Atomic Fountain Frequency Standard Research Articles

GPS PPP-AR frequency transfer and its application for comparing atomic fountain primary frequency standards between NRC and PTB

We report on the first characterization of the frequency transfer performance of the GPS precise point positioning with carrier-phase integer ambiguity resolution (PPP-AR) implemented by the Natural Resources Canada (NRCan) online service CSRS-PPP. We show that continuous PPP-AR links of multiple days can be formed by carefully fixing the day boundary phase discontinuity. The stability of the links can reach and (Allan deviation) at 1 d and 7 d averaging times, respectively, and mid to low 10−17 at 20 d–30 d. We compared the atomic fountain primary frequency standards (PFS) between NRC in Ottawa, Canada and PTB in Braunschweig, Germany for 135 d via GPS PPP-AR and precise point positioning links. The NRC and PTB PFS agree within a few parts in 1016, which is well below the combined systematic uncertainty. It is expected that with uncertainty at 10−17, the PPP-AR links will become a powerful tool for remote comparisons of atomic optical clocks, essential for the future redefinition of the SI second.

Bimetal Temperature-Compensated Ramsey Cavity for Atomic Fountain Frequency Standards

A typical Ramsey cavity, commonly used in atomic fountain frequency standards (AFFSs) for microwave interrogation, is sensitive to ambient temperature variation. Then, it becomes one of the main limiting factors for the AFFSs operating in a narrow operating-temperature range and a temperature-well-controlled environment up to now. In this article, a new design of bimetal temperature-compensated Ramsey cavity (BTCRC) based on the thermal expansion self-compensating mechanism is presented and experimentally demonstrated. This cavity is a typical TE011-mode cylindrical microwave resonator with a titanium tube and two oxygen-free copper (OFC) caps. The resonance frequency (RF) of the cavity can be immune to temperature variations if the dimensions of the tube and caps are properly designed. To validate this self-compensating mechanism, a BTCRC resonating at the Rb clock frequency with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> -factor of about 17000 is constructed. And its RF thermal coefficient is measured to be 0.73 kHz/°C, which is about 150 times superior to −115.9 kHz/°C of a typical OFC Rb cavity in the temperature range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$22.48~^{\circ }\text{C}$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$37.48~^{\circ }\text{C}$ </tex-math></inline-formula> . During the operation of AFFSs, this BTCRC ensures that the microwave field amplitude inside the cavity, which dictates the transition probability and thus the contrast of the Ramsey fringes, will be stable in time. In addition, the potential temperature variation-induced frequency shifts due to cavity pulling are significantly minimized.

Time: Astronomical time - atomic time - redefining second based on constant

The evolution history of modern time from the astronomical time to the atomic second is briefly reviewed herein. The progress and status of realizing the current definition of the second using cesium cold atomic fountain primary frequency standards, the process of generating the international atomic time (TAI) and coordinated universal time (UTC), and trend of releasing the tolerance on inserting leap-seconds into the UTC are sketched. The present situation on steering the TAI regularly from January 2016 by cesium fountain frequency standards from five countries is reported. Research progress of local and global optical frequency standards is summarized, and steering TAI by six optical frequency standards starting from 2016 is reported. Requirements and responses on steering TAI regularly by optical frequency standards are introduced according to the roadmap, which is under discussion internationally, or the future redefinition of the second. Furthermore, two types of optical frequency standards, namely the basic-research and the metrological type are suggested, and the performance and technical qualifications for the metrological type of optical frequency standards are recommended. For the optical frequency standards of the metrological type, with uncertainties better than 3×10−17, not only static relative gravity but also the alternative tide frequency shifts should be considered in their frequency evaluations. Finally, a scheme employing frequency comparison, across a long longitude-distance, between two highly stable optical frequency standards to demonstrate the relativistic tidal frequency shift directly and experimentally is proposed.

