Cs Clock Research Articles

Spacetime variation of the s and c quark masses

Space-time variation of fundamental physical constants in expanding Universe is predicted by a number of popular models. The masses of second generation quarks are larger than first generation quark masses by several orders of magnitude, therefore space-time variation in quark masses may significantly vary between each generation. We evaluate limits on variation in the s and c quark masses from Big Bang nucleosynthesis, Oklo natural nuclear reactor, Yb+, Cs and Rb clock data. The construction of 229Th nuclear clock is expected to enhance these limits by several orders of magnitude. Furthermore, constraints are obtained on an oscillating scalar or pseudoscalar cold dark matter field, as interactions of the field with quarks produce variations in quark masses.

Femtosecond-Level Frequency Transfer at 10 GHz over Long Fiber Link with Optical–Electronic Joint Compensation

We report a fiber-optic 10 GHz frequency transfer technique based on an optical–electronic joint phase compensator. A highly stable frequency signal at 10 GHz was transferred in a 50-km long fiber link by using this technique. Two key parameters of the frequency dissemination, the timing fluctuation and frequency stability were both measured. The experimental results show the root-mean-square timing fluctuation of the transferred microwave is about 103 fs within 10,000 s, and the frequency stability for the transmission link is 2.2 × 10−14 at 1 s and 8.5 × 10−17 at 2000 s. The technique proposed in this paper provides a powerful tool which can be used to transfer atomic clocks (e.g., commercial H-master and Cs clocks) in a long fiber link.

High accuracy spectroscopic parameters of the 1.27 µm band of O2 measured with comb-referenced, cavity ring-down spectroscopy

We present broadband spectroscopic parameters in the a1Δg−X3Σg−(0,0) band of 16O2 centered near 1.27 µm. High-fidelity absorption spectra were acquired on room-temperature air samples over the pressure range of 3.3 kPa to 100 kPa using a length-stabilized cavity ring-down spectrometer, with spectrum frequencies linked to a Cs clock via an optical frequency comb. Parameters were determined by multi-spectrum fitting of advanced line profiles to the measured spectra. This analysis yielded line intensities, positions, collisional broadening and shifting coefficients, Dicke narrowing and speed dependent line shape parameters, first-order line mixing coefficients, and O2-air binary absorption coefficients for collision-induced absorption. We report relative standard uncertainties in the line intensities and broadening coefficients of 0.16 % and 0.05 %, respectively, and line position uncertainties of 60 kHz (2 × 10−6 cm−1). Our analysis reveals strong numerical correlation in fits of the Dicke narrowing and speed dependent widths and shows that inclusion of a recently proposed modification to the Hartmann Tran line profile can increase fitted Dicke narrowing frequencies by as much as 50 %.

Enhancing the frequency stability of national time scale using EMD

Almost all time keeping laboratories maintain the 5071A Cesium (Cs) frequency standard for generating their national time scales. The frequency stability of this standard is limited by different types of noise, especially the White Frequency Modulation noise. These noise types affect the stability of the resultant average Time Scale (TS). Kalman Filter (KF) is still applied until now within the time scale algorithm for de-noising and predicting improved resultant TS frequency stability. But, this method is very complicated and is based on difficult estimation techniques. In 2013, the Empirical Mode Decomposition (EMD) technique was applied for the first time on Cs atomic clock signal de-noising and frequency prediction. In this paper, the EMD technique is embedded in the TS generation algorithm for studying its effect on the stability of the resultant TS. Results show that the frequency stability of the resultant TS is improved for averaging times up to nearly 40 days due to EMD. These results are verified by comparing the effect of the EMD to that of the KF on the resultant TS frequency stability. Results show that the frequency stability of the resultant average TS is improved due to using EMD or KF for Cs clock signal de-noising for averaging times up to nearly 40 days. In addition, EMD is found to be more effective and simpler than the complicated KF.

Highly-stable laser-based underwater radio-frequency transfer with electronic phase compensation

We demonstrate a highly-stable laser-based underwater radio frequency transfer with electronic phase compensation technique. A 100 MHz radio-frequency signal has been transferred over a 5-m underwater link with this frequency transfer scheme. To evaluate the quality of the transferred frequency signal, timing jitter power spectral, timing fluctuations and instabilities were all measured. The transferred frequency signal over 5-m underwater link with electronic phase compensation achieved 2.1 ps of root-mean-square (RMS) timing fluctuations, meanwhile, the relative fractional frequency instability is 5 ×10−13 at 1 s and 7 ×10−16 at 1000 s for phase-compensated link. The transmission link achieved higher stability than Cs and H-master clocks, which gives this proposed transfer scheme the potential of disseminating these atomic clocks signal over the underwater transmission link.

