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How does an atomic clock work?

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Quick answer

An atomic clock works by tuning a microwave oscillator until it exactly matches the natural vibration frequency of atoms like caesium. A detector watches how the atoms respond, and feedback keeps the oscillator locked to that frequency. Counting the oscillator's cycles then gives a second that is defined by atomic physics, not by any mechanical part.

An atomic clock keeps time by locking an electronic oscillator to the exact frequency at which atoms transition between two energy states. Because that frequency is fixed by quantum mechanics and identical everywhere, the clock drifts by less than a second over millions of years.

Step by step

  1. Atoms of caesium or rubidium are heated and formed into a beam or trapped cloud.
  2. Microwave radiation is applied to the atoms at a frequency that is slowly swept across the expected resonance.
  3. A detector measures how many atoms absorb the radiation and change energy state.
  4. The detector signal is strongest when the microwave frequency exactly matches the atomic transition.
  5. Control electronics adjust a quartz oscillator so its frequency stays at that peak.
  6. An electronic counter tallies the oscillator's cycles to produce the clock's seconds.

Key things to understand

  1. Atoms such as caesium or rubidium absorb and emit electromagnetic radiation only at sharply defined frequencies set by their energy levels.
  2. A microwave cavity bathes the atoms in radiation whose frequency is swept until the atoms respond most strongly.
  3. A detector measures how many atoms changed state, producing a signal that peaks at the exact atomic resonance frequency.
  4. Feedback electronics continuously adjust a quartz oscillator so its frequency stays locked to that atomic resonance.
  5. Counting the oscillator's cycles converts the steady frequency into seconds, making the clock's tick rate independent of temperature, wear or location.

Labelled diagram

  1. Atom source

    Caesium or rubidium atoms are vaporised and directed into a beam or trap.

  2. Microwave cavity

    The atoms pass through a tuned microwave field that can flip their energy state.

  3. Detector

    A hot-wire or laser detector counts how many atoms changed state.

  4. Feedback loop

    Electronics adjust the microwave frequency to keep the detector signal at its peak.

  5. Quartz oscillator

    Its frequency is steered by the feedback signal and counted to mark seconds.

How an atomic clock locks onto an atomic frequency

Real-world examples

  • The international second is defined by a caesium-133 transition, and a bureau near Paris combines hundreds of clocks worldwide into International Atomic Time.
  • GPS satellites carry rubidium and caesium clocks that keep time to within a few nanoseconds so receivers can calculate position.
  • Network Time Protocol servers linked to atomic clocks or satellite time signals synchronise computers and financial trading systems worldwide.
  • Optical lattice clocks in metrology labs are so stable they could detect the tiny time shift caused by a change in height of one centimetre.

Common mistake

✗ Myth: Atomic clocks are radioactive and work by measuring the decay of atoms.

✓ Fact: Atomic clocks do not use radioactivity. They use the vibration frequency of electrons changing energy levels in stable atoms, which is a reversible quantum transition, not a decay process.

People also ask

What is an atomic clock used for?
Atomic clocks provide the timing backbone for satellite navigation, telecommunications, financial trading and the internet. They define the second, keep computer networks synchronised, and let GPS receivers calculate position by comparing signal travel times from multiple satellites.
How accurate is an atomic clock?
The best caesium fountain clocks lose less than one second in tens of millions of years. Optical clocks in laboratories are even more precise, drifting by less than a second over the age of the universe. Commercial caesium clocks are accurate to about one second in thousands of years.
Why do atomic clocks use caesium?
Caesium-133 has a convenient microwave transition frequency that is easy to measure with electronics, and its atoms are heavy enough to be slowed and probed precisely. The international definition of the second is based on this transition, so caesium clocks set the global time standard.
Why is caesium used in atomic clocks?
Caesium was chosen because its resonance frequency is easy to measure with microwave electronics and the atoms can be prepared and detected reliably. The second is officially defined by the caesium-133 transition, so caesium clocks set the global time standard.
How accurate are atomic clocks compared with quartz watches?
A quartz watch may drift by about a second per day, while a caesium atomic clock drifts by roughly a second in millions of years. The difference comes from the atomic resonance being fixed by physics, whereas a quartz crystal's vibration changes with temperature and age.
What is the difference between a caesium clock and an optical clock?
Caesium clocks use microwave frequencies around 9 billion cycles per second, while optical clocks use visible-light frequencies tens of thousands of times higher. Higher frequency means more cycles counted per second, so optical clocks can be more precise.
Do atomic clocks work in space and on satellites?
Yes. Satellite navigation systems carry atomic clocks because timing errors of even a millionth of a second would translate into position errors of hundreds of metres. Their signals let receivers on the ground synchronise to the same time scale.

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