Inverse problem approach to evaluate quadratic Zeeman effect for an atomic fountain frequency standard KRISS-F1

The evaluation of the quadratic Zeeman shift in atomic fountain frequency standards requires information on the magnetic field distribution along the drift region. Plotting Zeeman frequencies against various launching heights provides the temporally averaged magnetic field values seen by the flying atoms. Using those data, we were able to deduce a reasonable field map by taking the approach of solving an inverse problem. We employed a regularization method after establishing a linear set of equations. The analysis enabled us to evaluate the quadratic Zeeman shift and its uncertainty against various launching heights. Our method encompasses and generalizes the deconvolution method. This paper covers the mathematical tricks to solve the inverse problem and its application to the atomic fountain frequency standard KRISS-F1.

Measuring atom positions in a microwave cavity to evaluate distributed cavity phase shifts

Distributed cavity phase (DCP) frequency shifts are a leading systematic effect in atomic fountain frequency standards. They originate from the phase variations of the field in the microwave cavity combined with different positions of the atoms in the cavity on the ascent and descent. Here we demonstrate techniques to precisely determine the position of the cloud of atoms in the microwave cavity, using either the approximately linear variation of the transverse components of the microwave field or the quadratic variation of the longitudinal microwave field amplitude in the cavity. We also show that shifting the initial position of the atoms gives a significantly higher sensitivity to DCP variations than the often-used tilting of fountains. A demonstrated centring precision of order 50 μm will enable DCP frequency shift uncertainties to be reduced to less than 10–17 and thereby contribute insignificantly to the accuracy budget of a standard. These techniques to vertically align a fountain are straightforward to automate for routine operation and require a negligible fraction of the standard’s averaging time.

10-15 Frequency Stability Microwave Synthesizer for Atomic Fountain Frequency Standard

This paper presents the design and phase noise measurement of microwave frequency synthesizer for atomic fountain frequency standard. In this synthesizer, a voltage-controlled DRO is locked to 100MHz hydrogen maser signal through a divider chain. The required interrogation frequency 9192631770 Hz is generated by mixed with 7368230 Hz and 400 MHz signals. The experimental results show that the phase noise of interrogation signal at 9.192 GHz is -88dBc/Hz@ offset 1Hz. With such performances, the expected Dick effect contribution to atom fountain frequency standard is reported at a level of 1.0×10-15 at 1 s integrating time, that is a factor 10 higher than quantum noise limit of atom frequency standard. The carrier spectral spur of microwave synthesizer signal impacted on atomic fountain frequency standard is also discussed.

Atomic fountain frequency standard: principle and development

The principle and development of fountain frequency standard are introduced in this paper. Fountain frequency standard is an atomic clock technology developed in recent 20 years. It is based on laser cooling technology, and realizes the trapping and projection of the cold atom medium with laser cooling technology. In the process of launching upward and falling back, the cold atom medium first completes the preparation of atomic state, then passes through the microwave cavity twice to achieve the Ramsey interaction; between the two interactions it undergoes free evolution, and finally the Ramsey interference fringes are obtained by detecting the atomic interference probability with the two-level fluorescence detection method in the detection region, and the frequency is locked with a line width of the central fringe being about 1 Hz. The stability and uncertainty of the frequency are two important indexes of the fountain frequency standard. The factors influencing the stability of the fountain clock frequency mainly are quantum projection noise and electronic noise. At present, the short term stability of the fountain clock is (10-13-10-14)τ-1/2, and the long term stability is (10-16-10-17). The frequency uncertainty of the fountain frequency standard is mainly influenced by the two-order Zeeman frequency shift, the blackbody radiation frequency shift, the cold atom collisional frequency shift, and the frequency shift relating to the microwave. The uncertainty of the fountain clock is around 10-16 currently. As a reference frequency standard, the working media of the fountain clock mainly are 133Cs and 87Rb. All international metrology institutions have been developing the fountain frequency standard, and it plays a more and more important role in establishing the coordinated universal time and the calibration of the international atomic time. In addition, the fountain frequency standards are also used to study high-precision time-frequency reference and time comparison chain, and verify basic physical theories.

