NOISE OF A SINGLE-ION FREQUENCY STANDARD

A frequency source supposed to serve as a standard requires feedback f or short-term and long-term frequency control. A spectral discriminant for short-term control may be provided, in the microwave regime, by a hyperfine resonance of a multi-ion ensemble confined in an electrodyn amic trap, and in the optical regime by an eigenmode of a focal Fabry- Perot cavity. For long-term frequency control, resonances of single tr apped ions are discriminants of superior accuracy although the control led frequency shows random fluctuations caused by noise on the feedbac k signal. Its fundamental contribution is quantum projection noise. Th e detection of fluorescence for the generation of feedback is incompat ible with the ions being prepared in a superposition state in the exci tation by the wave emitted from the oscillator. Attempts of circumvent ing this limitation by preparing ensembles of con-elated ions prior to the measurement (spin squeezing) are reviewed.