Evaluation of a ^{88}Sr^{+} Optical Clock with a Direct Measurement of the Blackbody Radiation Shift and Determination of the Clock Frequency

We report on an evaluation of an optical clock that uses the ^{2}S_{1/2}→^{2}D_{5/2} transition of a single ^{88}Sr^{+} ion as the reference. In contrast to previous work, we estimate the effective temperature of the blackbody radiation that shifts the reference transition directly during operation from the corresponding frequency shift and the well-characterized sensitivity to thermal radiation. We measure the clock output frequency against an independent ^{171}Yb^{+} ion clock, based on the ^{2}S_{1/2}(F=0)→^{2}F_{7/2}(F=3) electric octupole (E3) transition, and determine the frequency ratio with a total fractional uncertainty of 2.3×10^{-17}. Relying on a previous measurement of the ^{171}Yb^{+} (E3) clock frequency, we find the absolute frequency of the ^{88}Sr^{+} clock transition to be 444 779 044 095 485.277(59) Hz. Our result reduces the uncertainty by a factor of 3 compared with the previously most accurate measurement and may help to resolve so far inconsistent determinations of this value. We also show that for three simultaneously interrogated ^{88}Sr^{+} ions, the increased number causes the expected improvement of the short-term frequency instability of the optical clock without degrading its systematic uncertainty.

Out-of-Band Fibre-Optic Time and Frequency Transfer Using Asymmetric and Symmetric Opto-Electronic Repeaters

Signal repeaters for fibre-optic communication can be realized with back-to-back connected transceivers. This configuration can provide high gain (≈30 dB) at low cost, and the needed semiconductor lasers and modulators can be realized for practically any relevant wavelength. Unfortunately, for time and frequency transfer the uncorrelated wavelength drifts in the transceiver lasers can compromise transfer stability, and device replacement due to failure may result in large time offsets that have to be measured via global navigation satellite services. This work demonstrates that good results can nevertheless be obtained with standard telecom DWDM tranceivers over long time periods (years). More importantly, a simple wavelength-symmetric repeater is proposed that can be used to cancel the detrimental effects of wavelength drifts and which lessens the need for link re-calibrations after transceiver replacements. In a proof-of-concept test setup, timing drift due to wavelength drift of a repeater laser is reduced by approximately two orders of magnitude.

Spin Resonance Clock Transition of the Endohedral Fullerene ^{15}N@C_{60}

The endohedral fullerene ^{15}N@C_{60} has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

Stochastic ordering of order statistics

If Xi, i = 1, ···, n are independent exponential random variables with parameters λ1, · ··, λ n, and if Yi, i = 1, ···, n are independent exponential random variables with common parameter equal to (λ1 + · ·· + λ n)/n, then there is a monotone coupling of the order statistics X(1), · ··, X(n) and Y(1), · ··, Y(n); that is, it is possible to construct on a common probability space random variables X′i, Y′i, i = 1, ···, n, such that for each i, Y′(i)≦X′(i) a.s., where the law of the X′i (respectively, the Y′i) is the same as the law of the Xi (respectively, the Yi.) This result is due to Proschan and Sethuraman, and independently to Ball. We shall here prove an extension to a more general class of distributions for which the failure rate function r(x) is decreasing, and xr(x) is increasing. This very strong order relation allows comparison of properties of epidemic processes where rates of infection are not uniform with the corresponding properties for the homogeneous case. We further prove that for a sequence Zi, i = 1, ···, n of independent random variables whose failure rates at any time add to 1, the order statistics are stochastically larger than the order statistics of a sample of n independent exponential random variables of mean n, but that the strong monotone coupling referred to above is impossible in general.

A simple coupling of renewal processes

We use a simple coupling to prove the classical result that the renewal function U of a zero-delayed renewal process satisfies U(t) – λ . t→λ2μ2/2 as t→∞ if the life-length distribution is of non-lattice type and has finite first and second moments μ and μ2 respectively; λ is the renewal intensity, and is equal to 1/μ.

Broadband, unpolarized repumping and clearout light sources for Sr(+) single-ion clocks

Future transportable optical clocks require compact and reliable light sources. Here, broadband, unpolarized repumper and state clearout sources for Sr⁺ single-ion optical clocks are reported. These turn-key devices require no frequency stabilization or external modulators. They are fiber based, inexpensive, and compact. Key characteristics for clock operation are presented, including optical spectra, induced light shifts, and required extinction ratios. Tests with an operating single-ion standard show a clearout efficiency of 100%. Compared to a laser-based repumper, the achievable fluorescence rates for ion detection are a few tens of percent lower. The resulting ion kinetic temperature is 1-1.5 mK, near the Doppler limit of the ion system. Similar repumper light sources could be made for Ca⁺ (866 nm) and Ba⁺ (650 nm) using semiconductor gain media.

Environmentally induced changes in sensory sensitivities

For studying changes in the sensory sensitivities, psychophysical methods offer a variety of models. The psychophysical approach also holds for applications on indoor air quality. The human senses can be used for characterizing environments in terms of effects on health and comfort, and have been used by the WHO for determining recommended exposure limits. The use of psychophysical methods for air pollution applications is illustrated by two studies on effects on sensory functions in smokers and passive smokers. Olfactory as well as auditory effects were found. Furthermore, methodological findings are presented, such as obtaining information on detectability and perceived intensity in the very same estimation procedure. The results also demonstrate the need for calibrating scales with regard to individual scaling behavior in perceived intensity measurements as well as the capability of the master scale principle for performing such a calibration.

