Primary Frequency Standards Research Articles

Near- to mid-IR spectral purity transfer with a tunable frequency comb: Methanol frequency metrology over a 1.4 GHz span

We report the upgrade and operation of a frequency-comb-assisted high-resolution mid-infrared molecular spectrometer, allowing us to combine high spectral purity, International System of Units (SI)-traceability, wide tunability, and high sensitivity. An optical frequency comb is used to transfer the spectral purity of a SI-traceable 1.54 μm metrology-grade frequency reference to a 10.3 μm quantum cascade laser (QCL). The near-infrared reference is operated at the French time/frequency metrology institute, calibrated there to primary frequency standards, and transferred to Laboratoire de Physique des Lasers via the REFIMEVE fiber network. The QCL exhibits a linewidth of δν ∼ 0.1 Hz and a sub-10−15 relative frequency stability from 0.1 to 10 s, and its frequency is traceable to the SI with a total relative uncertainty better than 4 × 10−14 after 1 s averaging time. We have developed the instrumentation allowing comb modes to be continuously tuned over 9 GHz, resulting in a QCL of record spectral purity uninterruptedly tunable at the precision of the reference over an unprecedented span of Δν = 1.4 GHz. We have used our apparatus to conduct sub-Doppler spectroscopy of methanol in a multi-pass cell, demonstrating state-of-the-art frequency uncertainties down to the few kilohertz level (∼10−10 in relative value). We have observed weak intensity resonances unreported so far, resolved subtle doublets never seen before, and brought to light discrepancies with HITRAN. This demonstrates the potential of our apparatus for probing subtle internal molecular processes, building accurate spectroscopic models of polyatomic molecules of atmospheric or astrophysical interest, and carrying out precise spectroscopic tests of fundamental physics.

First evaluation of the primary frequency standard NTSC-CsF2

Caesium fountain primary frequency standards have been used to realize the definition of the second in the International System of units for more than two decades. This paper reports the structure and uncertainty evaluation of the NTSC-CsF2, developed by National Time Service Center (NTSC), Chinese Academy of Sciences. The short-term stability at high atomic density is 2.1 × 10−13 at 1 s of averaging time. The fractional combined type B uncertainty is evaluated at 4.3 × 10−16. A frequency comparison has been conducted between NTSC-CsF2 and the SI second through a Global Navigation Satellite System link for eight months whose results show a fractional frequency difference of 4.2(5.2) × 10−16. With a reduced chi-square of 0.8, NTSC-CsF2 is found to be consistent with that of the other fountains within the uncertainty.

Continuous optical generation of microwave signals for fountain clocks.

For the optical generation of ultrastable microwave signals for fountain clocks, we developed a setup based on a cavity stabilized laser and a commercial frequency comb. The robust system, in operation since 2020, is locked to a 100MHz output frequency of a hydrogen maser and provides an ultrastable 9.6GHz signal for the interrogation of atoms in two cesium fountain clocks, acting as primary frequency standards. Measurements reveal that the system provides a phase noise level that enables quantum projection noise limited fountain frequency instabilities at the low 10-14(τ/s)-1/2 level. At the same time, it offers largely maintenance-free operation.

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.

Resilient timekeeping algorithm with multi-observation fusion Kalman filter

The timescales incorporated into the Primary Frequency Standard (PFS) exhibit excellent stability and accuracy. However, during the dead time of PFS, the reliability of the timescale can be compromised. To address this issue, a resilient timekeeping algorithm with a Multi-observation Fusion Kalman Filter (MFKF) is proposed. This algorithm fuses the frequency measurements from hydrogen masers with various reference frequency standards, including PFS and commercial cesium beam atomic clocks. The simulation results show that the time deviation and instability of the timescale generated by MFKF are improved compared to those with Kalman filtering. The experimental results demonstrate that even within 70 days of PFS dead time the resilient timescale generated by MFKF can operate reliably. Furthermore, it is theoretically proven that MFKF produces a smaller post-covariance than that with single-observation Kalman filtering.

