Cesium Frequency Standards Research Articles

Micro mercury trapped ion clock prototypes with 10^{-14} frequency stability in 1-liter packages

Modern communication and navigation systems are increasingly relying on atomic clocks. As timing precision requirements increase, demands for lower SWaP (size, weight, and power) clocks rise. However, it has been challenging to break through the general trade-off trend between the clock stability performance and SWaP. Here we demonstrate micro mercury trapped ion clock (M2TIC) prototypes integrated with novel micro-fabricated technologies to simultaneously achieve high performance and low SWaP. The M2TIC prototypes could reach the 10^{-14}-stability level in 1 day with a SWaP of 1.1 L, 1.2 kg, and under 6 W of power. This stability level is comparable to the widely used rack-mount Microchip 5071A cesium frequency standard. These standalone prototypes survived regular commercial shipping across the North American continent to a government laboratory, where their performance was independently tested. The M2TIC sets a new reference point for SWaP and performance and opens opportunities for high-performance clocks in terrestrial and space applications.

On the efficiency of laser pumping of hyperfine structure sublevels of Rubidium-87 and Cesium-133 atoms

The evolution of the hyperfine structure magnetic sublevel populations of 87Rb and 133Cs atoms in resonant linearly polarized laser fields is theoretically analyzed. Analytical expressions are obtained for stationary populations of the magnetic sublevels of the ground state 52S1/2 Fg = 1, Ff = 2 in 87Rb and 62S1/2 Fg = 3, Ff = 4 in 133Cs under optical pumping at the Fg ↔ Fe = Fg and Ff ↔ Fe = Ff transitions of the D2 lines (n 2S1/2 n 2P3/2) versus the initial populations. The characteristic population stabilization time are obtained by numerical solution for the density matrix equations, depending on the intensity of the laser fields and the detuning of the optical resonance. It is shown that the alternation of the polarization direction of the laser field at the transition Ff ↔ Fe = Ff almost completely transfers atoms to the lower sublevel of the “clock” M1 transition Fg M = 0 ↔ Ff M = 0 in rubidium and cesium frequency standards, which proportionally increases the signal of the recording system.

Ti-doped MgO thin film by magnetron sputtering for cesium frequency standard

The main component of magnetically selected cesium frequency standard is electron multiplier. Its working principle is to magnify cesium beam signal by emitting secondary electron through surface of material. Magnesium oxide (MgO) has been adopted due to its excellent capability to emit secondary electron. There are many ways to prepare MgO thin film, including evaporation, pulsed laser deposition and MOCVD. It has been studied that doping can be used to enhance its emission capability and decrease its working potential of the thin film. In this paper, Ti-doped MgO thin films were deposited by magnetron sputtering method. Titanium metal and MgO ceramic were co-sputtered in oxygen containing argon ambient with DC and RF power source, respectively. XRD results show that the deposited film has the texture of (100) and (110) orientation. The ratio of Ti/Mg were consistent in XPS and EDS. SEM and AFM show that the film is polycrystalline. Electron multiplier device was assembled, and the secondary electron emission coefficient of the Ti-doped MgO film is greater than that of the undoped one, especially 36% higher at low primary electron energy of 200 eV which is meaningful to cesium atomic frequency standard.

The Hyperfine Transition for the Definition of the Second

AbstractThe unit of time of the International System of Units (SI), the “atomic second” was defined through a constant of physics in 1967. It is derived from the frequency of the hyperfine transition of the atom of cesium 133. From the astronomical definition of the second until today, the accuracy of the realization of the second has improved by eight orders of magnitude, with a rate that has increased since the development of the cesium frequency standards, to reach parts in 1016 for the best clocks today. In 2018, when the new SI was adopted, the time metrology community proved that a new generation of frequency standards operating in optical wavelengths has uncertainties at the level of 10–18, and challenge the implementation of high accurate frequency and time comparison techniques to decide on a revision of the definition of the second. Herein, the progress in the definition and realization of the second from astronomy until today is reviewed, an inventory of the present resources is assembled and a brief view for the future given.

