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We report on a transportable and easy-to-operate optical clock utilizing the 2S1/2 - 2D3/2 transition of a single trapped 171Yb+ ion at 436 nm. Developed within a pilot project for quantum technology in Germany lead by industry, the clock is set up in two 19″ racks. In this way, transportation can easily be realized, and the large degree of automatization allows for operation outside highly specialized laboratories for applications beyond basic research. Comparisons to existing high-accuracy optical clock systems enable a verification of the clock's stability and uncertainty budget at the low 10−17 level. During these tests, operation with 99.8% availability over more than 14 days has been achieved.

Everyone is trying to make frequency measurements as accurate as possible. But what if the measurement process itself becomes a major source of error? This issue is particularly relevant in the context of state-of-the-art atomic clocks where errors at the 18th decimal place are not negligible anymore. In order to spectroscopically interrogate an atomic clock transition without compromising the clock’s accuracy, we devise a perturbation-immune version of Ramsey’s method of separated oscillatory fields. By actively balancing the spectroscopic responses from phase-congruent Ramsey probe cycles of unequal durations, we eliminate a wide variety of interrogation-induced line shifts often encountered in high precision spectroscopy, among them, in particular, light shifts, phase chirps, and transient Zeeman shifts. We experimentally demonstrate autobalanced Ramsey spectroscopy on the light shift prone 171Yb+ electric octupole optical clock transition and show that interrogation defects are not turned into clock errors. This opens up frequency accuracy perspectives below the 10-18 level for the Yb+ system and for other types of optical clocks. Further applications of this simple and universal approach include next generation coherent population trapping clocks and perturbation-immune readout of quantum sensors. Future long-term clock comparisons searching for physics beyond the standard model will also benefit from this technique.

We used precise point positioning, a well-established GPS carrier-phase frequency transfer method to perform a direct remote comparison of two optical frequency standards based on single laser-cooled $^{171} {\text {Yb}}^+$ ions operated at the National Physical Laboratory (NPL), U.K. and the Physikalisch-Technische Bundesanstalt (PTB), Germany. At both institutes, an active hydrogen maser serves as a flywheel oscillator which is connected to a GPS receiver as an external frequency reference and compared simultaneously to a realization of the unperturbed frequency of the ${^2S_{1/2}(F=0)-{^2D_{3/2}}(F=2)}$ electric quadrupole transition in $^{171} {\text {Yb}}^+$ via an optical femtosecond frequency comb. To profit from long coherent GPS-link measurements, we extrapolate the fractional frequency difference over the various data gaps in the optical clock to maser comparisons which introduces maser noise to the frequency comparison but improves the uncertainty from the GPS-link instability. We determined the total statistical uncertainty consisting of the GPS-link uncertainty and the extrapolation uncertainties for several extrapolation schemes. Using the extrapolation scheme with the smallest combined uncertainty, we find a fractional frequency difference $y({\text {PTB}})-y({\text {NPL}})$ of $-1.3\times 10^{-15}$ with a combined uncertainty of $1.2\times 10^{-15}$ for a total measurement time of 67 h. This result is consistent with an agreement of the frequencies realized by both optical clocks and with recent absolute frequency measurements against caesium fountain clocks within the corresponding uncertainties.

Questioning the presumably most basic assumptions about the structure of space and time has revolutionized our understanding of Nature. State-of-the-art atomic clocks make it possible to precisely test fundamental symmetry properties of spacetime, and search for physics beyond the standard model at low energy scales of just a few electron volts. Here, we experimentally demonstrate for the first time agreement of two single-ion clocks at the $10^{-18}$ level and directly confirm the validity of their uncertainty budgets over a half-year long comparison period. The two clock ions are confined in separate ion traps with quantization axes aligned along nonparallel directions. Hypothetical Lorentz symmetry violations would lead to sidereal modulations of the frequency offset. From the absence of such modulations at the $10^{-19}$ level we deduce stringent limits on Lorentz symmetry violation parameters for electrons in the range of $10^{-21}$, improving previous limits by two orders of magnitude.