Characteristics of a real-time ensemble time scale corrected by the KRISS-PSFS using a reduced Kalman filter (Kred) algorithm

An ensemble time, which is indicated by TA (temps atomique, in French), was generated every minute by combining four active hydrogen masers with the reduced Kalman filter (Kred) algorithm. The TA frequency was averaged over 12 h and then the mean frequency was steered to the primary and secondary frequency standards developed by KRISS with a micro-phase stepper (MPS). The steered and physically realized TA frequency is called KRISS ensemble time (KET). We investigated the frequency-state estimate outputted from the Kred algorithm and the control signal applied to the MPS, according to the abnormal temperature change of the room where the hydrogen masers were placed. When the frequency of the TA was corrected by the KRISS-F1, the Allan deviation of KET was 1.72 × 10−15 at the average time τ = 1 d, and the variation of [UTC–KET] was stable within 2 ns for 75 d.

Absolute frequency measurement of the 87Sr optical lattice clock at NTSC using international atomic time

We report the absolute frequency measurement of the transition in 87Sr optical lattice clock (Sr1) at National Time Service Center. Its systematic frequency shifts are evaluated carefully with a total relative uncertainty of . The measured absolute frequency is 429 228 004 229 872.91(18) Hz with a relative uncertainty of , with reference to the ensemble of primary and secondary frequency standards published in the Circular T bulletin by BIPM through a global navigation satellite system link.

Reliability and Reproducibility of the Cryogenic Sapphire Oscillator Technology

The cryogenic sapphire oscillator (CSO) is a highly specialized machine, which delivers a microwave reference signal exhibiting the lowest frequency fluctuations (Allan deviation σy(τ)) for the integration time τ between 1 s and 104 s, indeed 4 decades. In such interval, good units feature σy(τ) < 10-15, with < 10-14 drift in one day. The oscillator is based on a sapphire monocrystal resonating at 10 GHz in a whisperinggallery mode, cooled at ≈ 6 K for zero thermal coefficient and optimal quality factor Q.We report on the progress accomplished implementing eleven CSOs in about 10 years since the first sample delivered to the Malargüe station of the European Space Agency (ESA) in Argentina. Short-term stability is improved by a factor of 3.10, depending on τ, and the refrigerator’s electric power is reduced to 3 kW single-phase line. Frequency stability and overall performances are reproducible, with unattended operation between scheduled maintenance every two years. The CSO is now a semicommercial product suitable to scientific applications requiring extreme frequency stability with reliable unattended long-term operation, like the flywheel for primary frequency standards, the ground segment of Global Navigation Satellite System (GNSS), astrometry, Very Long Baseline Interferometry, and radio astronomy stations.

Fano-like Resonance due to Interference with Distant Transitions.

Narrow optical resonances of atoms or molecules have immense significance in various precision measurements, such as testing fundamental physics and the generation of primary frequency standards. In these studies, accurate transition centers derived from fitting the measured spectra are demanded, which critically rely on the knowledge of spectral line profiles. Here, we propose a new mechanism of Fano-like resonance induced by distant discrete levels and experimentally verify it with Doppler-free spectroscopy of vibration-rotational transitions of CO_{2}. The observed spectrum has an asymmetric profile and its amplitude increases quadratically with the probe laser power. Our results facilitate a broad range of topics based on narrow transitions.

Measurements of Majorana transition frequency shift in caesium atomic fountain clocks

The caesium atomic fountain clock is a primary frequency standard. During its operation, a Majorana transition frequency shift will occur once a magnetic field at some special locations along the atomic trajectory is singular. In this study, by developing a physical model, we analyzed the magnetic field requirements for atomic adiabatic transition and calculated the influence of the Majorana atomic transition on the atomic state via a quantum method. Based on the simulation results for the magnetic field in the fountain clock, we applied the Monte Carlo method to simulate the relationship between the Majorana transition frequency shift and the magnetic field at the entrance of the magnetic shielding, as well as the initial atomic population. Measurement of the Majorana transition frequency shift was realized by state-selecting asymmetrically populated atoms. The relationship between the Majorana transition frequency shift and the axial magnetic field at the entrance of the magnetic shielding was obtained. The measured results were essentially consistent with the calculated results. Thus, the magnetic field at the entrance of the magnetic shielding was configured, and the Majorana transition frequency shift of the fountain clock was calculated to be 4.57 × 10−18.

