science

NIST-F1

NIST-F1 is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand NIST-F1 rather than just read about it. In short: 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 served 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.

Key takeaways

NIST-F1 belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect NIST-F1 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of NIST-F1 from memory before moving on to harder problems.

Reference excerpt

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 served 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. 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. The most recent contribution of NIST-F1 to BIPM TAI was in March 2016.

Frequency measurement 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. 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.

Accuracy As of 2013, the clock's uncertainty was about 3.1 × 10−16 (0.00031 ppt). It is expected to neither gain nor lose a second in more than 100 million years.

Evaluated accuracy 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. It used a model developed by NIST to evaluate Doppler frequency shifts, known as distributed cavity phase, some believe to be incorrect. 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. Beginning in 2020, NIST-F1's microwave cavity was rebuilt to create NIST-F4. which as of April 2025 is undergoing certification by BIPM. On December 19, 2025, a power outage at NIST combined with failure of a backup generator caused a disruption in signals sent out from the Boulder clock. The clock's precision was later restored.

References

External links U.S. Atomic Time NIST-F1 Cesium Fountain Clock S R Jefferts; J Shirley; T E Parker; T P Heavner; D M Meekhof; C Nelson; F Levi; G Costanzo; A De Marchi; R Drullinger; L Hollberg; W D Lee; F L Walls (2002). "Accuracy evaluation of NIST F-1" (PDF). Metrologia. 39 (4): 321–336. Bibcode:2002Metro..39..321J. doi:10.1088/0026-1394/39/4/1. S2CID 250794497. Archived from the original (PDF) on 2005-03-18. Retrieved 2007-11-25.

Illustrations

Worked examples

Example 1 — a first encounter with NIST-F1

Start with the simplest possible case. Write down what NIST-F1 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to NIST-F1 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about NIST-F1 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of NIST-F1

In research: NIST-F1 appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses NIST-F1 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: NIST-F1 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Atomic clocks, National Institute of Standards and Technology, so understanding it makes those chapters shorter.
In everyday life: Look for NIST-F1 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study NIST-F1 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what NIST-F1 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain NIST-F1 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is NIST-F1 in simple terms?

Why does NIST-F1 matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study NIST-F1?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on NIST-F1.