We devise a perturbation-immune version of Ramsey's method of separated oscillatory fields. Spectroscopy of an atomic clock transition without compromising the clock's accuracy is accomplished by actively balancing the spectroscopic responses from phase-congruent Ramsey probe cycles of unequal durations. Our simple and universal approach eliminates a wide variety of interrogation-induced line shifts often encountered in high precision spectroscopy, among them, in particular, light shifts, phase chirps, and transient Zeeman shifts. We experimentally demonstrate auto-balanced Ramsey spectroscopy on the light shift prone $^{171}$Yb$^{+}$ electric octupole optical clock transition and show that interrogation defects are not turned into clock errors. This opens up frequency accuracy perspectives below the $10^{-18}$ level for the Yb$^{+}$ system and for other types of optical clocks., Comment: 8 pages, 6 figures

We used precise point positioning, a well-established GPS carrier-phase frequency transfer method to perform a direct remote comparison of two optical frequency standards based on single laser-cooled [Formula: see text] ions operated at the National Physical Laboratory (NPL), U.K. and the Physikalisch-Technische Bundesanstalt (PTB), Germany. At both institutes, an active hydrogen maser serves as a flywheel oscillator which is connected to a GPS receiver as an external frequency reference and compared simultaneously to a realization of the unperturbed frequency of the (2)S1/2(F=0)-(2)D3/2(F=2) electric quadrupole transition in [Formula: see text] via an optical femtosecond frequency comb. To profit from long coherent GPS-link measurements, we extrapolate the fractional frequency difference over the various data gaps in the optical clock to maser comparisons which introduces maser noise to the frequency comparison but improves the uncertainty from the GPS-link instability. We determined the total statistical uncertainty consisting of the GPS-link uncertainty and the extrapolation uncertainties for several extrapolation schemes. Using the extrapolation scheme with the smallest combined uncertainty, we find a fractional frequency difference [Formula: see text] of -1.3×10(-15) with a combined uncertainty of 1.2×10(-15) for a total measurement time of 67 h. This result is consistent with an agreement of the frequencies realized by both optical clocks and with recent absolute frequency measurements against caesium fountain clocks within the corresponding uncertainties.

A fundamental limit to the stability of a single-ion optical frequency standard is set by quantum noise in the measurement of the internal state of the ion. We discuss how the interrogation sequence and the processing of the atomic resonance signal can be optimized in order to obtain the highest possible stability under realistic experimental conditions. A servo algorithm is presented that stabilizes a laser frequency to the single-ion signal and that eliminates errors due to laser frequency drift. Numerical simulations of the servo characteristics are compared to experimental data from a frequency comparison of two single-ion standards based on a transition at 688 THz in 171Yb+. Experimentally, an instability sigma_y(100 s)=9*10^{-16} is obtained in the frequency difference between both standards., 15 pages, 5 figures, submitted to J. Phys. B

We have used a Ti:Sapphire-based fs-comb generator to measure the frequencies of several optical frequency standards available at PTB. The frequency values for HeNe- and frequency-doubled Nd:Yag lasers stabilized to iodine hyperfine transitions as well as the frequencies of the /sup 1/S/sub 0/-/sup 3/P/sub 1/ transition of /sup 40/Ca at 657.5 nm (456 THz) and the /sup 2/S/sub 1/2/-/sup 2/D/sub 3/2/ transition of /sup 171/Yb/sup +/ at 435.5 nm (688 THz) are presented. Comparing our results with previously published data for the frequency variation of transitions of the Hg/sup +/ ion and H atom, we have derived a new upper limit of the variation of the fine structure constant /spl alpha/.

The vasoconstrictor hormone angiotensin-II (AII) raises cytosolic free calcium and stimulates protein kinase-C (PKC) activity in vascular smooth muscle cells (VSMC). Phorbol-12-myristate-13-acetate (PMA) directly activates PKC in these cells. Both of these agonists stimulate prostacyclin production. Several studies have shown that atrial natriuretic factor (ANF) does not interfere with the AII-induced early calcium response. We, therefore, examined the effects of ANF on PKC activity and prostacyclin production in cultured rat aortic VSMC. PKC activity was determined in the membranous and cytosolic fractions after anion exchange chromatography. ANF (10(-7) M) inhibited by 44 +/- 3% and 39 +/- 8% the increase in membranous PKC activity induced by AII and PMA, respectively. ANF (10(-7) M) inhibited PMA-stimulated prostacyclin production, whereas AII-induced prostacyclin production remained unaffected. Thus, our results suggest that some biological effects induced by ANF in VSMC are mediated by an inhibition of membranous PKC activity.

