Cryogenic sapphire dielectric resonators: performance and potential

Sapphire dielectric resonators (SDR) at cryogenic temperatures have been used to produce oscillators of exceptional short- and medium-term stability. We discuss the demonstrated performance achieved at the National Physical Laboratory (NPL) as well as potential long-term capabilities of this type of oscillator both as a "fly-wheel" standard for atomic clocks and in a possible determination of the fundamental physical constants.