Continuous IPPP links for UTC

The technique of precise point positioning with integer ambiguity resolution (IPPP) has been developed for many years and has been shown to significantly improve the long-term performance of time and frequency transfer with respect to other GNSS-based techniques. In this paper, we present results of GPS IPPP links over a period of 22 months for a dozen time laboratories participating to UTC. We show that continuous links, in which the continuity of the GPS phase measurements is preserved, can be maintained for periods exceeding one year and further extended if data from two receivers per station are available. We quantify the frequency transfer uncertainty of IPPP by comparison to optical links and show how IPPP could improve UTC links to below 1 × 10−16 relative frequency uncertainty over averaging times of up to one month, i.e. the characteristic period of UTC publication. Comparisons of primary and secondary frequency standards reported for TAI indicate that IPPP could somewhat improve the accuracy of TAI/UTC. Comparisons of IPPP to two-way time transfer techniques reveal very long-term ns-size instabilities which must be further studied. Use of IPPP for UTC links is possible only if integer GNSS satellite products become available with a short delay and we report on such experimental products. Finally, we discuss the practical implications of using IPPP link in UTC and describe how the necessary steps could be implemented.

First uncertainty evaluation of the cesium fountain primary frequency standard NMIJ-F2

We report the first uncertainty evaluation of NMIJ-F2, the second atomic fountain primary frequency standard at the National Metrology Institute of Japan. To improve the frequency stability, we increase the number of detected atoms to 9 × 105 using high-power cooling laser beams for vapor-loaded optical molasses and optical pumping into the Zeeman sublevel m F = 0. We also employ an ultra-stable cryogenic sapphire oscillator as a local oscillator to prevent the degradation of frequency stability due to the Dick effect. After correcting the collisional frequency shift by alternating atom densities, the frequency stability typically reaches 2.5 × 10−13(τ/s)−1/2. Its value is 1.9 × 10−16 after 20 days of measurement. Type B uncertainty is typically evaluated at 4.7 × 10−16; the largest contribution is from a distributed cavity phase shift, followed by a microwave leakage shift. In long-term comparison, the frequency of NMIJ-F2 is found to be consistent with that of the other primary and secondary frequency standards within the uncertainty.

ПЕРСПЕКТИВНЫЙ ПЕРВИЧНЫЙ ЭТАЛОН ВРЕМЕНИ И ЧАСТОТЫ ДЛЯ СИСТЕМ ЧАСТОТНО-ВРЕМЕННОГО ОБЕСПЕЧЕНИЯ СЕТЕЙ СВЯЗИ

The article deals with the acquisition of primary time and frequency standards for time-frequency support systems as a reference node for the formation of the time scale for 4G and 5/6G communication networks. The basis of frequency and time synchronization systems, which together form the frequency-time support of communication networks, are the primary standards of time and frequency. For communication networks up to 4 generations in the frequency division mode (TDM, 3G, 4G FDD networks), only a network frequency synchronization system is needed, which is based on the use of primary reference generators. In fact, it is a part of the HDPE that provides the formation of the reference frequency. Taking into account the future requirements for synchronization systems of 5/6G communication networks, it is necessary to provide for the use of improved primary reference sources, as well as a secondary master oscillator with the functions of a master PTP frequency and a combiner of frequency and time signals. In the future, this will allow the transition to the introduction of improved HDPE (uHDTV). In connection with the current situation on the Russian market, the article discusses the possibility of using domestically produced equipment for time-frequency support systems.

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.

A 87Sr optical lattice clock with 2.9 × 10 −17 uncertainty and its absolute frequency measurement

We report the 87Sr optical lattice clock at NIM with a clock laser referenced to a 30 cm ULE cavity. Several improvements, such as the atomic temperature and density, the lattice laser frequency stabilization, the fiber noise cancellation, etc, have been made since its first evaluation in 2015. Its systematic frequency shifts are carefully evaluated with a total relative uncertainty of 2.9 × 10−17. The measured absolute frequency is 429 228 004 229 873.07(0.13) Hz with a relative uncertainty of 3.1 × 10−16, with reference to the ensemble of primary and secondary frequency standards published in the Circular T bulletin by BIPM through a satellite link.

