Atomic ion frequency standards

The history and status of trapped-ion frequency standards are reviewed. In a trapped-ion frequency standard, the frequency of an oscillator is servoed to a resonance which corresponds to a transition between two energy levels of an atomic ion. The ions are suspended in space by a combination of electric and magnetic fields. In a conventional rubidium cell, the atoms are surrounded by a buffer gas having a pressure of about 10³ Pa (approximately 10 torr). In an ion trap, the ions are held either in a vacuum or in a low-pressure buffer gas (less than 10^-3 Pa). In an atomic beam, the atoms also move through a vacuum, without collisions. However, the time available for interaction with the electromagnetic field is limited to their flight time through the apparatus, usually about 10 ms or less. Trapped ions can be observed for much longer periods. Consequently clocks based on ions trapped in electromagnetic fields portend orders-of-magnitude improvement in the development of new frequency standards. Prospects for future standards are discussed