We report on the implementation and the metrological characterization of a vapor-cell Rb frequency standard working in pulsed regime. The three main parts that compose the clock, physics package, optics and electronics, are described in detail in the paper. The prototype is designed and optimized to detect the clock transition in the optical domain. Specifically, the reference atomic transition, excited with a Ramsey scheme, is detected by observing the interference pattern on a laser absorption signal.The metrological analysis includes the observation and characterization of the clock signal and the measurement of frequency stability and drift. In terms of Allan deviation, the measured frequency stability results as low as 1.7 × 10 −13 τ −1/2 , τ being the averaging time, and reaches the value of few units of 10 −15 for τ = 10 4 s, an unprecedent achievement for a vapor cell clock. We discuss in the paper the physical effects leading to this result with particular care to laser and microwave noises transferred to the clock signal. The frequency drift, probably related to the temperature, stays below 10 −14 per day, and no evidence of flicker floor is observed.We also mention some possible improvements that in principle would lead to a clock stability below the 10 −13 level at 1 s and to a drift of few units of 10 −15 per day.1

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To significantly improve the frequency references used in radio-astronomy and the precision measurements in atomic physics, we provide frequency dissemination through a 642-km coherent optical fiber link.\ud On the frequency transfer, we obtained a frequency instability of 3x10^-\ud 19\ud at 1,000 s in terms of Allan deviation on a 5-mHz measurement bandwidth, and an accuracy of 5x10^-19. The ultimate link performance has been evaluated by doubling the link to 1,284 km, demonstrating a new characterization technique based on the double round\ud trip on a single fiber. This method is an alternative to\ud previously demonstrated techniques for link characterization. In particular, the use of a single fiber may be beneficial to long hauls realizations in view of a continental fiber network for frequency and time metrology, as it avoids the doubling of the amplifiers, with a subsequent reduction in costs and maintenance. A detailed analysis of the results is presented, regarding the phase noise, the cycle-slips detection and removal and the instability evaluation. The observed noise power spectrum is seldom found in the literature; hence, the expression of the Allan\ud deviation is theoretically derived and the results confirm the expectations

For almost two decades, caesium fountain primary frequency standards (PFS) have represented the best realization of the definition of the second in the International System of units. Their accuracy has progressively improved with time, reaching a few parts in 1016. In this paper, we present the accuracy evaluation of ITCsF2, the new Cs fountain PFS developed at the Italian National Metrological Institute, designed to be operated at cryogenic temperature to reduce the blackbody radiation shift. The short-term stability of the ITCsF2 fountain is 2 × 10−13τ−1/2 when operated at high atomic density, and the relative inaccuracy reaches 2.3 × 10−16. We also report four calibrations of International Atomic Time with a relative frequency agreement of (−1.7 ± 3.2) × 10−16, between ITCsF2 and the average of the other fountains operated in the world during the reference periods.

We reconsider the idea of a pulsed optically pumped frequency standard conceived in the early 1960s to eliminate the light-shift effect. The development of semiconductor lasers and of pulsed electronic techniques for atomic fountains and new theoretical findings allow an implementation of this idea which may lead to a frequency standard whose frequency stability is limited only by the thermal noise in the short term and by the temperature drift in the long term. We shall also show both theoretically and experimentally the possibility of doubling the atomic quality factor with respect to the classical Ramsey technique approach