We present work on a frequency standard based on the 2S1/2(F=0, mF=0)rarr2D3/2(F=2, mF=0) electric quadrupole transition in 171Yb+ at 688 THz, and future plans for a frequency standard based on the 2S1/2(F=0, mF=0)rarr2F7/2(F=3, mF=0) electric octupole transition in the same ion at 642 THz. Direct measurement of the ratio of these two frequencies, derived from a single ytterbium ion, constitutes a sensitive test for the temporal constancy of the fine structure constant.

Two single-ion optical frequency standards are compared in order to investigate systematic frequency shift effects. Without externally applied perturbations, a mean frequency difference of 0.26(42) Hz is observed, corresponding to a relative agreement of 3.8(6.1) /spl middot/10/sup -16/.

We describe experimental investigations of an optical frequency standard based on a single laser-cooled /sup 171/Yb/sup +/ ion confined in a radiofrequency Paul trap. The electric quadrupole transition from the /sup 2/S/sub 1/2 /(F=0) ground state to the /sup 2/D/sub 3/2/(F=2) state at 435.5 nm is used as the reference transition. The transition was resolved with a Fourier-limited width of approximately 30 Hz. In order to investigate the instability and accuracy of a future frequency standard based on the reference transition, a first experiment was done comparing two laser frequencies stabilized individually to two separate ions in different traps. As a result we found a relative frequency instability /spl sigma//sub y/(640 s)=1.5/spl middot/10/sup -15/ for the difference of the two laser frequencies. We also derived a first estimate /spl Theta/(/sup 2/D/sub 3/2/)=(3.9/spl plusmn/1.9)ea/sub 0//sup 2/ for the electric quadrupole moment of the /sup 2/D/sub 3/2/ state.

Optical transition frequencies of trapped laser-cooled ions have been measured so far with an uncertainty of 10 - 1 4 with reference to primary cesium clocks. Systematic frequency shifts in these systems should be controllable to significantly higher accuracy. We have performed comparisons between frequency standards based on two independently stored 1 7 1 Yb + ions. The present experimental results indicate a relative instability (Allan standard deviation) of the optical frequency difference between the two systems of 1.0.10 - 1 5 at an averaging time of 1000 s and a mean frequency difference of 0.2 Hz for the reference frequency of 688 THz. Monitoring the absolute transition frequency over a period of a few years at this level of accuracy will lead to a stringent limit on the present value of the temporal derivative of the fine-structure constant. First results have been obtained with a frequency standard based on 1 9 9 Hg + at NIST and, more recently, also with Yb + . The combination of results obtained with different transitions can be used to separate the electronic and nuclear contributions to a drift of a transition frequency. We propose optical spectroscopy of a nuclear transition in 2 2 9 Th 3 + as a means for a precision monitoring of possible temporal variations in the strength of the nuclear forces.

We describe experimental investigations on an optical frequency standard based on a single laser cooled 171 Yb + ion confined in a radiofrequency Paul trap. The 435.5 nm electric-quadrupole transition from the 2 S 1/2 (F=0) groudn state to the 2 D 3/2 (F=2) state is probed using a diode laser system whose short-term frequency stability is derived from an environmentally isolated ULE cavity. The transition is resolved with a Foruier-limited full halfwidth of approximately 30 Hz. The absolute optical transition frequency is determined using a femtosecond frequency comb generator and a Cs fountain clock as the frequency reference. The measured frequency value is 688 358 979 321 Hz at an ambient blackbody radiation field of 300 K. The 1σ fractional uncertainty of this frequency measurement is 1×10 -14 .

