We describe the operation of a laser-cooled rubidium 87Rb frequency standard. We present a new measurement of the 87Rb hyperfine frequency with a 1.3 × 10−14 relative accuracy, by comparison with a Cs fountain primary standard. The measured 87Rb ground-state hyperfine splitting is ν87 = 6 834 682 610.90429(9) Hz. This value differs from previously published values (see Essen L., Hope E. G. and Sutcliffe D., Nature 189 1961 298; Penselin S., Moran T., Cohen W. and Wscinkler G., Phys. Rev. 127 1962 524; Arditi M. and Cerez P. IEEE Trans. Instrum. Meas. IM-21 1972 391) by about 2 − 3 Hz and is 104 times more accurate. Because of the low collisional shift in 87Rb, future improvements may lead to a stability of 1 × 10−14τ−1/2 and a relative accuracy in the 10−17 range.

As part of a programme to implement scientific time and frequency metrology in Brazil, the caesiumbeam frequency standard of the Instituto de F ísica de São Carlos has been improved in order to obtain greater accuracy. Our most recent evaluation shows an Allan standard deviation of , corresponding to an improvement of two orders of magnitude over our previous evaluation. Estimates of the main shifts of the frequency standard, such as second-order Doppler shift, end-to-end cavity phase shift, and Rabi frequency value, have been performed employing two different methods that allow the results to be extracted from the Ramsey pattern.

For the past two years we have been implementing a program for the establishment of scientific time and frequency metrology in Brazil. The main objective of this program is to construct an atomic fountain and use it as a primary standard. As a first step toward this goal, we have constructed a (133)Cs beam optically pumped conventional clock. In this paper we describe the system and the results of its evaluation. The possible limitations of our short-term stability are discussed.

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Periodontitis is an inflammatory infection caused by bacterial plaque accumulation that affects the periodontal tissues. Current treatments lack bioactive signals to induce tissue repair and coordinated regeneration of the periodontium, thus alternative strategies are needed to improve clinical outcomes. Electrospun nanofibers present high porosity and surface area and are able to mimic the natural extracellular matrix, which modulates cell attachment, migration, proliferation, and differentiation. Recently, several electrospun nanofibrous membranes have been fabricated with antibacterial, anti-inflammatory, and osteogenic properties, showing promising results for periodontal regeneration. Thus, this review aims to provide an overview of the current state of the art of these nanofibrous scaffolds in periodontal regeneration strategies. First, we describe the periodontal tissues and periodontitis, as well as the currently available treatments. Next, periodontal tissue engineering (TE) strategies, as promising alternatives to the current treatments, are addressed. Electrospinning is briefly explained, the characteristics of electrospun nanofibrous scaffolds are highlighted, and a detailed overview of electrospun nanofibers applied to periodontal TE is provided. Finally, current limitations and possible future developments of electrospun nanofibrous scaffolds for periodontitis treatment are also discussed.

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In order to evaluate an atomic clock, it is important to determine the main frequency shifts caused by external fields, device imperfections, etc. Scanning the frequency of the main oscillator, the Ramsey fringe is obtained and used to determine the main shifts. The gravitational shift (10 −17 ), second order Doppler (1.65×10 −13 ), Black-Body radiation shift (2.9×10 −14 ), quadratic Zeeman effect (5×10 −13 ), Rabi Pulling (1.3×10 −13 ) and cavity pulling (1.3×10 −13 ) have been evaluated. The short term stability, (6.6±0, 2)×10 −13 , was obtained.

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