NIST Time and Frequency Seminar 2025

Once again I attended the fire hose of information that is the U.S. National Institute of Standards and Technology (NIST) Time and Frequency Seminar. This three day, typically annual, event, held at their Boulder Colorado laboratories (commuting distance for me), covered such wide ranging topics as optical atomic clocks, practical measurement techniques for time and frequency, how to characterize and analyze frequency and phase errors in data, ways in which television and radio broadcasters might augment GPS for timing and positioning, and much more.

In honor of the event I wore my Rolex Milgauss. Felt cute, might delete later.

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My understanding is that virtually all national time and frequency metrology laboratories, including NIST (civilian) and the USNO (military) in the U.S., use an ensemble of cesium beam atomic clocks and hydrogen maser atomic clocks, the average of which is used to determine their contribution to the measurement of the SI second and the international definition of UTC. These are commercial devices, not laboratory experiments, and aren't astronomically expensive.

They use a combination of both because, even though the cesium resonant frequency is the (current) definition of the second in the international system of units, cesium atomic clocks suffer from jitter (short term variation), while hydrogen masers are more stable. The jitter in commercial atomic clocks is well understood, and the difference between a rack-mounted commercial cesium beam clock and a much larger and far far more expensive cesium fountain atomic clock in labs at places like NIST is all the extra hardware to try to reduce that jitter.

The image below is of the NIST F-3 cesium fountain clock. The collection of commercial cesium beam standards are kept locked up in another room.

Here's the thing: all hydrogen maser clocks suffer from drift (long term variation). And they all drift by a different amount. And it is not understood why. One hypothesis is it's some mechanism of aging of the components. If the manufacturers could eliminate this, they certainly would (and charge more).

The image below is of a decommissioned commercial hydrogen maser clock that I saw at NIST in 2018. You can't typically find one of these at NIST where it can be photographed because the running ones are kept locked in temperature controlled chambers adapted from commercial egg incubators.

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The blinking 1Hz LEDs in this brief video clip literally represent the real-time manufacture of the UTC(NIST) time scale (the U.S. civilian time base) and the U.S. contribution to the international determination of UTC.

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Next to the toe of my shoe is a survey marker embedded in the floor of one of the NIST labs. Atomic clocks are so precise now that centimeter changes in altitude have to be adjusted for, thanks to general relativistic effects.

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This was my fourth (2018, 2023, 2024, and 2025) and probably last time attending the Time and Frequency Seminar. It is so popular that not only does it sell out, but the waiting list is lengthy too. Could be time to let someone else become a certified Time Lord.