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NIST-F1, source of the official time of the United States

NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and serves as the United States' primary time and frequency standard. The clock took fewer than four years to test and build, and was developed by Steve Jefferts and Dawn Meekhof of the Time and Frequency Division of NIST's Physical Measurement Laboratory.[1]

The clock replaced NIST-7, a cesium beam atomic clock used from 1993 to 1999. NIST-F1 is ten times more accurate than NIST-7. It has been succeeded by a new standard, NIST-F2, announced in April 2014. The NIST-F2 standard aims to be about three times more accurate than the NIST-F1 standard, and there are plans to operate it simultaneously with the NIST-F1 clock.[2] The most recent contribution of NIST-F1 to BIPM TAI was in March 2016.[3]

Frequency measurement

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The apparatus consists of an optical molasses made of counter-propagating lasers which cool and trap a gas of cesium atoms. Once trapped, the atoms are propelled upward by two vertical lasers inside a microwave chamber. Depending on the exact frequency of the microwaves, the cesium atoms will reach an excited state. Upon passing through a laser beam, the atoms will fluoresce (emit photons). The microwave frequency which produces maximum fluorescence is used to define the second.[1]

Similar atomic fountain clocks, with comparable accuracy, are operated by other time and frequency laboratories, such as the Paris Observatory, the National Physical Laboratory (NPL) in the United Kingdom and the Physikalisch-Technische Bundesanstalt in Germany.[1]

Accuracy

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As of 2013, the clock's uncertainty was about 3.1 × 10−16. It is expected to neither gain nor lose a second in more than 100 million years.[1]

Evaluated accuracy

[edit]

The evaluated accuracy uB reports of various primary frequency and time standards are published online by the International Bureau of Weights and Measures (BIPM).
In May 2013 the NIST-F1 cesium fountain clock reported a uB of 3.1 × 10−16. However, that BIPM report and the other recent reports are based on an evaluation that dates to 2005.[4] It used a model developed by NIST [5] to evaluate Doppler frequency shifts, known as distributed cavity phase, some believe to be incorrect.[6] The recent evaluation of NIST-F2 did not use the NIST model of distributed cavity phase used for NIST-F1 and, while NIST-F2 instead used an approach more aligned with other standards, that evaluation of distributed cavity phase was shown to have other shortcomings.[7]

Beginning in 2020, NIST-F1's microwave cavity was rebuilt to create NIST-F4. which as of April 2025 is undergoing certification by BIPM.[8]

References

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NIST-F1

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NIST-F1 is a cesium fountain atomic clock, a type of primary frequency standard developed and operated by the National Institute of Standards and Technology (NIST) in Boulder, Colorado, that served as the United States' official civilian time and frequency reference from 2000 until 2015. Developed by NIST researchers Steve Jefferts and Dawn Meekhof, NIST-F1 was completed and initially tested in under four years, beginning evaluations in 1999. The clock operates on the principle of laser cooling cesium atoms to near absolute zero, launching them upward in a fountain-like trajectory through a microwave cavity to measure the hyperfine transition frequency of 9,192,631,770 Hz, which defines the SI second. This method allows for interrogation times of about one second, significantly reducing measurement uncertainty compared to earlier beam-type cesium clocks. Upon its initial operation, NIST-F1 achieved a fractional uncertainty of 1.7 × 10^{-15}, meaning it would neither gain nor lose more than one second over approximately 20 million years, making it three times more accurate than its predecessor, NIST-7. By 2013, refinements had improved its stability to a systematic approaching 1 × 10^{-16}, equivalent to keeping time within one second over about 100 million years. As a key component in realizing Coordinated Universal Time (UTC) and UTC(NIST), NIST-F1 contributed data to international timekeeping efforts and served as a calibration reference for secondary standards, supporting applications in GPS navigation, , financial transactions, and scientific research. It was eventually succeeded by more advanced cesium fountains like , NIST-F3, and NIST-F4 (which became the U.S. primary standard in 2025), as well as emerging optical lattice clocks, reflecting ongoing advancements in atomic timekeeping technology.

