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Evaluation of Long Term Performance of Continuously Running Atomic Fountains

An ensemble of rubidium atomic fountain clocks has been put into operation at the U.S. Naval Observatory (USNO). These fountains are used as continuous clocks in the manner of commercial cesium beams and hydrogen masers for the purpose of improved timing applications. Four fountains have been in operation for more than two years and are included in the ensemble used to generate the USNO master clock. Individual fountain performance is characterized by a white-frequency noise level below $2\times 10^{-13}$ and fractional-frequency stability routinely reaching the low $10^{-16}$s. The highest performing pair of fountains exhibits stability consistent with each fountain integrating as white frequency noise, with Allan deviation surpassing $6\times 10^{-17}$ at $10^7$~s, and with no relative drift between the fountains at the level of $7.5 \times 10^{-19}$/day. As an ensemble, the fountains generate a timescale with white-frequency noise level of $1\times 10^{-13}$ and long-term frequency stability consistent with zero drift relative to the world's primary standards at $1 \times 10^{-18}$/day. The rubidium fountains are reported to the BIPM as continuously running clocks, as opposed to secondary standards, the only cold-atom clocks so reported. Here we further characterize the performance of the individual fountains and the ensemble during the first two years in an operational environment, presenting the first look at long-term continuous behavior of fountain clocks.