Optical clock

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Optical clocks are the most precise instruments ever developed.^[1] The precision of a clock is the smallest unit of time it can measure. Optical clocks reach record-breaking precision by counting oscillations of visible light, which oscillates up to 750 quadrillion times a second. By counting these oscillations, one can divide a second into 750 quadrillion pieces. Each of these pieces is roughly one femtosecond. This means that by counting oscillations of light, one can be certain of the time to within one femtosecond. Oscillations of light are counted using a frequency comb, and stabilized using atoms.

Optical clocks are a subset of atomic clocks, which typically measure microwaves. However, microwaves oscillate around 100,000 times slower than visible light. For this reason, optical clocks are expected to replace microwave caesium clocks as the definition of the second.^[2] Several elements have been used in optical clocks, including magnesium, aluminum, potassium, calcium, rubidium, strontium, indium, ytterbium, mercury, and radium. John L. Hall and Theodor W. Hansch shared the 2005 Nobel Prize in Physics for their contributions to optical clock development.