Atomic clock

The clocks that define the second itself: how they count atoms, how they went from a 1949 experiment to 19-decimal accuracy, and what the atomic clock on your wall really is.

What an atomic clock is

Every clock has two parts: something that ticks at a steady rate, and something that counts the ticks. In a grandfather clock the ticker is a pendulum; in a quartz watch it is a vibrating crystal. In an atomic clock the ticks come from atoms themselves.

Atoms of the same element absorb and give off energy at exactly the same frequencies, and they never wear out. That makes them the most reliable tick there is, and it is why the world's official time is now kept by atomic clocks rather than by watching the sky.

How it works

Scientists shine light or microwaves at a group of atoms and tune it until it matches the frequency the atoms respond to. When the tuning is right, many atoms absorb energy and jump to a different energy state, which the clock can detect.

The clock keeps its signal locked on that frequency and counts the waves as they pass. Because the atoms always respond at the same frequency, the count turns into extremely precise time. NIST notes that the accuracy of an atomic clock depends on how good its lasers, electronics and other parts are, not on the atoms, which are identical everywhere.

See also: the exact atomic time right now on OnlineClock.app.

From 1949 to the definition of the second

The first atomic clock was built in 1949 at the National Bureau of Standards in Washington, D.C., now called NIST. It was hardly better than the best electronic clocks of the day, but it proved the idea.

In 1955 Louis Essen and Jack Parry at Britain's National Physical Laboratory built what the Science Museum calls the first successful atomic clock, using cesium atoms. It was accurate to about a second in 300 years.

Cesium clocks became so dependable that in 1967 the world redefined the second itself: one second is the time taken by 9,192,631,770 cycles of the radiation tuned to cesium's resonant frequency.

The most accurate clock in the world

Clock records keep falling. In July 2025 NIST announced an aluminum ion clock with a systematic uncertainty of 5.5 x 10^-19, about 41 percent more accurate than the previous record, which NIST describes as keeping time to 19 decimal places. It traps a single aluminum ion and pairs it with a magnesium ion that cools it and reads out its state, a technique called quantum logic.

Clocks like this one tick at the frequency of light rather than microwaves, and they are part of an international effort to redefine the second using an optical clock instead of cesium. Because the title changes hands every few years, any most accurate clock claim needs a date next to it.

Why light makes better clocks

The cesium clocks that define the second tick at microwave frequencies, nearly 10 billion times a second. Visible light vibrates about 100,000 times faster, and more ticks per second means time can be split into finer pieces, like a ruler with more marks.

Counting light waves only became possible after the invention of the frequency comb in 1999, a laser tool that converts optical frequencies into signals electronics can count. NIST says the best optical clocks, using atoms such as aluminum, strontium and ytterbium, are now about 100 times more accurate and stable than cesium fountain clocks.

Some optical clocks trap thousands of atoms at once in a grid of laser light, called an optical lattice, and average them. Others hold a single ion. Scientists have even carried a lattice clock up a 14,000-foot Colorado mountain to measure how time runs faster at altitude, as Einstein's general relativity predicts.

Atomic clocks in your pocket

Every time a phone shows your position on a map, it is relying on atomic clocks. GPS satellites each carry their own atomic clocks, and a receiver works out where it is by comparing the time signals from at least four satellites.

The satellite clocks drift slightly, so more precise clocks on the ground correct them regularly. NIST says that without atomic clocks the errors would build up so quickly that GPS would be useless. The rubidium clock designed in the 1970s for the first GPS satellites is still typical of the kind flying today.

The atomic clock on your wall

Clocks sold as atomic for home use do not contain atoms. They are quartz clocks with a small radio receiver tuned to a time signal broadcast from an atomic clock. In the United States that signal is NIST's station WWVB near Fort Collins, Colorado, at 60 kHz.

Most of these clocks listen once a day, usually around midnight or early morning, when the signal travels best, and keep time with their quartz crystal in between. Metal buildings, electrical noise from nearby electronics and weak batteries can stop them syncing, and then they drift like any ordinary quartz clock.

See also: how atomic time reaches your phone and computer on OnlineClock.app.

Keep exploring

Sources

1. How do atomic clocks work?, NIST
2. NIST ion clock sets new record for most accurate clock in the world, NIST (July 2025)
3. Optical clocks: the future of time, NIST
4. Knowing where we are: atomic clocks and GPS, NIST
5. Caesium atomic clock, 1955, Science Museum Group Collection
6. Help with WWVB radio controlled clocks, NIST

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