Development and History

Origins and Construction

The development of NIST-F1, the first cesium fountain atomic clock at the National Institute of Standards and Technology (NIST), began in the mid-1990s under the leadership of physicists Steve Jefferts and Dawn Meekhof. Working within NIST's Time and Frequency Division at the laboratory in , the team drew on the cesium fountain principle, which uses laser-cooled cesium atoms launched upward in a vacuum to enable longer interrogation times for hyperfine transitions. This approach promised significant improvements over traditional cesium beam clocks by reducing perturbations from environmental factors. Construction of NIST-F1 was completed in under four years, reflecting efficient collaboration within , which provided the necessary resources and expertise for the project. The clock achieved its first operation in November 1998, marking a in NIST's efforts to advance primary frequency standards. NIST-F1 was designed to replace NIST-7, a room-sized cesium beam clock that had served as the U.S. primary since 1993. Initial evaluation tests conducted in 1999 confirmed that NIST-F1 offered approximately three times the accuracy of its predecessor, validating the fountain design's potential for enhanced precision. The project was funded through NIST's Time and Frequency Division, benefiting from the institution's long-standing commitment to atomic timekeeping research.

Operational Milestones

NIST-F1 was publicly announced by the National Institute of Standards and Technology (NIST) in December 1999 as a groundbreaking cesium fountain designed to serve as the U.S. primary frequency standard. It began operations in November 1998, with the first formal calibration report submitted to the International Bureau of Weights and Measures (BIPM) in November 1999. NIST-F1 served as the U.S. primary time standard from 2000 to 2015, providing regular frequency measurements to support (TAI) starting in 1999. By the early 2000s, it achieved an operational milestone of accuracy equivalent to one second in 20 million years, establishing it as one of the world's most precise timekeepers at the time. The clock operated continuously for over 15 years, from its initial startup through much of the 2010s, contributing consistently to TAI and serving as the backbone of national timekeeping . In April 2014, NIST introduced as a successor standard to operate alongside NIST-F1, enhancing the agency's timekeeping capabilities. NIST-F1's final contribution to BIPM's TAI calculations occurred in March 2016, after which it was taken offline for necessary upgrades and maintenance. Following its decommissioning, components of NIST-F1, including its rebuilt , were integrated into a new apparatus designated NIST-F4, representing a direct evolution of the original design. As of April 2025, NIST-F4 had completed initial evaluations, with data submitted to BIPM for certification; by November 2025, it serves as NIST's current primary frequency standard, ensuring continuity in ultra-precise frequency standards.

Design Principles

Cesium Fountain Mechanism

The cesium fountain mechanism in NIST-F1 employs a vertical where laser-cooled cesium-133 atoms are launched upward against , enabling them to pass through a twice—once ascending and once descending—thereby extending the interaction time with the interrogating field to approximately one second. This fountain-like begins with atoms tossed upward about 1 meter above the cavity, reaching apogee before falling back under , which maximizes the free-fall period for precise frequency measurement. To achieve this, cesium atoms are first cooled using optical molasses, a technique involving six counter-propagating infrared laser beams that reduce atomic velocities to a few centimeters per second, corresponding to temperatures of a few microkelvins above absolute zero. This laser cooling is essential for minimizing thermal motion and preparing the atoms for the controlled vertical launch. At the core of the mechanism is the hyperfine ground state transition in cesium-133 atoms, where the energy difference between the two hyperfine levels in the ground state produces a microwave resonance frequency of exactly 9,192,631,770 Hz, serving as the basis for defining the SI second. This transition is interrogated using the Ramsey method, with a free evolution time of about 0.5 seconds between microwave pulses, allowing for high-resolution determination of the frequency. Compared to traditional cesium beam clocks, the fountain design offers significant advantages, including greatly reduced Doppler shifts due to the symmetric upward and downward passages through the cavity at the same velocity, and an extended Ramsey interrogation time that improves frequency stability and accuracy by orders of magnitude. These features enable NIST-F1 to realize the SI second with exceptional precision, contributing to international time scales like UTC(NIST).

