: the moment in the cathedral of Pisa when he became aware of the isochronism of the pendulum, that the time it takes to swing back and forth is independent of the amplitude of the swing. The pendulum clock emerged from Galileo’s experiments, and Huygens’s implementation of the laws discovered by means of these experiments, into a working device during the seventeenth century. The second major technological innovation in timekeeping that is definitive of the modern era is the atomic clock. This clock brought time down to earth once and for all and initiated the complete decoupling of time measurement from astronomical time. Claude Audoin and Bernard Guinot note in The Measurement of Time that the cesium beam–frequency standard began regular operation in the United Kingdom in 1955, heralding the use of atomic properties for time measurement,1 and that the first commercial [End Page 133] cesium clock—the “atomichron”—was manufactured a year later.2 At the Thirteenth General Conference on Weights and Measures, the atomic definition of the second was officially ratified. No longer a fraction of an hour or a day, “the second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium 133 atoms.”3 Although one of the most radical events in the history of timekeeping, the invention of the atomic clock took place in the twentieth century with little public fanfare, and even years later the atomic clock remains widely unknown. Perhaps, as Audoin and Guinot speculate, this is due to “a reaction to the lack of poetry in atomic clocks, or their inscrutable accuracy.”4 Conditioned to think of time in terms of solar and lunar revolutions, could it be that we are neither ready nor easily able to integrate an alternative into our daily lives? In our social practices and internal awareness of time, are we not keeping up with the inventions of newer and more accurate modes of timekeeping driven by scientific and technological innovation? Part of the strangeness of atomic time lies precisely in this perceived detachment from the communal experience of time that informs our lives, despite the fact that relatively common objects, such as GPS devices, are implicated in the coordination of atomic clocks through satellites. Although atomic clocks synchronize our lives in basic ways, it is fair to say that they have not yet claimed a foothold in the popular imagination. Even if the cultural implications of the atomic clock are for future generations to discover, there is ample evidence that the history of modern timekeeping technology up to that point is far more than a simple record of its scientific discoveries and practical applications: a look at earlier cases quickly reveals that it includes far-reaching effects on the human experience and recording of time. In literary studies, for example, where the study of time in terms of the temporality of narration or the narrated time of events is a familiar topic, very recent scholarship has taken up the many connections between technological progress in timekeeping and the manifold strategies and preferences of time organization that inform a literary text. In Telling Time, a study of the relation between clocks and diurnal literary forms in seventeenth- and eighteenth-century England, [End Page 134] Stuart Sherman writes about the fundamental relationship between the “counting” and “telling” of time—between techniques of calculation and modes of expression: “clocks count time (OE tellan, to narrate, is cognate with Middle Low German tellen, to count, reckon), or rather, they make it available for human...

PHARAO laser source flight model: design and performances.

In this paper, we describe the design and the main performances of the PHARAO laser source flight model. PHARAO is a laser cooled cesium clock specially designed for operation in space and the laser source is one of the main sub-systems. The flight model presented in this work is the first remote-controlled laser system designed for spaceborne cold atom manipulation. The main challenges arise from mechanical compatibility with space constraints, which impose a high level of compactness, a low electric power consumption, a wide range of operating temperature, and a vacuum environment. We describe the main functions of the laser source and give an overview of the main technologies developed for this instrument. We present some results of the qualification process. The characteristics of the laser source flight model, and their impact on the clock performances, have been verified in operational conditions.

Development of automatic intercomparison system for generation of time scale ensembling several atomic clocks

National physical laboratory India (NPLI) has five commercial cesium atomic clocks. Till recently one of these clocks had been used to maintain coordinated universal time (UTC) of NPLI. To utilize all these clocks in an ensemble manner to generate a smoother time scale, it has been essential to inter-compare them very precisely. This has been achieved with an automatic measurement system with well-conceived software. Though few laboratories have developed such automatic measurement system by themselves based on the respective requirements; but they are not reported. So keeping in mind of the specific requirement of the time scale generation, a new system has been developed by NPLI. The design has taken into account of the associated infrastructure that exists and would be used. The performance of the new system has also been studied. It has been found to be quite satisfactory to serve the purpose. The system is being utilized for the generation of time scale of NPLI.