Laser-based atmospheric radio-frequency transfer with sub-picosecond timing fluctuation using single phase compensator

We demonstrate a 100-m long outdoor laser-based atmospheric transfer with sub-picosecond timing fluctuation using a single phase compensator setup. With this frequency transfer scheme, a 1 GHz radio-frequency signal has been transferred over a 100 m atmospheric link. Timing fluctuation and instability were measured to evaluate the quality of the distributed frequency signal. The experimental results show the root-mean-square timing fluctuation of the transferred frequency signal is 441 fs within 5000 s, with a relative fractional frequency instability of 2 × 10−13 @ 1 s and 2 × 10−16 @ 1000 s. The proposed frequency transfer technique with single phase compensator has a potential application that disseminating a commercial Cs clock and H-master signal in free space since the stability of the transmission link is superior to these atomic clocks.

Phase-stabilized 100\xa0mW frequency comb near 10\xa0\u03bcm

Long-wavelength mid-infrared (MIR) frequency combs with high power and flexible tunability are highly desired for molecular spectroscopy, including investigation of large molecules such as C60. We present a high power, phase-stabilized frequency comb near 10 μm, generated by a synchronously pumped, singly resonant optical parametric oscillator (OPO) based on AgGaSe2. The OPO can be continuously tuned from 8.4 to 9.5 μm, with a maximum average idler power of 100 mW at the center wavelength of 8.5 μm. Both the repetition rate (frep) and the carrier-envelope offset frequency (fceo) of the idler wave are phase-locked to microwave signals referenced to a Cs clock. We describe the detailed design and construction of the frequency comb, and discuss potential applications for precise and sensitive direct frequency comb spectroscopy.

Search for transient ultralight dark matter signatures with networks of precision measurement devices using a Bayesian statistics method

We analyze the prospects of employing a distributed global network of precision measurement devices as a dark matter and exotic physics observatory. In particular, we consider the atomic clocks of the Global Positioning System (GPS), consisting of a constellation of 32 medium-Earth orbit satellites equipped with either Cs or Rb microwave clocks and a number of Earth-based receiver stations, some of which employ highly-stable H-maser atomic clocks. High-accuracy timing data is available for almost two decades. By analyzing the satellite and terrestrial atomic clock data, it is possible to search for transient signatures of exotic physics, such as "clumpy" dark matter and dark energy, effectively transforming the GPS constellation into a 50,000km aperture sensor array. Here we characterize the noise of the GPS satellite atomic clocks, describe the search method based on Bayesian statistics, and test the method using simulated clock data. We present the projected discovery reach using our method, and demonstrate that it can surpass the existing constrains by several order of magnitude for certain models. Our method is not limited in scope to GPS or atomic clock networks, and can also be applied to other networks of precision measurement devices.

JY1 time scale: a new Kalman-filter time scale designed at NIST

We report on a new Kalman-filter hydrogen-maser time scale (i.e. JY1 time scale) designed at the National Institute of Standards and Technology (NIST). The JY1 time scale is composed of a few hydrogen masers and a commercial Cs clock. The Cs clock is used as a reference clock to ease operations with existing data. Unlike other time scales, the JY1 time scale uses three basic time-scale equations, instead of only one equation. Also, this time scale can detect a clock error (i.e. time error, frequency error, or frequency drift error) automatically. These features make the JY1 time scale stiff and less likely to be affected by an abnormal clock. Tests show that the JY1 time scale deviates from the UTC by less than ±5 ns for ~100 d, when the time scale is initially aligned to the UTC and then is completely free running. Once the time scale is steered to a Cs fountain, it can maintain the time with little error even if the Cs fountain stops working for tens of days. This can be helpful when we do not have a continuously operated fountain or when the continuously operated fountain accidentally stops, or when optical clocks run occasionally.

Frequency Measurement System of Optical Clocks Without a Flywheel Oscillator.