Key Components

The NIST-F1 cesium fountain atomic clock features a specialized constructed from oxygen-free high-purity to contain cesium atoms and minimize collisions with background gases, ensuring an environment essential for precise atomic interactions. This chamber integrates with the overall flight tube structure, featuring a single electrical connection to the exterior to reduce potential perturbations, and is actively temperature-controlled to maintain minimal thermal gradients. At the heart of the interrogation region is the , also made of oxygen-free high-purity , designed as a cylindrical operating in the TE011 mode and tuned to the cesium hyperfine of 9.192 GHz. The cavity measures 3 cm in radius and 2.18 cm in height, incorporating 8 cm below-cutoff waveguides with 1 cm diameter apertures and quarter-wave chokes to suppress unwanted TM111 modes, thereby optimizing field uniformity across the atomic cloud path. To mitigate external magnetic field perturbations, NIST-F1 employs a magnetically shielded environment consisting of four concentric layers of 79% nickel-iron alloy, forming cylindrical shields with welded upper end-caps and clamped lower end-caps. These shields include precisely sized apertures—3.5 cm diameter at the upper end and 4.2 cm at the lower—to accommodate the structure while maximizing attenuation of ambient fields in the C-field region. The system comprises a six-beam setup using at 852 nm for atom cooling and state selection, centered around an extended-cavity master amplified to 300 mW output via an , with acousto-optic modulators for frequency shifting. A supplementary repump , a 5 mW type, supports in the atomic ensemble. These components are arranged at right angles to form a , cooling cesium atoms to near . Post-interrogation fluorescence is captured by an optical detection system featuring two regions equipped with large 10 cm diameter spherical mirrors and silicon photodiodes, integrated with an for imaging the atomic cloud. getters are incorporated between detection zones to manage residual cesium vapor, enhancing signal clarity. Overall, NIST-F1's setup is notably compact relative to earlier thermal beam cesium clocks, with the fountain geometry enabling a vertical atomic flight path of approximately 1 meter, all housed within the NIST laboratories in .

Operational Process

Atom Cooling and Launch

In the initial preparation phase of NIST-F1, cesium atoms are loaded from a low-pressure vapor background into an in a lin⊥lin configuration formed by six intersecting infrared laser beams at 852 nm. This setup captures thermal atoms through repeated cycles of absorption and , confining approximately 10710^7 atoms in a cloud roughly 1 cm in diameter within about 0.4 seconds. The laser cooling process employs red-detuned lasers, tuned approximately 10-15 MHz below the cesium 2S1/2F=4→2P3/2F′=5^2S_{1/2} F=4 \to ^2P_{3/2} F'=5 cycling transition, to Doppler cool the atoms by imparting momentum opposite to their upon photon absorption. This reduces the atomic velocities, achieving an initial temperature of around 100 μK after the molasses phase, corresponding to a root-mean-square velocity of about 14 cm/s. Following this, the atoms undergo with circularly polarized light to prepare them in the specific hyperfine ∣F=4,mF=0⟩|F=4, m_F=0\rangle, ensuring a coherent ensemble for subsequent interrogation. For launch, the moving-molasses technique is applied using the two vertical laser beams, where the upward-propagating beam is detuned to higher frequencies (blue-detuned) and the downward beam to lower frequencies (red-detuned) relative to the cooling transition, creating a frequency chirp that imparts a net upward momentum. This propels a selected subset of approximately 10610^6 atoms upward at a velocity of about 4.25 m/s, optimizing the signal-to-noise ratio while minimizing collisional effects during the fountain flight. After launch, the laser intensity is reduced and further red-detuned to sub-Doppler cool the ensemble to below 2 μK.