UTC(OP) based on LNE-SYRTE atomic fountain primary frequency standards

UTC(OP), the French national realization of the international coordinated universal time, was redesigned and rebuilt. The first step was the implementation in October 2012 of a new algorithm based on a H-maser and on atomic fountain data. Thanks to the new implementation, the stability of UTC(OP) was dramatically improved and UTC(OP) competes with the best time scales available today. Then the hardware generation and distribution of the UTC(OP) physical signals were replaced. Part of the new hardware is composed of commercial devices, but the key elements were specifically developed. One of them is a special switch that allows the UTC(OP) signals to be derived from one of two time scales, based on two different H-masers, which are generated simultaneously. This insures the continuity of the UTC(OP) signal even when a change of the reference H-maser is required. With the new hardware implementation, UTC(OP) is made available through three coherent signals: 100 MHz, 10 MHz and 1 PPS. For more than 3 years, UTC(OP) remained well below 10 ns close to UTC, with a difference even less than 5 ns if we except a short period around MJD 56650.

NPLI Cesium Atomic Fountain Frequency Standard: Preliminary Results

National Physical Laboratory India has developed a cesium atomic fountain frequency standard. The fountain has now become operational and is ready for complete frequency evaluation. In this paper, we report for the first time, the preliminary results obtained with this fountain, which include Ramsey fringes, frequency stability analysis, with an H-maser, and magnetic field mapping in the atomic flight region.

Accurate rubidium atomic fountain frequency standard

The design, operating parameters and the accuracy evaluation of the NPL Rb atomic fountain are described. The atomic fountain employs a double magneto-optical arrangement that allows a large number of 87Rb atoms to be trapped, a water-cooled temperature-stabilized interrogation region and a high quality factor interrogation cavity. From the uncertainties of measured and calculated systematic frequency shifts, the fractional frequency accuracy is estimated to be 3.7 × 10−16. The fractional frequency stability, limited predominantly by noise in the local oscillator, is measured to be 7 × 10−16 after one day of averaging. Based on the proposed quasi-continuous regime of operation of the fountain, the accuracy of the Rb standard of 5 × 10−17 reachable in two days of averaging is predicted.

Cryogenic-Sapphire-Oscillator-Based Reference Signal at 1 GHz with 10-15 Level Instability

We describe a 1 GHz reference signal with 10-15 level fractional frequency stability for averaging times longer than 1 s, synthesized from a 10.8 GHz oscillation frequency of a cryogenic-sapphire-resonator-based ultra-stable oscillator and phase locked to a hydrogen maser with a time constant of about 1000 s. The degradation of the short term stability of the reference signal by phase locking to the hydrogen maser was evaluated. We also report on the implementation of the 1 GHz signal down converted to 5 MHz as a reference oscillator for a cesium atomic fountain frequency standard.

An experimental study of intermodulation effects in an atomic fountain frequency standard

The short-term stability of passive atomic frequency standards, especially in pulsed operation, is often limited by local oscillator noise via intermodulation effects. We present an experimental demonstration of the intermodulation effect on the frequency stability of a continuous atomic fountain clock where, under normal operating conditions, it is usually too small to observe. To achieve this, we deliberately degrade the phase stability of the microwave field interrogating the clock transition. We measure the frequency stability of the locked, commercial-grade local oscillator, for two modulation schemes of the microwave field: square-wave phase modulation and square-wave frequency modulation. We observe a degradation of the stability whose dependence with the modulation frequency reproduces the theoretical predictions for the intermodulation effect. In particular no observable degradation occurs when this frequency equals the Ramsey linewidth. Additionally we show that, without added phase noise, the frequency instability presently equal to 2×10-13 at 1 s, is limited by atomic shot-noise and therefore could be reduced were the atomic flux increased.

Design and metrological features of microwave synthesizers for atomic fountain frequency standard

In this paper we describe the improved redesign of the microwave frequency synthesizers for Laboratoire National d'Essais-Systèmes de Référence Temps-Espace (LNE-SYRTE) atomic fountains. The synthesizers use a cryogenic oscillator to generate both Cs and Rb hyperfine frequencies based on a new distribution frequency of 1 GHz. The main metrological features (phase noise, long-term phase stability, and spectral purity) of the synthesizers have been measured in situ connected to an atomic fountain and are compatible with an accuracy goal of 10(-16) for the atomic fountains. The simultaneous test of two different synthesizers on the FO2 atomic fountain at the 10(-16) level also is reported.

