Evaluation of long term performance of continuously running atomic fountains

Abstract

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