Phase noise analysis of voltage controlled oscillator used in cesium atomic clock

Coherent population trapping (CPT) cesium frequency standard plays a significant role in precision guidance of missile and global positioning system (GPS). Low noise 4.596 GHz voltage controlled oscillator (VCO) is an indispensable part of microwave signal source in cesium frequency standard. Low-phase noise is also the most important and difficult performance indicator of VCO. Starting from phase noise analysis method proposed by Leeson, the formulas about the relationship between phase noise of output signal of oscillator feedback model and phase fluctuation spectrum of amplifier, phase noise of oscillator are derived in this paper. Finally, the asymptote model of microwave oscillator is proposed based on the formula derivation. The experiment shows that when the reverse bias voltage of variode is 1.8 V, the designed oscillation frequency of VCO is 4.596 GHz, the power is −1 dBm and the DC power consumption is 19.6 mW. The tendency of phase noise simulation curve and actual test curve conform to asymptote model. The phase noise in 1 and 10 kHz is, respectively, −60.86 and −86.58 dBc/Hz. The significance of the paper lies in determining the main factors influencing oscillator phase noise and providing guiding direction for the design of low-phase noise VCO.

Measuring absolute frequencies beyond the GPS limit via long-haul optical frequency dissemination.

Global Positioning System (GPS) dissemination of frequency standards is ubiquitous at present, providing the most widespread time and frequency reference for the majority of industrial and research applications worldwide. On the other hand, the ultimate limits of the GPS presently curb further advances in high-precision, scientific and industrial applications relying on this dissemination scheme. Here, we demonstrate that these limits can be reliably overcome even in laboratories without a local atomic clock by replacing the GPS with a 642-km-long optical fiber link to a remote primary caesium frequency standard. Through this configuration we stably address the 1S0-3P0 clock transition in an ultracold gas of 173Yb, with a precision that exceeds the possibilities of a GPS-based measurement, dismissing the need for a local clock infrastructure to perform beyond-GPS high-precision tasks. We also report an improvement of two orders of magnitude in the accuracy on the transition frequency reported in literature.

Measuring the Flashing Time Interval of LED Lights in Toys

Abstract:We have developed two methods to measure the light flashing time interval in toys. One method uses high speed video recording and a synchronous counter. The method starts by taking a short video clip of the flashing Light Emitting Diodes (LEDs) under test, together with the reading of an in-house designed synchronous counter which is phase locked to a cesium frequency standard. The flashing time interval is obtained by viewing the recorded video to search frame-by-frame for the time interval between two successive LED state changes. This method has a measurement uncertainty (k = 2) of less than 2 ms. The second method is a direct measurement with a time interval counter. A circuit with a photo-transistor converts the light signal to an electrical signal. A variable resistor is connected in series with the photo-transistor to tune the detection sensitivity such that it can work well in low illuminance conditions. The output signal is then amplified and fed to a time interval counter that is phase ...

Calibration of Stopwatches by Utilizing High Speed Video Recordings and a Synchronous Counter

Abstract:A new method for the calibration of stopwatches, called video totalize method, has been developed at the Hong Kong Standards and Calibration Laboratory (SCL). The method starts with the taking of two short video clips of the display of the stopwatch under test, together with the reading of an in-house designed synchronous counter, with the two clips separated by an interval of six to seven hours. The 10-digit synchronous counter is driven by a 1 kHz clock which is phase locked to the cesium frequency standard of SCL. The elapsed times measured by the stopwatch and the synchronous counter are obtained by viewing the recorded video to search frame-by-frame for the instant at which the reading of the stopwatch changes. Using this method, the measurement uncertainty is no longer constrained by the display resolution of the stopwatch, but instead is limited only by the frame rate of the video recording. Digital cameras that can record video at 420 frames per second with usable image quality are commer...

Spectral characteristics of a laser emitter designed for pumping and detecting a reference quantum transition of a caesium frequency standard

Experimental and calculated spectral characteristics of a diode laser with a Bragg grating soldered on a thermoelectric cooler are presented. A model of the laser is developed, which takes into account the pressure arising after soldering the Bragg grating on the thermoelectric cooler, as well as temperature and dispersion. Theoretical and experimental spectral characteristics of the laser are compared and their satisfactory agreement is shown.