Absolute frequency of 87Sr at 1.8 × 10−16 uncertainty by reference to remote primary frequency standards

The optical lattice clock NICT-Sr1 regularly reports calibration measurements of the international timescale TAI. By comparing measurement results to the reports of eight primary frequency standards, we find the absolute frequency of the 87Sr clock transition to be , with a fractional uncertainty of less than 1.8 × 10−16 approaching the systematic limits of the best realization of the SI second. Our result is consistent with other recent measurements and further supported by the loop closure over the absolute frequencies of 87Sr, 171Yb and direct optical measurements of their ratio.

Measurement of the 27Al+ and 87Sr absolute optical frequencies

We perform absolute measurement of the 27Al+ single-ion and 87Sr neutral lattice clock frequencies at the National Institute of Standards and Technology and JILA at the University of Colorado against a global ensemble of primary frequency standards. Over an eight month period multiple measurements yielded the mean optical atomic transition frequencies Hz and ν Sr = 429 228 004 229 873.19(0.15) Hz, where the stated uncertainties are dominated by statistical noise and gaps in the observation interval (‘dead-time’ uncertainty).

Simple and compact diode laser system stabilized to Doppler-broadened iodine lines at 633 nm

We present a compact iodine-stabilized laser system at 633 nm, based on a distributed-feedback laser diode. Within a footprint of 27×15cm2, the system provides 5 mW of frequency-stabilized light from a single-mode fiber. Its performance was evaluated in comparison to Cs clocks representing primary frequency standards, realizing the SI unit Hz via an optical frequency comb. With the best suited absorption line, the laser reaches a fractional frequency instability below 10-10 for averaging times above 10 s. The performance was investigated at several iodine lines, and a model was developed to describe the observed stability on the different lines.

Broadband high-resolution molecular spectroscopy with interleaved mid-infrared frequency combs

Traditionally, there has been a trade-off in spectroscopic measurements between high resolution, broadband coverage, and acquisition time. Originally envisioned for precision spectroscopy of the hydrogen atom in the ultraviolet, optical frequency combs are now commonly used for probing molecular ro-vibrational transitions throughout broad spectral bands in the mid-infrared providing superior resolution, speed, and the capability of referencing to the primary frequency standards. Here we demonstrate the acquisition of 2.5 million spectral data points over the continuous wavelength range of 3.17–5.13 µm (frequency span 1200 cm−1, sampling point spacing 13–21 MHz), via interleaving comb-tooth-resolved spectra acquired with a highly-coherent broadband dual-frequency-comb system based on optical subharmonic generation. With the original comb-line spacing of 115 MHz, overlaying eight spectra with gradually shifted comb lines we fully resolve the amplitude and phase spectra of molecules with narrow Doppler lines, such as carbon disulfide (CS2) and its three isotopologues.

A strontium optical lattice clock with 1 × 10−17 uncertainty and measurement of its absolute frequency

We present a measurement of the absolute frequency of the 5 s2 1S0 to 5s5p 3P0 transition in 87Sr which is a secondary representation of the SI second. We describe the optical lattice clock apparatus used for the measurement, and we focus in detail on how its systematic frequency shifts are evaluated with a total fractional uncertainty of 1 × 10−17. Traceability to the International System of Units is provided via comparison to International Atomic Time (TAI). Gathering data over 5- and 15-day periods, with the lattice clock operating on average 74% of the time, we measure the frequency of the transition to be 429 228 004 229 873.1 (5) Hz, which corresponds to a fractional uncertainty of 1 × 10−15. We describe in detail how this uncertainty arises from the intermediate steps linking the optical frequency standard, through our local time scale UTC(NPL), to an ensemble of primary and secondary frequency standards which steer TAI. The calculated absolute frequency of the transition is in good agreement with recent measurements carried out in other laboratories around the world.