Stimulated by the development of optical frequency comb generators based on Kerr-lens modelocked femtosecond lasers [1,2], the absolute frequencies of several narrow transition lines in cold atoms or single stored ions, such as H, Ca, Yb+ were recently measured by phase- coherently linking optical signals to a cesium-clock controlled hydrogen maser [3-6]. This phase-coherent link is established via a frequency-comb generated by a femtosecond laser. In the frequency domain the femtosecond laser’s pulse train corresponds to a comb spectrum. This comb is spectrally broadened to extend over the whole visible wavelength region and into the near infrared. In our experiment this was achieved in a micro- structure/photonic crystal fiber from Lucent Technologies. This fiber has core diameter of 1.7 μm and its GVD ist zero at 780 nm [7], which is the center wavelength of the spectrum emitted by our Ti:Sa-laser similar to that described in ref. [8]. Its spectrum of some 170 nm width around 780 nm is broadened to extend from 500 nm to about 1100 nm, by passing approximately 30 mW through a 10 cm piece of the micro-structure fiber. During the propagation in the fiber, the comb nature of the spectrum is not altered, only its spectral width is extended. The result is an evenly spaced comb spectrum over almost the whole visible wavelength range and into the near infrared, where the individual comb mode m, has the frequency vi = mi×frep+vceo. The last term, the so called carrier envelope offset frequency is due to different group and phase velocity delay in the laser cavity. It results in a collective shift of all comb modes with respect to the frequency origin. Any external optical frequency Δi is measured by detecting a beat note A; with a suitable comb mode with the order number mi

We developed a novel technique for frequency measurement and synthesis, based on the operation of a femtosecond comb generator as transfer oscillator. The technique can be used to measure frequency ratios of any optical signals throughout the visible and near-infrared part of the spectrum. Relative uncertainties of $10^{-18}$ for averaging times of 100 s are possible. Using a Nd:YAG laser in combination with a nonlinear crystal we measured the frequency ratio of the second harmonic $\nu_{SH}$ at 532 nm to the fundamental $\nu_0$ at 1064 nm, $\nu_{SH}/\nu_0 = 2.000 000 000 000 000 001 \times (1 \pm 7 \times 10^{-19})$., Comment: 4 pages, 4 figures

This study investigates the diagnostic value of echocardiography in patients with suspected pulmonary embolism. Doppler-echocardiography was performed in fifty consecutive patients, predominantly presenting in the emergency ward, with clinically suspected pulmonary embolism. Patients were classified as having or not pulmonary embolism by a sequential non-invasive strategy including lung scan, D-dimer measurement and lower limb venous compression ultrasonography, pulmonary angiography being performed in case of an inconclusive non-invasive work-up. The prevalence of pulmonary embolism was 36% (18 of 50 patients). Right ventricular dilatation on 2-D echocardiography associated to a tricuspid regurgitation velocity ≥2.7 m/s, corresponding to a pulmonary systolic pressure ≥39 mmHg, were present in 12 of the 18 patients (67%) with and in two of the 32 patients (6.3%) without pulmonary embolism. They were, however, absent in five of the 18 patients (28%), in whom the definite diagnosis of pulmonary embolism was made. The combination of these both echocardiographic criteria yielded a sensitivity of 67% and a specificity of 94%, positive predictive value was 86% and negative predictive value was 83%. The diagnostic performance of these two combined echocardiographic criteria, when present, permitted to reach in patients with a high clinical pre-test probability of pulmonary embolism the post-test probability values above 90%. On the other hand, the absence of these two Doppler-echocardiographic criteria did not allow to exclude pulmonary embolism, except in presence of a low pre-test probability. The findings of our study show that Doppler-echocardiography in patients with high clinical suspicion of pulmonary embolism may represent a potentially useful screening technique for the diagnosis of the disease permitting prompt initiation of treatment. However, the method does not allow to exclude pulmonary embolism in all patients with intermediate or high clinical suspicion of the disease.