We developed a system for the remote frequency comparison of optical clocks. The system does not require a flywheel oscillator at the remote end, making it possible to evaluate optical frequencies even in laboratories, where no stable microwave reference, such as an Rb clock, a Cs clock, or a hydrogen maser exists. The system is established by the integration of several systems: a portable carrier-phase two-way satellite frequency transfer station and a microwave signal generation system by an optical frequency comb from an optical clock. The measurement was as quick as a conventional method that employs a local microwave reference. We confirmed the system uncertainty and instability to be at the low 10-15 level using an Sr lattice clock.

Double-modulation CPT cesium compact clock

Double-modulation coherent population trapping (CPT) is based on a synchronous modulation of Raman phase and laser polarization, which allows the atomic population to accumulate in a common dark state. The high contrast signal obtained on the clock transition with a relative compact and robust laser system is interesting as basis of a high performance microwave clock. Here we study the parameters of a double-modulation CPT Cs clock working in cw mode. The optimal polarization modulation frequency and cell temperature for maximum contrast of clock transition are investigated. The parameters of the detection are also studied. With the optimal parameters, we observe a CPT signal with contrast of 10% and linewidth of 492 Hz, which is well suited for implementing a cw atomic clock.

Line shapes, positions and intensities of water transitions near 1.28 μm

We present measurements of approximately 70 isolated, self-broadened, water vapor lines which are assigned to the (1,0,1)–(0,0,0), (0,0,2)–(0,0,0), (1,2,0)–(0,0,0), and (2,0,0)–(0,0,0) vibrational bands and which occur in the transparency window region from 7710cm−1 to 7920cm−1. We acquired absorption spectra on room-temperature, water samples over the pressure range 150–800Pa with the frequency-stabilized cavity ring-down spectroscopy technique. In order to optimize measurement accuracy, we integrated mK-level temperature control and SI-traceable pressure measurements into our cavity ring-down spectroscopy measurements. This technique yielded relative uncertainties of 0.04% and 0.20% in sample density and measured line intensity, respectively. We also referenced our spectrum frequency axes to a Cs clock, which provided vacuum line positions with a combined standard uncertainty of 3MHz. Comparison of our measured intensities, positions and self-broadening parameters with literature values reveals that the present work substantially reduces uncertainty in these line parameters. For the (1,0,1)–(0,0,0) vibrational band, the measured line intensities agree at the 1%-level with published ab initio calculations. Our spectra exhibited signal-to-noise ratios up to ≈20,000:1 to enable stringent tests of theoretical line profiles through multispectrum least-squares data analysis. We show that the partially correlated, quadratic-speed-dependent Nelkin–Ghatak profile gives a quality of fit that is commensurate with the high spectrum signal-to-noise ratio, and unlike most other profiles considered here, reproduces the measured line shapes without systematic residuals over the entire pressure range. Our results confirm that mechanisms of (1) collisional narrowing, (2) speed-dependent effects and (3) partial correlation between velocity-changing and dephasing collisions, contribute to the self-broadened line shape of isolated water vapor transitions.

Temperature Dependence Cancellation of the Cs Clock Frequency in the Presence of Ne Buffer Gas

The temperature dependence of the Cs clock transition frequency in a vapor cell filled with Ne buffer gas has been measured. The experimental setup is based on the coherent population trapping (CPT) technique and a temporal Ramsey interrogation allowing a high resolution. A quadratic dependence of the frequency shift is shown. The temperature of the shift cancellation is evaluated. The actual Ne pressure in the cell is determined from the frequency shift of the 895nm optical transition. We can then determine the Cs-Ne collisional temperature coefficients of the clock frequency. These results can be useful for vapor cell clocks and especially for future micro-clocks.

Composite clock including a Cs clock, an H-Maser clock, and a voltage-controlled oscillator

This article presents the project of creating a composite clock, the advances in its realization and the technical choices. The goal is to generate an output signal which combines the advantages of each clock, the long-term stability for the cesium clock, the mid-term stability for the hydrogen maser clock, and the short-term stability for the voltage-controlled oscillator (VCO). The control system is designed to transfer the stability of the best clock at a given averaging time to the VCO. This system is designed to reach a relative instability of approximately 10(-14) at 1 s, 10(-15) at 10(3) s, and 10(-14) at 10(6) s at 100 MHz, depending on the stability of the master clocks. To make this composite clock, we use a digital PLL with 2 references.

Composite clock: a new algorithm for servoing a VCO firstly to a hydrogen maser clock and then to a caesium clock

We present the theory and the development of an algorithm used for locking a voltage controlled oscillator (VCO) to a hydrogen maser clock (HM clock) and then to a caesium clock (Cs clock). This algorithm is the heart of a composite clock using these three oscillators.