Progress towards primary frequency standard

The cesium fountain clock as primary frequency standard is widely used in the areas, such as time-keeping system, satellite navigation, fundamental physics research, etc. The principle of operation of cesium fountain clock is introduced. The noise source and frequency shift term are ananlyzed. The major noise source influencing frequency stability are cold atom loading time, microwave phase noise related to Dick effect, and detection laser frequency noise. The major frequency bias influencing frequency uncertainty is blackbody radiation frequency shift,cold atom collision frequency shift,distributed cavity phase frequency shift and microwave leakage frequency shift.The key technique to achieve highperformance cesium fountain clock is sumerized. The application of cesium fountain clock is presented. The status of space cesium clock and future primary frequency standard of optical clock are shown.

Frequency comparison of lattice clocks toward the redefinition of the second

Strontium is the most popular species for optical lattice clocks. Recent reports of the accuracies from Boulder, U.S. and Tokyo reach 10−18 level, which is better than state-of-the-art caesium clocks more than one order of magnitude. While this achievement accelerates the discussion to redefine the second, the agreement of frequencies in separate laboratories is of critical importance. For this context, intercontinental comparison of Sr lattice clocks were demonstrated between Japan and Germany using a satellite-based technique. The frequency difference was consistent with zero with an uncertainty of 1.6 × 10−15.

Precision Spectroscopy of Atomic Hydrogen

Precise determinations of transition frequencies of simple atomic systems are required for a number of fundamental applications such as tests of quantum electrodynamics (QED), the determination of fundamental constants and nuclear charge radii. The sharpest transition in atomic hydrogen occurs between the metastable 2S state and the 1S ground state. Its transition frequency has now been measured with almost 15 digits accuracy using an optical frequency comb and a cesium atomic clock as a reference [1]. A recent measurement of the 2S – 2P3/2 transition frequency in muonic hydrogen is in significant contradiction to the hydrogen data if QED calculations are assumed to be correct [2, 3]. We hope to contribute to this so-called "proton size puzzle" by providing additional experimental input from hydrogen spectroscopy.

Integrated System for Monitoring and Control of the National Time and Frequency Standard

In Poland, the national time and frequency standard (NTFS) is realized by means of three caesium atomic clocks, a system for the generation of the Polish time scale UTC(PL), and measuring systems for comparing directly and indirectly the atomic clocks and time scale. In order to ensure the best realization of NTFS, the Institute of Electrical Metrology at the University of Zielona Gora (IME UZ), in cooperation with the Central Office of Measures (GUM), has developed and built an integrated monitoring and control system. This system significantly increases the functional capabilities of autonomous measuring systems used previously at GUM and adds new features that increase the reliability and safety of the NTFS. Examples of such features are: automatic detection of invalid operating conditions (including breakdowns) of the atomic clocks and remote notification of alarms, as well as a new approach to predicting corrections for UTC(PL) using neural networks (NNs). In this paper, the presentation of hardware and software of the NTFS is limited to their general features. However, the application of NNs for predicting corrections for UTC(PL) and automation in determining the standard radio frequency deviation from its nominal value are presented in more detail.

Congruence of two Kalman filter composite clocks

This paper compares the estimation results of two composite clock algorithms based on Kalman filtering of measured clock differences: the original Jones–Tryon algorithm and Greenhall's covariance-reduction modification. The theory is carried out for a general clock model that includes the model introduced by Davis et al: the sum of the three-state linear model and a set of Markov processes used to approximate flicker-frequency noise. Since measurement noise is allowed, at each measurement time the corrected clock offsets (the difference between the clock offsets and their estimates) do not coincide but form a set of values, different for the two algorithms. The core result is that these two sets are congruent, that is, they differ by a common scalar shift. As a function of time the value of the shift behaves as a random walk. Moreover, the two Kalman filters have the same measurement residuals. Therefore, any detector algorithm that uses the measurement residuals is not impacted by the covariance reduction. The congruency holds for time-varying measurement matrices. Therefore, the result is applicable to the measurement setup of laboratories as well as that of a Global Navigation Satellite System with varying visibilities of satellite clocks at ground stations. These results are illustrated by a simulated ensemble of three caesium clocks and two active hydrogen masers, measured against one of the masers.

