1949 – today

How do atomic clocks work?

An atomic clock tunes a microwave or laser signal until it exactly matches a natural vibration inside an atom, then counts the cycles. For cesium-133 that vibration is 9,192,631,770 cycles per second, which has defined the second since 1967. The best cesium clocks drift by about a second in 300 million years.

First atomic clock	Ammonia, US National Bureau of Standards, 1949
First accurate cesium clock	Louis Essen and Jack Parry, NPL (UK), 1955
Second defined by cesium	1967, 13th General Conference on Weights and Measures
Cesium frequency	9,192,631,770 Hz
Best cesium fountain clocks	about 1 second in 300 million years
Leap seconds added	27, from 1972 to 2016
TAI − UTC	37 seconds

Every clock counts something that repeats: a swinging pendulum, a vibrating quartz crystal. The trouble is that pendulums and crystals change slightly with temperature, wear and age. Atoms don't. Every cesium-133 atom in the universe has exactly the same internal "tick," which makes it an ideal standard.

How a cesium clock works

The outermost electron of a cesium atom can sit in one of two energy states, separated by a precise amount of energy. Microwaves at exactly the right frequency flip atoms from one state to the other. In a cesium clock:

Cesium atoms are heated into a beam, or in modern "fountain" clocks cooled with lasers and tossed upward.
They pass through a microwave cavity fed by a quartz oscillator.
A detector counts how many atoms flipped state.
Electronics nudge the oscillator's frequency until the number of flipped atoms is at its maximum, which means the microwaves are exactly on the cesium frequency.
Counting 9,192,631,770 cycles of that locked frequency gives one second.

Redefining the second

Until the 1950s the second was 1/86,400 of an average day. But Earth's rotation isn't steady: tides are slowly braking it, and it speeds up and slows down by milliseconds for reasons including earthquakes, ocean currents and the motion of Earth's core. After Louis Essen and Jack Parry built the first accurate cesium clock at the UK's National Physical Laboratory in 1955, the atom won. In 1967 the General Conference on Weights and Measures defined the second as 9,192,631,770 periods of the cesium-133 transition. See how the second is defined.

From many clocks to one time

About 450 atomic clocks in around 80 laboratories feed their readings to the International Bureau of Weights and Measures (BIPM) near Paris, which combines them into International Atomic Time (TAI). UTC, the time the world's clocks follow, is TAI with leap seconds added so that it never drifts more than 0.9 seconds from Earth's actual rotation. Since 1972, 27 leap seconds have been added, the last on December 31, 2016, and UTC is now 37 seconds behind TAI. See leap seconds and UTC vs GMT.

How the time reaches you

National labs such as NIST in the US broadcast time by radio (stations WWV and WWVB in Colorado) and over the internet. GPS satellites carry their own atomic clocks. Your phone and computer set themselves using the Network Time Protocol (NTP), which usually keeps them within a few milliseconds of UTC. That's what this site's clock checks against.

Better than cesium

Optical clocks use atoms such as strontium or ytterbium that tick at optical frequencies, around 400 to 500 trillion times a second, roughly 50,000 times faster than cesium. Finer ticks mean finer measurement: the best are stable to about one part in 10¹⁸, about a second over the age of the universe. Scientists are preparing to redefine the second in terms of an optical transition, possibly around 2030.

Atomic clocks aren't radioactive. Cesium-133 is the stable, non-radioactive form of the element.

Questions people ask

How accurate is an atomic clock?

The best cesium fountain clocks would gain or lose about one second in 300 million years. Optical clocks are about 100 times better.

Are atomic clocks radioactive?

No. They use cesium-133, the stable form of cesium, and measure an energy change in its electrons, not radioactive decay.

Is my phone's clock atomic?

Not directly. It sets itself from internet time servers and the cellular network, which are traced back to atomic clocks, so it's usually within a few milliseconds of UTC.

Sources

Last updated October 18, 2026. Found an error? Tell us.