Inhibition of fatty acid oxidation is an early event in myocardial ischemia that most likely contributes to tissue injury by the accumulation of potentially toxic intermediates such as acylCoA and acylcarnitine. After reperfusion both myocardial oxygen consumption and fatty acid oxidation may rapidly recover to preischemic levels, even when contractile function remains depressed. The mechanisms underlying the apparent dissociation between contractile function and oxidative metabolism early during reperfusion are still controversial. In isolated rat hearts subjected to 60 min of no-flow ischemia myocardial oxygen consumption and oxidation of palmitate were lowered during reperfusion by 3 mM of NiCl2 and by 6 μM of ruthenium red. The results provide indirect evidence for the hypothesis that intracellular calcium transport may be involved in the mechanisms responsible for the high oxidative metabolic rate early after reperfusion. (Mol Cell Biochem 116: 103–109, 1992)

For nonlinear optical resonators, it has been predicted that the interaction of transverse optical modes of the radiation field can lead to symmetry-breaking and Turing instabilities and to the formation of stationary transverse patterns /1, 2/. It can be concluded from these studies and also from one very early work /3/ that lasers operating in a state of “cooperative frequency locking” /2/ of transverse resonator modes may show bistability or multistability in the sense that stable oscillation is possible with a discrete set of different transverse field patterns.

We have measured the frequency of the $6s^2S_{1/2} - 5d^2D_{3/2}$ electric-quadrupole transition of $^{171}$Yb$^+$ with a relative uncertainty of $1\times 10^{-14}$, $\nu_{Yb}$ = 688 358 979 309 312 Hz $\pm$ 6 Hz. A femtosecond frequency comb generator was used to phase-coherently link the optical frequency derived from a single trapped ion to a cesium fountain controlled hydrogen maser. This measurement is one of the most accurate measurements of optical frequencies ever reported, and it represents a contribution to the development of optical clocks based on an $^{171}$Yb$^+$ ion standard.

Incubation experiments with eggs of a population of Isotoma viridis, which is exposed to annual long-term flooding from about April to July, as well as field observations show that temperature controls both, underwater egg dormancy and immediate postflood hatching. The population is located at the Eder Freshwater Reservoir in Germany.If constant experimental temperatures are above 14°C, almost all eggs are nondormant. Dormancy is established at temperatures below 15°C, but embryonic development is completed. Experiments indicate that of the environmental factors that change drastically at the end of submergence (light, turgor pressure, oxygen, a.o.), only temperature acts as a hatching trigger. Hatching of the previously dormant eggs occurs at a constant threshold temperature of 16°C, mainly within 2 to 20 days after temperature elevation, but most of these eggs need even higher temperatures to hatch. Remaining eggs were partly stimulated to hatch by recooling them at 7°C for some days and then rewarming them again.The threshold temperatures observed are unusually high for Collembola and seem to be the result of selection by the special floodplain conditions. During normal years, the surface temperatures of submerged soil usually do not exceed threshold limits before summer drainage. This allows both, protection from under-water hatching and an optimal timing of hatching at the very beginning of the main terrestrial period. The experiments show that above the threshold temperature (in warm summers), individuals can hatch under water and survive submerged for 10-15 days. They can survive even longer in the water habitat, if emergent structures enable them to climb onto the water surface. Furthermore, a considerable polymorphism observed in some hatching properties improves the chance to survive under the unpredictable floodplain conditions.

Sampling experiments show that some terrestrial arthropod species which predominate on the flood plains of Eder Reservoir (Germany) survive long-term inundations in the egg stage under water, hatching immediately after the floods: the Collembola Isotoma viridis, Anurida tullbergi, Sminthurides malmgreni, Sminthurinus aureus, and Sminthurus nigromaculatus; the land mites Cheylostigmaeus pannonicus and Zerconopsis müstairi; and the ephydrid fly Scatella stagnalis. Some other dominant species seem to have the same survival strategy, because they show the same characteristic pattern of appearance in the study area (bdellid and oribatid mites, ephydrid and sphaerocerid flies).

It is well known that the output field of lasers oscillating in a superposition of higher-order transverse modes of the optical resonator typically is characterized by a complex spatiotemporal structure /1/. The “cooperative frequency locking” /2/ of near-degenerate transverse resonator modes is accompanied by the formation of stationary transverse patterns of the optical field.

The transmission of frequency information via fiber optic telecommunication networks offers an attractive option for a long distance frequency comparison and future public dissemination of ultra-precise frequencies. Here, mode locked erbium-doped fiber lasers that combine the field of frequency metrology with that of optical telecommunication, enable a new and attractive opportunity for a direct frequency comparison of distant optical clocks which circumvents a down-conversion of the optical frequency into the microwave domain.