Frequency standards based on stored ions

The state of development of frequency standards based on stored ions is reviewed. Several preliminary demonstrations of the concept have already shown a level of performance approaching that of today´s cesium-beam standards (accuracy of one part in 10¹³). The potential for accurately measuring or reducing all known systematic effects suggests that frequency standards based on stored ions with inaccuracies of one part in 10¹⁵are obtainable and eventually they could be orders of magnitude better than this. This performance is a result of extremely high-Q resonances (e.g., millihertz linewidths at microwave frequencies) and a very small second-order Doppler shift which follows with the addition of techniques for ion cooling.