Single-frequency tunable laser for pumping cesium frequency standards

A single-frequency tunable laser for pumping the cesium frequency standard is studied. It is shown experimentally that the laser emits at a single frequency despite the fact that a few longitudinal modes of the external cavity fall within the reflection band of a fibre Bragg grating (FBG) written in the optical fibre. The laser wavelength can be tuned by varying the pump current of the laser, its temperature, and the FBG temperature. The laser linewidth does not exceed 2 MHz for 10 mW of output power.

Mapping of the Decameter Radio Sources of Cassiopeia A Using a 100 km Range Long-Baseline Interferometer: Verification of a New Method of Phase Calibration of Interferometers

Abstract A new method is proposed for detecting the phase difference of cesium frequency standards facilitated at coupling stations of an interferometer of 100 km range baselines for observations of decameter wavelength radio waves. In this method, the Earth’s rotation is utilized. That is, a time-depending variation of the phase differences of arriving radio waves from a given source forms a so-called fringe function due to the Earth’s rotation. By generating a reference fringe function corresponding to the position of a selected radio source, the signals can be detected being selected from widely and continuously distributed radio sources in the sky, when the correlation is analyzed between the generated reference fringe function and the fringe data detected by the interferometer. The feasibility of the proposed method has been verified by applying the method to observations of a 100 km range baseline interferometer for decameter wavelength radio waves at Tohoku University selecting the radio-wave sources in the Cassiopeia A supernova remnant as the objective. The results of the analyzed correlation values averaged for the observation data of 30 hr, at 22.158 MHz, are determined being scattered around average values with a distribution of less than 25%; this distribution corresponds to a phase determination error of $\pm 12^\circ$ for the cesium frequency standards of the interferometer at 10 MHz. That is, we can calibrate the phase shift of the cesium frequency standards facilitated at each member station of the interferometer system with an accuracy of $\pm 12^\circ$, which corresponds to an accuracy of $\pm 1 ''$ for determining the direction of the radio-source position in the sky in the case of 100 km range long baseline interferometer. This accuracy is, however, only possible when an integration time longer than 30 hr is applied for analyses of the fringe correlation values.

Cryogenic dual-mode resonator for a fly-wheel oscillator for a caesium frequency standard.

A dual-mode, sapphire-loaded cavity (SLC) resonator has been designed and optimized with the aid of finite element software. The resonance frequency was designed to be near the frequency of a Cs atomic frequency standard. Experimental tests are shown to agree very well with calculations. The difference frequency of two differently polarized modes is shown to be a highly sensitive temperature sensor in the 50 to 80 K temperature range. We show that an oscillator based on this resonator has the potential to operate with fractional frequency instability below 10(-14) for measurement times of 1 to 100 seconds. This is sufficient to operate an atomic clock at the quantum projection noise limit.

Relativistic Time Transformations in GPS

Since Selective Availability was permanently switched off on 7 May 2000, most of the GPS satellite clocks have been well behaved. During a 24-h period precise satellite clock solutions, corrected for GPS conventional relativistic corrections, follow straight lines within a few nanoseconds. The linear clock fit RMS for the best satellite clocks are well below the 1-ns level, which is consistent with the nominal stability of the GPS frequency standards. Typically, the GPS satellite clocks show an Allan variance at or below one part in 1011/100 s for the Cesium frequency standards and a few parts in 1012/100 s for the Rubidium frequency standards. These results correspond to clock RMSs for 15-min sampling at or below 3 and 0.3 ns, respectively. This already confirms experimentally that the conventional periodic relativity correction of the GPS system, also adopted for all the IGS clock solution products, is precise and correct to 0.6 ns or better. To establish the precision limits of the GPS conventional relativity treatment, the relativistic time transformations of GPS satellite frequency and clocks are critically reviewed, taking into account all the contributions larger than the 10−18 (or 0.001 ns). The conventional GPS relativity treatment was found to be accurate, i. e., correctly modeling the actual relativistic frequency (clock rate) effects of GPS satellites at about the 10−14 level. However, it is also affected by small periodic errors of the same magnitude. The integration of these small periodic frequency relativistic errors gives the approximation errors of the conventional periodic relativistic clock correction with amplitudes of about 0.1 ns and a predominant period equal to a half of the orbital period (∼ 6 h). These approximation errors of the conventional GPS relativistic clock correction are at about the same level as the current precision of the IGS clock solutions. © 2002 Wiley Periodicals, Inc.