Microwave Interrogation

The microwave interrogation in NIST-F1 employs a two-zone Ramsey scheme, where cold cesium atoms are subjected to two spatially separated π/2 microwave pulses during their free-flight trajectory through a dedicated cavity. As the atoms are launched upward, they first encounter the microwave cavity, which applies the initial π/2 pulse at approximately 9.192631770 GHz to create a coherent superposition of the ground hyperfine states |F=3, m_F=0⟩ and |F=4, m_F=0⟩. This pulse partially transfers the atomic population from the |F=4, m_F=0⟩ state into the |F=3, m_F=0⟩ state, initiating the Ramsey interrogation process. Following the launch, the atoms rise to an apex and fall back under , traversing the same again after a Ramsey time of about 0.56 seconds to receive the second π/2 pulse. This second interaction probes the phase evolution accumulated during free flight, enabling precise measurement of the 0-0 hyperfine transition frequency without direct continuous excitation. The cavity operates in the TE_{011} mode, constructed from oxygen-free high-conductivity with a radius of 3 cm and height of 2.18 cm, ensuring uniform field distribution across the atomic beam. The interrogation occurs in a magnetically shielded region with a uniform C-field of around 0.5 μT to define the quantization axis and mitigate Zeeman shifts. After the second pulse, the atoms continue downward into the detection zone, where their final state populations are determined via . Two orthogonal laser beams tuned to the D2 cycling transition (852 nm) excite the atoms; those in the |F=4, m_F=0⟩ state (unshelved) fluoresce brightly and are detected by photodiodes in an upper region, while atoms in the metastable |F=3, m_F=0⟩ state (shelved) show minimal until an additional pulse transfers them for detection in a lower region. This dual-zone setup captures approximately 10^6 atoms per cycle with high efficiency, normalizing the signal to account for atom number fluctuations. Each measurement cycle, from launch to detection, lasts about 1.03 seconds, with NIST-F1 performing thousands of such interrogations daily to accumulate data for locking.

Frequency Realization

Hyperfine Transition Measurement

The hyperfine transition measurement in NIST-F1 realizes the defining frequency of the unperturbed ground-state hyperfine splitting in cesium-133 atoms, corresponding to the transition between the |F=3, m_F=0⟩ and |F=4, m_F=0⟩ states. This frequency is exactly 9,192,631,770 Hz, as established by the SI second definition, which specifies the duration of 9,192,631,770 periods of the radiation corresponding to this transition in cesium-133 atoms at rest and at a temperature of 0 K. A servo system locks a local oscillator to the atomic resonance detected during cycles. The synthesizer, operating near 9.193 GHz and phase-locked to a via a high-quality , generates the signal, while a line-center servo iteratively adjusts the based on measurements of atom state transitions to maximize and maintain lock. This feedback ensures the oscillator tracks the hyperfine , with an atom-number servo additionally stabilizing power to control atomic density and minimize perturbations. The measured is averaged over multiple interrogation cycles, typically with a cycle time of about 2.2 seconds, to achieve high fractional stability, such as σ_y(τ) ≈ 4 × 10^{-16} at averaging times of 24 days. Using the Ramsey method for precision interrogation, this process enables long-term stability suitable for primary standard operation. Derived from the locked , NIST-F1 contributes to UTC(NIST) by ensemble clocks, yielding output signals including a 10 MHz and 1 pulse per second (1 PPS) for time and dissemination.

Ramsey Method Application

The Ramsey–Bordé variant of the method is employed in NIST-F1 to achieve high-precision interrogation of the cesium hyperfine transition. This adaptation features symmetric two-zone microwave pulses, where atoms pass through a first π/2 pulse in the lower zone, experience free evolution during the ballistic flight, and then a second π/2 pulse in the upper zone on the way down, effectively creating a Ramsey fringe pattern for frequency determination. The resulting fringe pattern represents the transition probability as a function of microwave frequency detuning from resonance, exhibiting a series of oscillatory fringes with the central fringe occurring precisely at the exact resonance frequency. The transition probability PP for atoms from the ground to the excited hyperfine state is given by P=12[1−cos⁡(Δϕ)],P = \frac{1}{2} \left[1 - \cos(\Delta \phi)\right], where Δϕ\Delta \phi is the phase difference accumulated due to detuning during the free evolution time. This sinusoidal dependence allows for sensitive detection of small frequency offsets by analyzing the position of the central fringe. The method's key benefits stem from its ability to leverage extended times—up to 0.5 seconds in NIST-F1—while maintaining coherence, thereby enhancing sensitivity to frequency deviations and narrowing the effective linewidth to approximately 1 Hz, far superior to continuous-wave interrogation techniques. In , each π/2 has a duration of about 0.1 ms to optimize contrast and minimize perturbations. This configuration contributes significantly to NIST-F1's fractional below 10^{-15}.

