HomeAtomic Clock
The World's Most Accurate Timekeeping

Atomic Clock — How Atomic
Time Works

Atomic clocks are the most accurate timekeeping devices ever built, losing less than one second over 300 million years. They underpin GPS satellites, the internet, financial systems, and every time standard on Earth.

Live Atomic Clock
NIST Time Standards
Clock Technology
10⁻¹⁸
seconds/day error
Accuracy
~9,000
atomic clocks in orbit
GPS Clocks
+37s
TAI ahead of UTC
TAI Offset
1955
NPL cesium clock
First Built

What Is an Atomic Clock?

An atomic clock is a precision timekeeping instrument that uses the electromagnetic resonance frequency of atoms as its oscillator. Unlike quartz clocks, which rely on a vibrating crystal, atomic clocks count quantum transitions — the frequency at which electrons flip between energy levels — to measure time.

The most common type uses cesium-133 atoms. The SI second is officially defined as 9,192,631,770 cycles of the cesium-133 hyperfine transition. This definition has been in place since 1967 and forms the basis of all international timekeeping through International Atomic Time (TAI) and Coordinated Universal Time (UTC).

The accuracy of an atomic time clock is extraordinary. A modern cesium fountain clock at NIST would neither gain nor lose a second over 300 million years — making it the world's most stable periodic phenomenon measurable by humans. Optical atomic clocks are already 100× better still.

How Does an Atomic Clock Work?

1
Atom Preparation

Cesium atoms are heated in an oven and fired as a beam (or laser-cooled into a ball in fountain clocks). Atoms in a specific hyperfine state are selected using magnetic fields.

2
Microwave Interrogation

Atoms pass through a microwave cavity tuned to ~9.19 GHz. If the microwave frequency exactly matches the atomic transition frequency, maximum atoms flip state.

3
Detection & Feedback

A detector counts how many atoms changed state. A servo loop adjusts the microwave frequency to maximize state-flips — locking the oscillator to the atom's natural resonance.

4
Second Output

Every 9,192,631,770 counted cycles produces exactly 1 SI second. The clock outputs a 1 pulse-per-second (1PPS) signal synchronized to this count.

Types of Atomic Clocks

From everyday GPS receivers to bleeding-edge research instruments — each type trades size and cost for accuracy. Full technical breakdown →

Cesium-133 Atomic Clock

SI Definition

The gold standard of timekeeping — cesium clocks define the SI second. The hyperfine transition of cesium-133 atoms oscillates at exactly 9,192,631,770 Hz, serving as the world's official time reference.

Accuracy: 10⁻¹⁴ s/day
Use: National labs, GPS

Rubidium Clock

Compact and affordable, rubidium clocks are widely used in telecom base stations, GPS receivers, and military systems. They drift slightly more than cesium but are significantly cheaper and smaller.

Accuracy: 10⁻¹¹ s/day
Use: Telecom, GPS devices

Optical Atomic Clock

The most accurate clock ever built. Optical clocks use laser-cooled atoms (strontium or ytterbium) oscillating at visible light frequencies — 100,000× higher than cesium. They won't lose a second in 15 billion years.

Accuracy: 10⁻¹⁸ s/day
Use: Research labs, future GPS

Atomic Fountain Clock

Cesium fountain clocks like NIST-F2 and SYRTE-FO2 toss cooled cesium atoms upward in a vacuum, allowing longer interrogation times. NIST-F2 is so precise it would not gain or lose a second in 300 million years.

Accuracy: 10⁻¹⁶ s/day
Use: NIST primary standard

Chip-Scale Atomic Clock (CSAC)

Miniaturized atomic clocks roughly the size of a matchbook, consuming only 120 mW. Used in military navigation systems, submarines, and portable GPS receivers where size and power constraints matter.

Accuracy: 10⁻¹⁰ s/day
Use: Military, portable GPS

Hydrogen Maser

Hydrogen masers provide extremely stable short-term performance, making them ideal for VLBI (Very Long Baseline Interferometry) radio telescope networks. They are less accurate long-term than cesium but unbeatable over seconds to hours.

