Optical atomic clock

An atomic clock that measures an optical-frequency transition in trapped atoms or ions. Its high-frequency reference enables exceptionally precise timekeeping.

Optical frequency comb: A laser spectrum with evenly spaced frequency lines that links optical frequencies to microwave or radio frequencies. It counts optical cycles by connecting the clock laser’s frequency to an electronic reference.
Atomic clock: A clock that keeps time by referencing a frequency associated with an atomic transition. Optical clocks are a high-frequency branch of this broader timekeeping technology.
Relativistic geodesy: The use of gravitational time dilation measured by precise clocks to determine differences in gravitational potential. Comparing clocks at different heights can reveal changes in Earth’s gravitational potential.
Cesium atomic clock: An atomic clock that uses the microwave transition of cesium-133 to realize the SI second. It provides the microwave-frequency standard against which optical clocks are evaluated and compared.
Clock comparison: The measurement of frequency ratios between two clocks, often to assess performance or test physical theories. Agreement between independent clocks is essential for validating accuracy and searching for new physics.
Atomic transition: A change between discrete energy states of an atom or ion, accompanied by absorption or emission of a photon. The clock’s reference frequency is set by a specific transition between atomic energy states.
Quality factor: A measure of how many oscillations occur during an oscillator’s characteristic linewidth. A narrow transition relative to its optical frequency gives the clock a high quality factor.
Gravitational time dilation: The difference in elapsed time between clocks at different gravitational potentials, as predicted by general relativity. Optical-clock comparisons can measure this effect across small height differences.
Microwave atomic clock: An atomic clock that references a microwave-frequency transition in atoms or ions. Its lower reference frequency generally permits fewer resolvable oscillations than an optical clock.
Systematic uncertainty: Uncertainty caused by persistent measurement biases rather than random variation. Unaccounted shifts can make a clock’s claimed accuracy differ from its actual performance.

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