High performance of a DMTD system used in a composite clock including a Cs clock, a H-maser (clock) and a VCO

We present a composite clock based on servoing a voltage controlled oscillator (VCO) by both a caesium beam clock and a hydrogen maser. We wish to obtain a physical realization of a new reference timescale with an output signal which combines the long term stability of the caesium clock, the middle term stability of the H maser and the short term stability of the VCO. The system contribution at 100 MHz to the middle term frequency instabilities is expected to be around 10−15 at 1 s. This paper describes the dual-mixer time-difference (DMTD) system used to obtain the information needed for the control of the VCO.

Characterization of periodic variations in the GPS satellite clocks

The clock products of the International Global Navigation Satellite Systems (GNSS) Service (IGS) are used to characterize the timing performance of the GPS satellites. Using 5-min and 30-s observational samples and focusing only on the sub-daily regime, approximate power-law stochastic processes are found. The Block IIA Rb and Cs clocks obey predominantly random walk phase (or white frequency) noise processes. The Rb clocks are up to nearly an order of magnitude more stable and show a flicker phase noise component over intervals shorter than about 100 s. Due to the onboard Time Keeping System in the newer Block IIR and IIR-M satellites, their Rb clocks behave in a more complex way: as an apparent random walk phase process up to about 100 s and then changing to flicker phase up to a few thousand seconds. Superposed on this random background, periodic signals have been detected in all clock types at four harmonic frequencies, n × (2.0029 ± 0.0005) cycles per day (24 h coordinated universal time or UTC), for n = 1, 2, 3, and 4. The equivalent fundamental period is 11.9826 ± 0.0030 h, which surprisingly differs from the reported mean GPS orbital period of 11.9659 ± 0.0007 h by 60 ± 11 s. We cannot account for this apparent discrepancy but note that a clear relationship between the periodic signals and the orbital dynamics is evidenced for some satellites by modulations of the spectral amplitudes with eclipse season. All four harmonics are much smaller for the IIR and IIR-M satellites than for the older blocks. Awareness of the periodic variations can be used to improve the clock modeling, including for interpolation of tabulated IGS products for higher-rate GPS positioning and for predictions in real-time applications. This is especially true for high-accuracy uses, but could also benefit the standard GPS operational products. The observed stochastic properties of each satellite clock type are used to estimate the growth of interpolation and prediction errors with time interval.

Precise control of femtosecond Ti:sapphire laser frequency comb

Phase controlled Femtosecond Ti:sapphire laser frequency comb have been widely used in measurement of absolute optical frequency, which is the revolutionary progress in the optical metrology. In this paper, based on the home-made 90MHz femtosecond Ti:sapphire laser, we firstly broadened the spectrum output directly from the laser to more than one octave with a piece of photonic crystal fiber，then locked precisely both the repetition rate and the carrier-envelope phase offset frequency to the same Cs clock with the stability of 6×10－14 by use of two sets of different phase-locked loops. As a result, we obtained the phase controlled Femtosecond laser frequency comb with the stability of the Cs clock.

Improvement of frequency change at Japan standard time

AbstractJapan Standard Time is an average atomic time generated by a group of Cs clocks at NICT. Several years ago, a problem was found in the timescale, that is, the frequency change by the removal of a clock from the group. The reason turned out to be improper clock‐rate estimation of each Cs clock. We fixed the problem by revising the clock‐rate estimation method. After this revision, the frequency change on clock removal became small. We also derived a formula to express the frequency change of the timescale by using the data of the removed clock. In some simulations and actual Japan Standard Time, this formula demonstrated its validity for estimating the frequency change. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 157(1): 29–37, 2006; Published online in Wiley InterScience (http://www.interscience.wiley.com). DOI 10.1002/eej.20384

Continuous Nondemolition Measurement of the Cs Clock Transition Pseudospin

We demonstrate a weak continuous measurement of the pseudospin associated with the clock transition in a sample of Cs atoms. Our scheme uses an optical probe tuned near the D1 transition to measure the sample birefringence, which depends on the component of the collective pseudospin. At certain probe frequencies the differential light shift of the clock states vanishes, and the measurement is nonperturbing. In dense samples the measurement can be used to squeeze the collective clock pseudospin and has the potential to improve the performance of atomic clocks and interferometers.