then undergo Ramsey excitation in a microwave cavity. The fraction of excited atoms are then detected by laser beams. These clocks have systematic uncertainty, which is equivalent to 50 picoseconds per day. A system of several fountains worldwide contribute to International Atomic Time. These caesium clocks also underpin optical frequency measurements. Optical clocks are based on forbidden optical transitions in ions or atoms. They have frequencies around , with a natural linewidth of typically 1 Hz, so the Q-factor is about , or even higher. They have better stabilities than microwave clocks, which means that they can facilitate evaluation of lower uncertainties. They also have better time resolution, which means the clock "ticks" faster. Optical clocks use either a single ion, or an optical lattice with – atoms. Rydberg constant A definition based on the Rydberg constant would involve fixing the value to a certain value: . The Rydberg constant describes the energy levels in a hydrogen atom with the nonrelativistic approximation . The only viable way to fix the Rydberg constant involves trapping and cooling hydrogen. This is difficult because it is very light and the atoms move very fast, causing Doppler shifts. The radiation needed to cool the hydrogen –– is also difficult. Another hurdle involves improving the uncertainty in QED calculations. Requirements A redefinition must include improved optical clock reliability. TAI must be contributed to by optical clocks before the BIPM affirms a redefinition. A consistent method of sending signals must be developed before the second is redefined, such as fiber-optics. SI multiples SI prefixes are commonly used for times shorter than one second, but rarely for multiples of a second. Instead, certain non-SI units are permitted for use with SI: minutes, hours, days, and in astronomy Julian years. See also Caesium standard Orders of magnitude (time) Seconds pendulum Time standard Notes References External links National Physical Laboratory: Trapped ion optical frequency standards High-accuracy strontium ion optical clock; National Physical Laboratory (2005) National Research Council of Canada: Optical frequency standard based on a single trapped ion NIST: Definition of the second; notice the cesium atom must be in its ground state at 0 K Official BIPM definition of the second The leap second: its history and possible future What is a Cesium atom clock? SLAC: Scales of Time – Our universe from 1018 to 10−18 seconds Centimetre–gram–second system of units Orders of magnitude (time) SI base units