50 Fun Facts About Atomic Clocks

An atomic clock keeps time by monitoring what property of atoms?

Transitions between quantised energy levels are driven by very specific frequencies of radiation, which is what the clock locks onto.

What kind of energy-level split in the caesium-133 atom defines the SI second?

It is the tiny energy split in the atom's ground state, driven by microwaves at about 9.19 GHz.

Exactly how many periods of caesium-133's clock transition make one second?

The other numbers are rubidium's frequency, hydrogen's 21-cm line and the speed of light in metres per second.

Before the caesium definition, the second was defined as a fraction of what?

There were deemed to be 31,556,925.9747 seconds in that year.

Whose 1873 Treatise first proposed measuring time by the vibrations of light waves?

He argued it would beat the Earth's rotation, which then defined the mean solar second.

Who built the first practical caesium atomic clock in 1955 at Britain's National Physical Laboratory?

Ironically, Essen was a lifelong critic of Einstein's relativity, publishing a book attacking it in 1971.

The first atomic-clock prototype, built in 1949, measured transitions in which molecule?

It was accurate but not more so than the best quartz clocks; caesium took over in 1955.

Which 1944 Nobel physicist built atomic-beam magnetic resonance equipment that led toward atomic clocks?

His Nobel citation was 'for his resonance method for recording the magnetic properties of atomic nuclei'.

Norman Ramsey won the 1989 Nobel Prize in Physics for inventing what?

Now called Ramsey interferometry, it gives narrower resonances; he developed it in 1949.

What was the first commercial atomic clock, sold by the National Radio Company in the 1950s, called?

Hewlett-Packard followed in 1964 with its rack-mounted 5060 caesium clock.

Which caesium clock became a US primary time standard when brought online in 2014?

Cooling its microwave region with liquid nitrogen removed the black-body radiation error that limited its predecessor.

NIST-F2 was rated to neither gain nor lose a second in at least how long?

Its predecessor NIST-F1 was rated at 100 million years.

Roughly what timing error corresponds to a positional error of about 30 cm in satellite navigation?

Light travels about 30 cm in a billionth of a second, which is why GPS satellites carry atomic clocks.

Relativity predicts GPS satellite clocks run how much faster per day than ground clocks?

Uncorrected, that would send positions drifting by kilometres within a day.

GPS Time runs at a constant offset from International Atomic Time of what size?

Like TAI, GPS Time never inserts extra seconds; receivers subtract a broadcast offset to get UTC.

Europe's Galileo satellites carry two rubidium clocks and two of which other kind?

The second-generation Galileo masers are about two feet long and weigh 40 pounds.

International Atomic Time (TAI) is a weighted average of roughly how many clocks worldwide?

They sit in about 80 institutions, and the scale is published a few weeks in arrears.

What is added to atomic time to keep civil time (UTC) within 0.9 s of Earth's rotation?

The first was introduced in 1972 and the 27th, at the end of 2016, was the last one added through 2025.

After 27 leap seconds since 1972, the last in 2016, by how many seconds did UTC trail TAI?

That is the initial 10-second offset of 1972 plus 27 one-second insertions, the last at the end of 2016.

In 2022 the General Conference on Weights and Measures voted to abandon the leap second by which year?

After that the difference between TAI and UTC will stay fixed.

In the 1971 Hafele-Keating experiment, four caesium clocks were flown around the world aboard what?

They flew east then west and confirmed relativity's predicted time differences against clocks at the US Naval Observatory.

What is the clock transition frequency of rubidium-87 used in rubidium clocks, roughly?

Rubidium standards are cheap, small and used everywhere from telecoms to satellites, but count only as secondary representations of the second.

Which type of atomic clock has the best short-term stability, making it a favourite for radio astronomy?

It uses hydrogen's 1.4 GHz hyperfine transition but drifts over the long term as its cavity ages.

A chip-scale atomic clock demonstrated by NIST in 2004 was about the size of what?

It drew just 125 milliwatts and became commercially available in 2011.

Optical clocks tick around 10^15 Hz. Microwave clocks like caesium run near what frequency?

Higher frequency means more 'ticks' to average, which is why optical clocks are so much more stable.

