High-Stability Comparison of Atomic Fountains Using Two Different Cryogenic Oscillators

We present a detailed characterization of two atomic clock interrogation systems based on two different cryogenic sapphire oscillators operated simultaneously. We use them as references for two accurate fountain clock frequency standards participating in international atomic time and operating both at the quantum projection noise frequency limit. The two fountain comparison down to a few 10
^-16
over 28 days demonstrates the potential of a cryocooled oscillator to replace a He-refilled cryogenic oscillator.