Accuracy: 10⁻¹⁵ s/day
Use: Radio astronomy, VLBI

Nuclear Clock

The next frontier — nuclear clocks use transitions in atomic nuclei (thorium-229) rather than electron shells. Theoretically 1,000× more stable than optical clocks, they could detect dark matter and test fundamental physics.

Accuracy: 10⁻¹⁹+ s/day
Use: Future research (2025+)

Atomic Clock Accuracy Comparison

How do atomic clocks compare to quartz and mechanical timepieces? The difference is staggering.

Clock TypeMechanismDrift / AccuracySizeExamples
Nuclear ClockNuclear transition (Th-229)1 s per 30 billion yrsLab benchJILA, PTB (prototype)
Optical ClockLaser-cooled Sr / Yb lattice1 s per 15 billion yrsLab benchNIST Sr, SYRTE
Cs Fountain (NIST-F2)Cesium-133 hyperfine1 s per 300 million yrs2-meter towerNIST-F2, PTB-CSF2
Hydrogen MaserHydrogen 1420 MHz maser1 s per 3 million yrsCabinetEffelsberg, VLBI networks
Cesium Beam ClockCesium thermal beam1 s per 300,000 yrsRack unitSymmetricom 5071A
Rubidium ClockRb-87 hyperfine, VCXO lock1 s per 300 yrsModuleGPS satellites, telecom
Chip-Scale (CSAC)Miniaturized Rb / Cs cell1 s per 10 yrsMatchbookMicrosemi SA.45s
Quartz OscillatorPiezoelectric crystal1 s per 30 daysChipWatches, computers
Mechanical WatchBalance wheel / escapement±30 s per dayWrist-wornRolex, Patek Philippe

Accuracy values are approximate and depend on implementation, environment, and maintenance intervals.

Why Do We Need Atomic Clocks?

GPS Navigation

GPS position accuracy requires nanosecond timing. Each GPS satellite carries 2–4 atomic clocks.

Internet & NTP

All internet time servers (NTP) trace their accuracy back to national atomic clock laboratories.

Finance

Financial regulators require microsecond-accurate timestamps on every trade. MiFID II mandates 100μs sync.

5G / LTE

Mobile base stations need nanosecond synchronization for hand-offs and frequency management.

Radio Astronomy

VLBI telescopes combine signals from dishes thousands of miles apart — requiring atomic clock precision.

Science

Gravitational wave detectors, dark matter searches, and relativity tests all depend on atomic timing.

History of Atomic Timekeeping

1879

Lord Kelvin proposes using molecular vibrations to measure time.

1945

Isidor Rabi proposes atomic clocks based on molecular beam magnetic resonance.

1949

NIST (then NBS) builds the first atomic clock using ammonia as the reference.

1955

Louis Essen and Jack Parry at NPL build the first cesium atomic clock, accurate to 1 second in 300 years.

1967

The SI second is officially redefined in terms of cesium-133 hyperfine transitions — 9,192,631,770 Hz.

1972

UTC is established with leap seconds to keep atomic time aligned with Earth's rotation (UT1).

1983

GPS satellites carry rubidium and cesium atomic clocks. Navigation depends on atomic time.

1991

NIST-7 cesium fountain clock achieves accuracy of 5 × 10⁻¹⁵.

2002

SYRTE Paris develops optical lattice clocks using strontium atoms, 1000× better than cesium.

2014

NIST-F2 cesium fountain clock set as US time standard — accurate to 1 second per 300 million years.

2023

JILA and NIST demonstrate optical clocks with 10⁻¹⁸ precision — best ever achieved.

2025

First thorium nuclear clock prototype demonstrated; paves way for 10⁻¹⁹ accuracy.

Explore Atomic Time Resources

Live Atomic Clock Online

View the current atomic time with milliseconds, synchronized across time zones. Shows UTC, TAI, and DST status.

View live clock →

NIST Time & UTC Standards

Learn about UTC vs TAI, International Atomic Time, NIST-F2, leap seconds, WWVB, and official US time.

View standards →

Atomic Clock Technology

Deep dive into cesium, rubidium, optical lattice, chip-scale, hydrogen maser, and nuclear clock technologies.

View technology →

Frequently Asked Questions