Performance Characteristics

Accuracy Evaluation

The accuracy of NIST-F1 as a primary frequency standard is formally assessed through evaluations submitted to the International Bureau of Weights and Measures (BIPM), which incorporate detailed budgets to ensure to the SI second. These evaluations, conducted periodically by NIST and reviewed by BIPM, quantify systematic frequency biases and their uncertainties, enabling NIST-F1's contributions to (TAI). Key reports include the initial formal evaluation in June/July 2001, with a combined standard uncertainty of 1.1 × 10^{-15}, followed by refinements documented in 2013 and 2016 assessments. The 2013 BIPM evaluation reported a total uncertainty of 4.2 × 10^{-16} (1σ, including dead-time effects), reflecting improvements in corrections and environmental control. This level of accuracy means NIST-F1 would neither gain nor lose more than 1 second over approximately 80 million years of continuous operation. By the 2016 evaluation, the uncertainty remained comparable at 4.4 × 10^{-16}, demonstrating sustained performance despite operational demands. Between 2017 and 2020, NIST-F1 continued regular evaluations with uncertainties around 4.4 × 10^{-16}, contributing over 65 formal reports to BIPM before the rebuild. Central to these budgets are key uncertainty components, including the shift, which dominates at the 10^{-16} level due to from the interrogation chamber affecting levels. In the 2013 assessment, this shift had a of -21.85 × 10^{-15} and an of 2.8 × 10^{-16}, mitigated through stabilization but limited by inhomogeneities. Cavity pulling, arising from non-uniform microwave fields in the Ramsey cavity, contributed a smaller of 0.02 × 10^{-15} with an of 2 × 10^{-17}. Relativistic effects, primarily the due to the laboratory's height above the (approximately 1650 meters in ), introduced a of +179.95 × 10^{-15} with an of 3 × 10^{-17}, calculated using precise models. A smaller second-order Doppler shift from the atoms' motion and the differential gravitational shift across the fountain height (~1 meter) are included with negligible additional . Compared to the preceding NIST-7 cesium beam standard, which achieved an uncertainty of 5 × 10^{-15}, NIST-F1 provided roughly a 10-fold improvement in accuracy, enabling finer realizations of the cesium hyperfine transition frequency. In 2025, following a rebuild of NIST-F1's to address aging components, the upgraded system—designated NIST-F4—was evaluated with an uncertainty of 2.2 × 10^{-16} (as of April 2025). NIST submitted the data to BIPM, and by mid-2025, it was certified as a contributing to TAI.

Stability and Uncertainty

The short-term frequency stability of NIST-F1 is characterized by an Allan deviation of σ_y(τ) ≈ 2 × 10^{-13} τ^{-1/2}, where τ is the averaging time in seconds, achieving white frequency modulation noise limited by quantum projection noise up to integration times of several days. This performance enables statistical uncertainties on the order of 5 × 10^{-16} in a few days of operation. Key noise sources limiting this stability include quantum projection noise from the atomic detection , which sets the fundamental limit at approximately 3 × 10^3 detected atoms, laser intensity fluctuations in the detection system, and servo errors arising from in the microwave synthesis chain. These are mitigated through optimized , high-quality oscillators for the local oscillator, and normalization techniques that reduce detection noise. The total uncertainty budget incorporates contributions from the distributed cavity phase shift, which is minimized to below 10^{-16} due to the symmetric design of the Ramsey cavity, and cold collision shifts, primarily spin-exchange effects that are reduced to negligible levels (uncertainty ~0.1 × 10^{-15}) by operating at low atomic densities of around 10^7 atoms per launch. Operationally, NIST-F1 maintains an accuracy equivalent to losing or gaining 1 second every 20 million years, reflecting its combined Type A and Type B uncertainties of approximately 5.3 × 10^{-16}. Stability is monitored continuously through comparisons with an ensemble of active hydrogen masers, which provide a reference timescale with mid-10^{-16} stability over 1–10 days and allow detection of any drifts or biases.

