Cryogenic sapphire oscillator with exceptionally high long-term   frequency stability
  We report on the development of a sapphire cryogenic microwave resonator oscillator long-term fractional frequency stability of 2x10^-17Sqrt[\tau] for integration times \tau>10^3 s and negative drift of about 2.2x10^-15/day. The short-term frequency instability of the oscillator is highly reproducible and also state-of-the-art: 5.6x10^-16 for an integration time of \tau ~ 20 s. 
J.G. Hartnett, C.R. Locke, E.N. Ivanov, M.E. Tobar, P.L. Stanwix