An alternative cold cesium frequency standard: the continuous fountain

We report on the primary frequency standard now under construction at the Observatoire de Neuchatel (ON). The design is based on a continuous fountain of laser-cooled cesium atoms, which combines two advantages: the negligible contribution of collisions to the inaccuracy and the absence of stability degradation caused by aliasing effects encountered in pulsed operation. The design is reviewed with special emphasis on the specific features of a continuous fountain, namely the source, the microwave cavity (TE(021) mode), and the microwave modulation scheme. The possible sources of frequency biases and their expected contributions to the error budget are discussed. Based on present data, an accuracy in the low 10(-15) range and a short-term stability of 7.10(-14) are attainable simultaneously under the same operating conditions.

Article

The accelerated development of laser methods for manipulating the internal state and motion of atoms, and the recent primary evaluation in France of a cesium frequency standard using cold atoms in an atomic fountain configuration, allow the frontiers of time metrology, accuracy, and stability to be pushed near parts in the 1016 level, a factor of 100 over the present primary laboratory clocks. These positive results revived the development of more accurate cesium atomic clocks, and the Institute of National Measurement Standards (INMS), of the National Research Council of Canada (NRC) is undertaking a cesium-fountain project aimed at reaching these new levels of accuracy and stability. This paper reviews its present status. To appreciate the considerable impact of cold atoms in time and frequency metrology, we begin by recalling the background principles of traditional cesium frequency standards, their typical performance and main limitations. After introducing the important theoretical results essential to understanding the techniques of laser cooling and magneto-optical trapping, we focus on describing the characterization and control of our magneto-optical trap (MOT) and report the parameters that allow us to fill the trap with a few million atoms in less than a second and cool them to a temperature lower than 5 muK using optical molasses. We will describe the specifics of our cesium-fountain prototype, which operates with a 2-dimensional moving molasses. This special configuration is applicable for multipulse operation of the fountain, which can greatly relax the local oscillator requirements for time keeping. Finally, progress toward our objective of launching cooled atoms, at a rate of one ensemble per second, at speeds of 7 m/s, for a fountain with an interrogation time of about 1 s, will be analysed.PACS Nos.: 06.20, 06.30, and 32.80P

Measurement of the Stark shift of the Cs hyperfine splitting in an atomic fountain

We have used an atomic fountain frequency standard to perform a new measurement of the frequency shift induced by a static electric field $E$ on the clock transition of the cesium $6{}^{2}{S}_{1/2}$ hyperfine splitting. The result in units of ${\mathrm{H}\mathrm{z}/(\mathrm{V}/\mathrm{m})}^{2}$ is $\ensuremath{\delta}{\ensuremath{\nu}}_{0}=\ensuremath{-}2.271(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{E}^{2}.$ The accuracy of this measurement represents a tenfold improvement over previous values. This result is of importance for testing ab initio theoretical calculations of the cesium atomic structure as well as semiempirical models used in parity violation experiments. It will also allow a better evaluation of the frequency shift induced by the blackbody radiation in cesium frequency standards.