Legacy and Successors

Contributions to Time Standards

NIST-F1 served as the primary frequency standard for realizing UTC(NIST), the ' official time scale, which is maintained by the National Institute of Standards and Technology (NIST) in . As the backbone of UTC(NIST), NIST-F1 provided the essential frequency calibrations to an ensemble of atomic clocks and oscillators, ensuring the time scale's alignment with international standards. This realization directly supported the U.S. contribution to (TAI), with NIST submitting frequency measurements from NIST-F1 to the International Bureau of Weights and Measures (BIPM) for incorporation into TAI calculations. Over its operational period, NIST-F1 enabled UTC(NIST) to remain within 100 nanoseconds of (UTC), facilitating its role in global time coordination. The clock's high accuracy underpinned the precise dissemination of UTC(NIST) through multiple channels, including GPS satellite signals and time servers such as time.nist.gov. These methods allowed widespread access to traceable time for in networks, global navigation satellite systems like GPS, and scientific experiments requiring sub-nanosecond precision. For instance, NIST-F1's stability supported calibrations in applications, including frequency references for telecom equipment and timing for research, where even minor drifts could compromise results. NIST-F1 significantly advanced the realization of the SI second by progressively reducing measurement uncertainties through hardware and evaluation improvements. Early evaluations achieved fractional uncertainties around 5 × 10^{-16}, later refined to below 4 × 10^{-16}, which minimized discrepancies in international comparisons and enhanced the overall fidelity of TAI. These advancements contributed to a more robust global definition of the second, with NIST-F1's data influencing BIPM's key comparisons and reducing systematic errors in cross-laboratory validations. Operationally, NIST-F1 functioned as the U.S. national time standard for over 15 years, from 2000 until its retirement in 2015, during which it provided uninterrupted contributions to UTC and supported critical infrastructure reliant on precise timing. Its long-term reliability ensured consistent influence on Coordinated Universal Time, bolstering applications from financial transactions to space missions.

Transition to NIST-F2 and Beyond

Following the long-term operation of NIST-F1 from 1999 to 2015, NIST transitioned to its successor, , which became operational in 2014 as the new U.S. civilian . employs a similar cesium fountain design but incorporates significant enhancements, including cryogenic cooling of the interrogation region to 80 K, which reduces the uncertainty in the (BBR) shift by a factor of more than 50 compared to NIST-F1. This improvement results in an overall fractional frequency uncertainty of approximately 1 × 10^{-16}, making about three times more accurate than the final configuration of NIST-F1, such that it would neither gain nor lose a second in over 300 million years. Components from NIST-F1 were later repurposed to extend its legacy, with the original microwave cavity rebuilt and upgraded into NIST-F4, a primary standard that achieved operational status by early 2025. The rebuild involved installing a new cylindrical in December 2022 to minimize shifts from atomic trajectory variations, alongside upgrades to heating coils, , and magnetic shielding for better environmental control. NIST-F4 now operates approximately 90% of the time, delivering a fractional of 2.2 × 10^{-16}, positioning it among the world's most precise cesium . Complementing this, NIST-F3—a next-generation clock—became fully integrated into routine operations in April 2025, providing stable referencing with accuracy approaching 10^{-16} to support the NIST time scale alongside NIST-F4. Lessons from NIST-F1's operational challenges, particularly the dominant BBR shift uncertainty, directly informed upgrades in its successors, leading to more precise corrections for effects. For instance, NIST-F2's cryogenic environment and NIST-F4's refined cavity design both stem from analyses of NIST-F1's BBR sensitivities, enabling uncertainties in this correction to drop below 10^{-17} in modern fountains. These advancements ensure continued reliability in primary standards. Looking ahead, NIST's cesium fountain clocks, including , F3, and F4, remain central to realizing the current SI second while serving as benchmarks for calibrating emerging optical lattice clocks, which promise uncertainties below 10^{-18}. This role supports ongoing international efforts toward redefining the second in terms of an optical transition by the , where cesium fountains will validate the stability of optical references before a potential shift away from microwave-based definitions.