ASF 예측모델과 실측치를 이용한 영일만 해상 ASF 맵 생성기법

In order to establish eLoran system it needs the betterment of a receiver and a transmitter, the add of data channel to loran pulse for loran system information and the differential Loran for compensating Loran-c signal. Precise ASF database map is essential if the Loran delivers the high absolute accuracy of navigation demanded at maritime harbor entrance. In this study we developed the ASF mapping method using predicted ASFs compensated by the measured ASFs for maritime in the harbor. Actual ASF is measured by the legacy Loran signal transmitted from Pohang station in the GRI 9930 chain. We measured absolute propagation delay between the Pohang transmitting station and the measurement points by comparing with the cesium clock for the calculation of the ASFs. Monteath model was used for the irregular terrain along the propagation path in the Yeongil Bay. We measured the actual ASFs at the 12 measurement points over the Yeongil Bay. In our ASF-mapping method we estimated that the each offsets between the predicted and the measured ASFs at the 12 spaced points in the Yeongil. We obtained the ASF map by adjusting the predicted ASF results to fit the measured ASFs over Yeungil bay.

Absolute 12C16O2 transition frequencies at the kHz-level from 1.6 to 7.8 µm

Absolute transition frequencies were measured for a series of transitions in the (30013)←(00001) near-infrared 12C16O2 band. These measurements were referenced to a cesium atomic clock through the use of an optical frequency comb. Combined standard uncertainties as low as 18kHz (6×10−7cm−1) were achieved. Importantly, deviations as large as 5MHz were observed relative to the HITRAN 2008 database. These measurements were then included in a global fit of 416 CO2 mid-infrared and near-infrared measurements each of which was absolute. The resulting spectroscopic parameters provide a series of secondary frequency standards with kHz-level uncertainties across a wide frequency range and should significantly improve spectroscopic retrieval algorithms for space-based measurements of atmospheric CO2.

Time and frequency transfer over optical networks

This paper aims to summarize and express network requirements of time and frequency transfer over optical networks performed in National Research and Education networks of France and Czech Republic. Time transfer experiment in Czech Republic converts microwave output frequency of cesium atomic clocks into train of optical pulses that carries accurate time over all-optical network with two network domains. Frequency transfer experiment references optical comb etalon to frequency of atomic clock. An optical frequency of optical comb is then sent throughout optical network in a single bidirectional channel. The paper highlights advantages and challenges of both transfer approaches.

Precision measurement of the hydrogen 1S-2S frequency via a 920-km fiber link.

We have measured the frequency of the extremely narrow 1S-2S two-photon transition in atomic hydrogen using a remote cesium fountain clock with the help of a 920 km stabilized optical fiber. With an improved detection method we obtain f(1S-2S)=2466 061 413 187 018 (11) Hz with a relative uncertainty of 4.5×10(-15), confirming our previous measurement obtained with a local cesium clock [C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011)]. Combining these results with older measurements, we constrain the linear combinations of Lorentz boost symmetry violation parameters c((TX))=(3.1±1.9)×10(-11) and 0.92c((TY))+0.40c((TZ))=(2.6±5.3)×10(-11) in the standard model extension framework [D. Colladay, V. A. Kostelecký, Phys. Rev. D. 58, 116002 (1998)].

Two-Color One-Way Frequency Transfer in a Metropolitan Optical Fiber Data Network

Two-color one-way frequency transfer through one strand of optical fiber is an alternative method to two-way frequency transfer, and is useful if unknown asymmetries exist in the link due to different paths for each direction. The term “two-color” refers to the ability to send signals at two different wavelengths utilizing the same fiber in one direction. The method is suitable for implementation in existing urban Single Mode Fiber networks, for instance in networks that are utilized for data and television communication. It is therefore able to coexist with data channels in wavelength-division multiplexing (WDM) systems. It performs as a dynamical control of transit time and simultaneously enables a real-time phase stabilized output signal. This paper presents results from a comparison of two cesium beam frequency standards separated by about 3 km over an optical fiber network located in a metropolitan area in Sweden. The cesium standards were simultaneously compared to each other with a Global Positioning System (GPS) satellite link and over optical fibers, so that the optical fiber technique could be evaluated with respect to the GPS technique. The difference in frequency stability between the two methods is shown to be about 3 × 10-15 over an averaging interval of 10 000 s

Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420 km

In this paper we present the results of our work concerning the long-distance fibre optic dissemination of time (1 PPS) and frequency (10 MHz) signals generated by atomic sources, such as caesium clocks, hydrogen masers or caesium fountains. For these purposes we developed dedicated hardware (a fibre optic system with active stabilization of the propagation delay and bidirectional fibre optic amplifiers) together with a procedure to enable calibration of the time transfer. Our laboratory measurements performed over fibre lengths of up to 480 km showed an Allan deviation of the order of 4 × 10−17, time deviation below 1 ps (both at one-day averaging) and the possibility of calibration with picosecond accuracy even for the longest from evaluated links. After successful laboratory evaluation the system was next installed on a 421.4 km long route between the Central Office of Measures (GUM) in Warsaw, Poland, and the Astrogeodynamic Observatory (AOS) in Borowiec near Poznań, Poland. Experiments comparing the UTC(PL) and UTC(AOS) atomic timescales using the fibre optic link and TTS-4 dual-frequency GNSS time transfer receivers showed that the consistency of the results is within the calibration accuracy of the GPS receivers and with much better noise performance. The field operation of the system proved its full functionality and confirmed our previous laboratory evaluation to the maximum extent possible using the methods for comparing distant clocks available at GUM and AOS.

Norman Ramsey and his method

While searching for a way to boost the resolution of an atomic spectrometer, Ramsey hit on a simple solution: Replace a single oscillating magnetic field with two separated ones.

Optical Atomic Clocks Could Redefine Unit of Time

Optical atomic clocks now outperform the best microwave cesium atomic clocks in terms of precision.

A Distributed-Feedback Diode Laser Frequency Stabilized on Doppler-Free Cs $D_{1}$ Line

This paper describes a distributed-feedback laser diode system frequency stabilized by simple saturated absorption technique at 895 nm on the Cs D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> line with a linewidth narrower than 1 MHz. This laser source was developed to be used in a compact cesium atomic clock based on coherent population trapping. The frequency stability of the laser system, evaluated by measuring the beat note of two similar laser systems and characterized by Allan deviation, is measured to be lower than 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> for integration times of up to 2000 s, even achieving 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> at 200 s. This laser diode system, mounted in a mechanically stable cage system using easy-to-find commercially available components, remains in locked operation for several weeks without any problem.

Frequency and Intensity Control of Lasers to Cool and Control Caesium Atoms

Lasers are now being routinely used to control the atomic motion. We describe the usage of narrow linewidth lasers to capture, cool, manipulate and detect the caesium atoms. This control helps in increasing the interaction time between atoms and microwave field in a cesium atomic clock. This paper describes the optics in detail for Cesium Fountain of NPLI. The aim of this work is to build a fountain frequency standard with a relative uncertainty below 10−15.

Frequency Transfer in Electronically Stabilized Fiber Optic Link Exploiting Bidirectional Optical Amplifiers

In this paper, we present the experimental result for the fiber optic frequency transfer link with optical regenerators exploiting the same Er-doped fiber for amplifying the signals in both directions. The measurements performed at the distance of 124 km with one amplifier and 224 km with three amplifiers showed that the presented solution is adequate for transferring the frequency signals generated by cesium clocks, hydrogen masers, and cesium fountain clocks. The Allan deviation around 9 ·10-18 and 5.5 ·10-17 was found for the averaging time of 105 s for 124- and 224-km-long links, respectively.

The evolution of time measurement, Part 5: radio controlled clocks [Recalibration

Frustrating as it is to time metrologists, a group of independent clocks can't keep the exact same time. To prove this, synchronize a group of wristwatches and then check them again a month later. You'll likely find that they disagree with each other by at least a few seconds. If you attempt the same exercise with a group of cesium clocks, the time differences will be tiny, nanoseconds instead of seconds, but again, the clocks won't keep the exact same time. The British horologist Frank Hope-Jones recognized these problems more than a century ago and became an advocate for collectivism, rather than individualism in clocks. The clocks of Hope-Jones' era were typically self-wound, were synchronized from different sources, and ran at very different rates. Therefore, none of them would come close to agreeing with the others.