Which element's optical lattice clock first beat caesium's precision, ticking at 429 terahertz?

JILA demonstrated a strontium clock precise to one part in 10^18 in 2015.

Who proposed the optical lattice clock in 2001 and built the first one in 2003?

His insight was trapping atoms in a laser lattice at a 'magic wavelength' that does not disturb the clock transition.

In an optical lattice clock, atoms are held in place by what?

The beams form a stable 'egg-crate' pattern of light with an atom in each pocket.

An optical lattice clock drifts one second in about how long, a thousand times longer than a caesium clock?

That is more than twice the current age of the universe.

NIST's 2010 'quantum logic' clock, first to beat caesium's precision, used which element's ions?

In 2019 a single-aluminium-ion version reached a frequency uncertainty of 9.4 parts in 10^19.

In 2021 a JILA optical clock measured time dilation between atom clouds separated by how much?

General relativity says clocks tick slower deeper in a gravitational field, and the effect showed up over that tiny height difference.

A proposed 'nuclear clock' would use a transition in the nucleus of which isotope?

Its unusually low-energy nuclear excitation emits in the ultraviolet, where lasers can reach; the frequency was pinned down in 2024.

NASA's Deep Space Atomic Clock, launched in 2019, was based on ions of which element?

It was designed to drift no more than a nanosecond in ten days and flew on a Falcon Heavy.

Which physicist, who demonstrated laser cooling of atoms in 1978, shared the 2012 Nobel Prize in Physics?

He shared it with Serge Haroche for methods of measuring and manipulating individual quantum systems.

What is the biggest source of uncertainty in the NIST-F1 caesium clock?

Its successor fixed this by cooling the microwave interaction region with liquid nitrogen.

Why is caesium a particularly good choice for an atomic clock, compared with hydrogen?

At room temperature a caesium atom moves at about 130 m/s versus 1,600 m/s for hydrogen, and its 9.19 GHz frequency is higher.

Roughly what does a national-standard caesium tube cost, and how long does it last?

A rubidium standard tube, by contrast, can cost as little as $50 and last more than ten years.

Which German institute in Braunschweig is one of the main keepers of the world's atomic clocks?

In 2016 it and Paris's LNE-SYRTE compared two optical clocks over 1,415 km of fibre-optic cable.

What was NIST called before 1988, when it built the NBS-1 to NBS-6 caesium clocks?

NBS-1 in 1959 was accurate to one part in 10^11; NIST-7 reached one part in 10^15.

Consumer 'atomic' wall clocks and watches actually stay accurate how?

Cheap receivers are accurate to about a tenth of a second.

What is a clock called in which laser-cooled atoms are tossed up through a microwave cavity and fall back?

The technique was introduced by Jerrold Zacharias and powers NIST-F1 and F2.

The 'Dick effect', a stability limit in sequential-interrogation clocks, is what phenomenon?

One major review lamented 'the pernicious influence of the Dick effect'.

The instability of an atomic clock is conventionally specified using which statistic?

It captures how the clock performs when averaged over time to reduce noise.

Around what year did 2020s roadmaps target redefining the SI second via optical clocks?

Optical clocks must first prove accuracy of about 2 parts in 10^18 and agree with each other worldwide.

Which body publishes evaluations of primary frequency standards and the monthly Circular T?

The International Bureau of Weights and Measures sits at Sèvres, outside Paris.

Which navigation system's time scale, unlike GPS Time, tracks UTC by inserting leap seconds?

The Russian system's time is generated by its Central Synchroniser to better than 1,000 nanoseconds.

Which lanthanide element is used in leading optical lattice clocks?

Other leading designs use single trapped ions rather than clouds of neutral atoms.

In July 2025 NIST reported a record trapped-ion clock accurate to roughly how many decimal places?

It was 41% better than the previous record and 2.6 times more stable than any other ion clock.

A radio time-signal clock incurs about 1 millisecond of delay for every how far from the transmitter?

Radio waves cover 300 km in a thousandth of a second.

Which French physicist developed optical pumping with Jean Brossel in 1949?

A side effect, the light shift of the resonant frequency, was later tamed by Claude Cohen-Tannoudji.