References

  1. https://www.nist.gov/pml/time-and-frequency-division/time-realization/cesium-fountain-atomic-clocks
  2. https://www.nist.gov/news-events/news/1999/12/nist-f1-cesium-fountain-clock
  3. https://www.nist.gov/news-events/news/2014/02/new-era-atomic-clocks
  4. https://www.nist.gov/news-events/news/2025/04/new-atomic-fountain-clock-joins-elite-group-keeps-world-time
  5. https://www.nist.gov/pml/time-and-frequency-division/time-services/brief-history-atomic-clocks-nist
  6. https://www.bipm.org/documents/20126/28437858/working-document-ID-2291/5780c815-5aca-bed9-e92c-40bed452b6f8
  7. https://www.bipm.org/documents/20126/28431211/working-document-ID-10210/f22dc39d-a1c3-a95b-7067-22e7d5337498
  8. https://www.bipm.org/documents/20126/28438307/working-document-ID-1239/45ba9031-eca5-4701-9b7c-2d8580b41866
  9. https://www.nist.gov/atomic-clocks/bringing-atomic-clock-back-life
  10. https://webtai.bipm.org/ftp/pub/tai/annual-reports/bipm-annual-report/annual_report_2016.pdf
  11. https://www.nist.gov/atomic-clocks/how-atomic-clocks-work/fountains-atoms-exquisite-timekeepers
  12. https://tf.nist.gov/general/pdf/1823.pdf
  13. https://tf.nist.gov/general/pdf/1404.pdf
  14. https://tf.nist.gov/general/pdf/2278.pdf
  15. https://www.nist.gov/publications/realization-si-second-and-generation-utcnist-time-and-frequency-division-national
  16. https://tf.nist.gov/general/pdf/2189.pdf
  17. https://tf.nist.gov/general/pdf/2035.pdf
  18. https://www.nist.gov/pml/time-and-frequency-division/time-realization/utcnist-time-scale-0/how-utcnist-works
  19. https://www.nist.gov/programs-projects/time-measurement-and-analysis-service-tmas
  20. https://webtai.bipm.org/ftp/pub/tai/data/PSFS_reports/nist-f1_56489-56534.pdf
  21. https://webtai.bipm.org/ftp/pub/tai/data/PSFS_reports/nist-f1_57419-57439.pdf
  22. https://www.bipm.org/documents/20126/28431211/working-document-ID-10210/f22dc39d-a1c3-a95b-7067-22e7d5337498/
  23. https://tf.nist.gov/general/pdf/1068.pdf
  24. https://iopscience.iop.org/article/10.1088/1681-7575/adc7bd
  25. https://tf.nist.gov/general/pdf/2066.pdf
  26. https://www.nist.gov/pml/time-and-frequency-division/how-utcnist-related-coordinated-universal-time-utc-international
  27. https://tf.nist.gov/general/pdf/2228.pdf
  28. https://tf.nist.gov/general/pdf/1383.pdf
  29. https://tf.nist.gov/general/pdf/2428.pdf
  30. https://iopscience.iop.org/article/10.1088/0026-1394/42/5/012
  31. https://tf.nist.gov/general/pdf/1968.pdf
  32. https://tf.nist.gov/general/pdf/2704.pdf
  33. https://phys.org/news/2014-04-nist-standard-nist-f2-atomic-clock.html
  34. https://www.nist.gov/si-redefinition/second-future
  35. https://www.nist.gov/blogs/taking-measure/think-you-know-what-second-it-will-likely-change-next-decade
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