Atomic clock

An atomic clock measures time by locking an oscillator to a characteristic atomic transition frequency. Its stability enables precise time standards, navigation, and tests of fundamental physics.

Atomic transition: A change in an atom’s quantum state caused by absorbing or emitting energy at a characteristic frequency. The clock’s reference frequency comes from a transition between atomic energy levels.
SI second: The SI base unit of time, defined by fixing the cesium-133 hyperfine transition frequency at 9,192,631,770 hertz. The current definition of the second is realized by cesium atomic clocks.
Global Positioning System: A satellite navigation system that determines position and time using signals from orbiting satellites. Satellite timing relies on onboard atomic clocks to calculate signal travel times.
Cesium standard: A frequency standard based on the cesium-133 hyperfine transition used to realize the SI second. Cesium fountain clocks have traditionally defined the most accurate realization of the second.
Resonance: A strong response that occurs when a periodic stimulus matches a system’s natural frequency. Clocks identify the atomic transition by finding the frequency at which atoms respond most strongly.
Frequency: The number of repetitions of a periodic event per unit time, measured in hertz. Atomic clocks count cycles of a reference frequency to measure elapsed time.
International Atomic Time: A continuous international time scale calculated from readings of atomic clocks worldwide. Atomic clocks maintained by laboratories contribute to this global timescale.
Hydrogen maser: A microwave oscillator that uses stimulated emission from hydrogen atoms to produce a highly stable signal. Hydrogen masers offer excellent short-term stability, though they do not define the SI second.
Rabi oscillation: The periodic exchange of population between two quantum states driven by a resonant electromagnetic field. These oscillations describe how clock fields drive atoms between the states being compared.
Phase-locked loop: A feedback system that adjusts an oscillator to maintain a chosen phase or frequency relationship with a reference signal. Feedback keeps the clock oscillator aligned with the atomic reference.

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