Cesium-fountain development at the National Research Council of Canada

The accelerated development of laser methods for manipulating the internal state and motion of atoms, and the recent primary evaluation in France of a cesium frequency standard using cold atoms in an atomic fountain configuration, allow the frontiers of time metrology, accuracy, and stability to be pushed near parts in the 10 level, a factor of 100 over the present primary laboratory clocks. These positive results revived the development of more accurate cesium atomic clocks, and the Institute of National Measurement Standards (INMS), of the National Research Council of Canada (NRC) is undertaking a cesium- fountain project aimed at reaching these new levels of accuracy and stability. This paper reviews its present status. To appreciate the considerable impact of cold atoms in time and frequency metrology, we begin by recalling the background principles of traditional cesium frequency standards, their typical performance and main limitations. After introducing the important theoretical results essential to understanding the techniques of laser cooling and magneto-optical trapping, we focus on describing the characterization and control of our magneto-optical trap (MOT) and report the parameters that allow us to fill the trap with a few million atoms in less than a second and cool them to a temperature lower than 5 >K using optical molasses. We will describe the specifics of our cesium-fountain prototype, which operates with a 2-dimensional moving molasses. This special configuration is applicable for multipulse operation of the fountain, which can greatly relax the local oscillator requirements for time keeping. Finally, progress toward our objective of launching cooled atoms, at a rate of one ensemble per second, at speeds of 7 m/s, for a fountain with an interrogation time of about 1 s, will be analysed.

Pulsar Astrometry and Improved Terrestrial Clocks

AbstractRecent improvements in cesium and hydrogen terrestrial frequency standards have brought the frequency precision of International Atomic Time (TAI) to a value of 2.5E-15 s/s over an averaging time of a month. In this paper we illustrate the improvement graphically, and discuss the state of the art for frame ties between the radio, dynamical, and optical frames. In a larger paper, available via the World Wide Web, we illustrate the measured accuracy curves of the frequency standards, show their effect on the ensemble time scales, explain the reasons for the confusing array of available time scales, and discuss the inverse problem of using pulsar data to correct the terrestrial time scale.

Preliminary evaluation of time scales based on hydrogen masers

Two experimental time scales based on ensembles of hydrogen masers were generated and compared with a variety of other references both internal and external to NIST. The masers all had some type of active cavity control to reduce frequency drift due to cavity changes, The first experimental time scale, TA2M, was generated over the interval from MJD 49050 to 49190 (March 4, 1993-July 22, 1993) using measurements between the masers every 6 hours, TA2M was compared to the NIST AT1 time scale, primarily based on commercial cesium frequency standards, and via once-per-day GPS common-view measurements to the USNO unsteered master clock and to UTC. The linear frequency drift of TA2M relative to UTC was small compared to the uncertainty of 2/spl middot/10/sup -16//d in estimating linear frequency drift. The masers at NIST were then linked to two masers at USNO using GPS common-view time transfer to study the performance of a maser ensemble, TA2M1, at measurement times from a few days to a few months. Again we found that the frequency drift of TA2M1 relative to UTC was smaller than the estimation uncertainty of 0.6/spl middot/10/sup -16//d. The stability of the ensemble at 1 month appeared to be about 1-2 parts in 10/sup 15/. From this work we see that the use of autotuned hydrogen masers in time scales is very promising. A single such maser rivals the 1993 stability of UTC. The typical fractional frequency stability of the masers showed flicker frequency modulation at 4/spl middot/10/sup -15/ or less for measurement times of 10 d-20 d. Linear frequency drift was measured for the masers against the TA2M1 time scale at levels under 1/spl middot/10/sup -16//d with uncertainties of 0.3/spl middot/10/sup -16//d. Measuring frequency drift against the definition of the SI second at this level pushes the limits of current technology.

Velocity of sound measurement in liquids

Sound velocities in pure water at various temperatures were measured. A PZT transducer sends a 2-MHz pulse towards a target stationed at an accurately known distance of 120 mm away. The transit time between the forward pulse and the reflected pulse was measured precisely with statistical averaging and a reference timebase derived from the NRC cesium frequency standard. The transducer-target probe and liquid chamber was immersed in a temperature controlled bath that was under the control of a computer. Measurements were repeated with known concentrations of alcohol and distilled water. Uncertainties of the measurements will be discussed. The authors would like to thank Lu-Jie of the Tongji University for his contribution to the construction of the measurement circuits.