Trapped ion optical frequency standards

Weak transitions in single cold trapped ions are highly reproducible as optical frequency standards on account of the high quality factors and stabilities achievable. In recent years, there has been a quickening pace in the development of these optical frequency standards, particularly due to the emergence of femtosecond laser combs for relating the optical frequency directly to the Cs standard. This paper reports progress on optical frequency standards based on the quadrupole transition in a single cold ion of Sr-88(+) and the octupole transition in a single cold ion of Yb-171(+).