Ambient nitrogen dioxide concentrations increase bronchial responsiveness in subjects with mild asthma

Twenty subjects with mild asthma were exposed at rest in a body plethysmograph, to NO2 at 0, 260, 510 and 1,000 micrograms.m3, for 30 min on four separate days. Bronchial responsiveness (histamine inhalation test) was measured after each exposure session. Airway resistance (Raw), thoracic gas volume (TGV) and specific airway resistance (sRaw) were measured before, during and after exposure, and the breathing pattern was monitored during the whole session. Bronchial responsiveness increased significantly after 30 min exposure to 510 micrograms.m3 NO2 (p less than 0.01). There were also tendencies to an increased bronchial responsiveness after exposure to 260 and 1,000 micron.m3 NO2, but these changes were not statistically significant. Effects on airway resistance and breathing pattern were not demonstrated by exposure to 0-1,000 micrograms.m3 NO2. We conclude that short-term NO2 exposure at about 500 micrograms.m3 slightly affects human bronchial responsiveness in subjects with mild asthma.

Ambient nitrogen dioxide concentrations increase bronchial responsiveness in subjects with mild asthma

Twenty subjects with mild asthma were exposed at rest in a body plethysmograph, to NO2 at 0, 260, 510 and 1,000 micrograms.m3, for 30 min on four separate days. Bronchial responsiveness (histamine inhalation test) was measured after each exposure session. Airway resistance (Raw), thoracic gas volume (TGV) and specific airway resistance (sRaw) were measured before, during and after exposure, and the breathing pattern was monitored during the whole session. Bronchial responsiveness increased significantly after 30 min exposure to 510 micrograms.m3 NO2 (p less than 0.01). There were also tendencies to an increased bronchial responsiveness after exposure to 260 and 1,000 micron.m3 NO2, but these changes were not statistically significant. Effects on airway resistance and breathing pattern were not demonstrated by exposure to 0-1,000 micrograms.m3 NO2. We conclude that short-term NO2 exposure at about 500 micrograms.m3 slightly affects human bronchial responsiveness in subjects with mild asthma.

A comparison of odor perception in smokers, nonsmokers, and passive smokers

The olfactory perception in matched groups of 26 smokers, 26 nonsmokers, and 15 passive smokers was examined psychophysically with two substances, n-butane and pyridine. Different psychophysical functions were obtained from these groups using the method of magnitude estimation. The smokers and passive smokers perceived all concentrations of n-butane to be weaker than did the nonsmokers. In the case of pyridine, a substance in tobacco smoke to which smokers are more exposed, there is an indication of a perceptual deficit similar to loudness recruitment. There were no appreciable differences between smokers and nonsmokers for high concentrations; the sensitivity was less and the psychophysical functions steeper for smokers than nonsmokers for relatively low concentrations. The results are evaluated in terms of information about the subjects' response criteria, and the authors consider the possibility of sensory deficit versus habituation.

Impaired odor perception in tank cleaners

The olfactory perception of 20 men (tank cleaners) exposed to petroleum products (while cleaning oil tanks) was examined. Office workers and watchmen were used as referents (N = 20 + 20). They were matched with regard to sex, age, and smoking habits. Odor detection thresholds and the perceived odor intensity of four odorous stimuli, pyridine, dimethyl disulfide (DMDS), n-butanol, and heating oil vapor (gas phase of heating oil heated to +40 degrees C), were determined. The results suggested that the tank cleaners had higher absolute odor thresholds for n-butanol and oil vapor than the referents. The psychophysical function of the tank cleaners and referents differed for all the tested substances in respect to odor intensity. The tank cleaners displayed an odor deficit analogous to the hearing loss known as "loudness recruitment," ie, normal perception of strong stimuli but impaired perception of weak stimuli. This odor deficit was therefore named "odor intensity recruitment" and seems, in tank cleaners, to be associated with occupational exposure to oil vapor.

Effects of short-term exposure to ambient nitrogen dioxide concentrations on human bronchial reactivity and lung function

Eight normal and 8 asthmatic subjects were exposed to NO2 in a modified body box for plethysmography during 20 min at 0,230,460 and 910 micrograms/m3 on 4 separate days. Bronchial reactivity (histamine inhalation test) was measured after exposure to air alone and to 910 micrograms/m3NO2. Airway resistance (Raw), thoracic gas volume (TGV) and specific airway resistance (SRaw) were measured before, during and after exposure. The bronchial reactivity of the asthmatic subjects increased significantly (p = 0.04) by 20 min exposure to 910 micrograms/m3 NO2. In the non-asthmatic group the airway resistance increased significantly (p = 0.03) after 20 min exposure to 460 micrograms/m3 NO2 and decreased significantly (p = 0.01) after 20 min exposure to 910 micrograms/m3 NO2. In the asthmatic group the trend in airway resistance was the same but not statistically significant. In the latter group TGV was significantly decreased (p = 0.02) during exposure to 910 micrograms/m3 NO2. Short term NO2-exposure in concentrations even below 1000 micrograms/m3 seems to have effects on human bronchial reactivity and lung function.

Olfactory self- and cross-adaptation: effects of time of adaptation on perceived odor intensity

The time-course of self- and cross-adaptation of the olfactory system was investigated for two constant concentrations of three odorous substances. The substances (hydrogen sulfide, dimethyl disulfide, and pyridine) were matched, in a pilot experiment, with regard to perceived odor intensity. The time of adaptation was controlled by the number of inhalations (1-10). A two-step scaling method, involving cross-modality matching and numerical scaling of the matching continuum, was used for measuring perceived odor intensity during adaptation. The results show that the time-course function for self-adaptation seems to be an exponential function for two of the substances (H2S, DMDS), while for the third (pyridine) the form of the function is less distinct. Cross-adaptation between substances was found for the high concentrations, while for the low concentrations, hydrogen sulfide and dimethyl disulfide gave rise to pronounced cross-facilitation. The latter effect increased with time of adaptation.