Microwave leakage-induced frequency shifts in the primary frequency Standards NIST-F1 and IEN-CSF1

In atomic fountain primary frequency standards, the atoms ideally are subjected to microwave fields resonant with the ground-state, hyperfine splitting only during the two pulses of Ramsey's separated oscillatory field measurement scheme. As a practical matter, however, stray microwave fields can be present that shift the frequency of the central Ramsey fringe and, therefore, adversely affect the accuracy of the standard. We investigate these uncontrolled stray fields here and show that the frequency errors can be measured, and indeed even the location within the standard determined by the behavior of the measured frequency with respect to microwave power in the Ramsey cavity. Experimental results that agree with the theory are presented as well.

Power dependence of the frequency bias caused by spurious components in the microwave spectrum in atomic fountains

The presence of spurious spectral components in the microwave excitation may induce frequency shifts in an atomic fountain frequency standard. We discuss how such shifts behave as a function of power variations of the excitation carrier and in the spur-to-carrier ratio. The discussion here is limited to the case of single-sideband spurs, which are generally much more troublesome due to their ability to cause frequency shifts. We find an extremely rich and unintuitive behavior of these frequency shifts. We also discuss how pulsed operation, typical of today's fountain frequency standards, relates to frequency shifts caused by spurs in the microwave spectrum. The conclusion of these investigations is that it is, at best, difficult to use elevated power microwaves in fountain frequency standards to test for the presence of spurs in the microwave spectrum.

Role of atomic and light polarization in an adiabatic passage experiment

Adiabatic passage with optical pulses has been used as an efficient tool for the transfer of atomic populations between Zeeman sublevels. The composition of the final state after adiabatic passage is investigated in the context of applications to atomic fountain frequency standards. Theoretical and experimental studies on the influence of the purity of the initial state on the result of the transfer are presented. Similarly, the role of the quality of polarization of the laser pulses in the process of adiabatic passage is discussed.

Power dependence of distributed cavity phase-induced frequency biases in atomic fountain frequency standards

We discuss the implications of using high-power microwave tests in a fountain frequency standard to measure the frequency bias resulting from distributed cavity-phase shifts. We develop a theory which shows that the frequency bias from distributed cavity phase depends on the amplitude of the microwave field within the cavity. The dependence leads to the conclusion that the frequency bias associated with the distributed cavity phase is typically both misestimated and counted twice within the error budget of fountain frequency standards.

Numerical Simulation of Distributed Cavity Phase Shift in Atomic Fountain Frequency Standard

The spatial phase variations in feeding a microwave cavity with asymmetrical amplitudes and phases were calculated using the finite element method (FEM) that was applied to the three-dimensional (3D) model of the microwave cavity developed at the National Metrology Institute of Japan (NMIJ)/AIST. The distributed cavity phase shift (DCPS) in an atomic fountain frequency standard was calculated for atomic interaction with the position-dependent amplitudes and phases inside the cavity. The tolerances of asymmetry in the phases and amplitudes of microwaves were determined in order to achieve an uncertainty of 5×10-16 with respect to the DCPS.

Preliminary Evaluation of the Cs Atomic Fountain Frequency Standard at NMIJ/AIST

With the aim of contributing to the calibration of the International Atomic Time Scale (TAI), a cesium atomic fountain frequency standard is developed at the National Metrology Institute of Japan. The short-term stability is measured to be /spl sigma/(/spl tau/)=4.7/spl times/10/sup -13//spl times//spl tau//sup -1/2/ by comparison with a hydrogen maser. Preliminary results of the accuracy evaluation, including measurement of the second-order Zeeman shift, are presented.

Development of a cesium atomic fountain frequency standard

We report on the development of a cesium atomic fountain frequency standard at the Korea Research Institute of Standards and Science (KRISS). Cesium atoms were launched upwards by using an optical moving molasses method after they were cooled in a magnetooptical trap. The temperature of launched atoms was about 2.5 /spl mu/K. From our cesium atomic fountain, we were able to observe Ramsey fringes of 1 Hz linewidth on the 9.2 GHz clock transition of cesium atoms.