Implementation of the beam reversal technique on compact cesium clocks: towards an improvement in accuracy.

We report the evaluation of the residual phase difference deltaphi in a short (18 cm) Ramsey cavity by implementing the beam reversal technique to an optically pumped cesium beam clock. Deltaphi is measured to be 21 +/- 1.5 microrad, allowing a more accurate evaluation of the frequency performances of this small cesium clock. Finally, the clock accuracy is equal to 1.1 x 10(-13).

Analysis tools for the accurate evaluation of a small frequency standard.

The short, optically pumped cesium beam tube developed at Laboratoire de l'Horloge Atomique has been carefully evaluated. For that purpose, we have developed a digital servo system that controls three parameters: the frequency of the ultra stable oscillator (USO), the microwave power of the signal experienced by the cesium atoms, and the static magnetic field applied to the atoms. The frequency standard shows a very satisfactory level of short- and medium-term frequency stabilities. A relative frequency offset, measured to be 4.10(-12 ), results mainly from the residual phase difference between the oscillatory fields in the two interaction regions, which is due to imperfection in cavity symmetry. We present two different means of analyzing the causes of this spurious frequency offset using theoretical and experimental considerations. First, a numerical simulation of the beam tube response is performed as a function of the microwave field amplitude for different values of the residual phase difference DeltaPhi. Results include the cavity-pulling effect. Compared with the measured frequency offset, the numerical simulation leads to a second-order Doppler shift of -3.3 mHz and a residual phase difference, DeltaPhi, between the fields interacting with the atoms in the second and first regions of the Ramsey cavity, amounting to +150 murad. Second, an experimental method of measurement of DeltaPhi without beam reversal is implemented. The latter yields DeltaPhi=155+/-17 murad. Finally, the clock accuracy is determined. It is equal to +/-14.10(-13).

Frequency performances of a miniature optically pumped cesium beam frequency standard.

The optically pumped cesium beam clock named Cs IV is operated with a new short Ramsey cavity satisfying strict requirements on the microwave leakage level. The most relevant characteristics of the device are presented. Cs IV is presently driven by standard electronics coming from a HP 5061 B clock that provides a sinusoidal modulation of the interrogation microwave signal and a microwave power stability of about 1% at a temperature of 20+/-1 degrees C. The short- and medium-term frequency stability measurement gives sigma(y)(1 day)=2x10(-14): this value holds up to 3 days. The accuracy evaluation results in an uncertainty of 10(-12), and the repeatability is evaluated to 3x10(-13). It appears that the flicker floor is beginning at 2x10(-14) and is mainly due to both the power fluctuations of the free running microwave interrogating signal and the fluctuations of the external static magnetic field. The accuracy is limited by the lack of knowledge of the end-to-end cavity phase shift.

Frequency shifts in cesium beam clocks induced by microwave leakages.

The anomalous sensitivity of an optically pumped cesium beam clock to microwave power and the unexpected frequency shifts observed are demonstrated to result from microwave leakages outside the arms of the Ramsey cavity. A new design of the cavity associated with a careful realisation provides very weak microwave leakages and negligible related frequency offsets. We have established a theoretical model that allows us to calculate the frequency shifts due to microwave field components propagating along the beam axis in regions which are free-field in an ideal Ramsey cavity. This results in first order Doppler effect shifts. The order of magnitude of the frequency shift can be predicted and agrees with the measured one when the amplitude of the leakage magnetic field is about a 1000 times smaller than the amplitude of the microwave interrogation field in the cavity.

New design for a high performance optically pumped cesium beam tube.

An optically pumped cesium beam resonator has been designed including three successive magnetic field regions. The optical interactions take place in the first and third regions, where the magnetic field has the required value of 3x10(-5) T. The microwave interaction occurs in the intermediate region, where the value of the C-field is typically set to 4x10(-6) T. It has been verified that the magnetic field profile along the cesium beam does not induce Majorana transitions. Using a single laser diode emitting at 852 nm with a linewidth of about 30 MHz, the resonator gives an excellent amplitude signal to noise ratio equal to 20000 in a 1-Hz bandwidth.

Stress-sensitivity mapping for surface acoustic waves on quartz.

A model is presented, relating the velocity shifts of surface acoustic waves (SAW) to the six tensor components of quasistatic stresses. Stress sensitivity is then defined through six independent coefficients, whatever the origin of the stress (direct external forces, thermoelastic stresses) might be. These coefficients, depending on crystal anisotropy, are computed for different cut angles and propagation directions of quartz crystal, and represented as a contour-line mapping. The determination of SAW quartz cuts compensated for both planar isotropic stresses and first-order temperature effects make it possible to define a family of quartz cuts with potentially low stress and temperature sensitivities for oscillator applications.

Limitation of work performance in normal adult males in the presence of beta-adrenergic blockade.

The effect on work performance of a single oral dose of the cardio-selective beta-adrenoreceptor blocking agent, metoprolol, was compared with an equipotent dose of the non-selective agent, propranolol, in the same subjects. A number of biochemical and physiological variables including heart rate, oxygen consumption, ventilation, lactate, free fatty acid and glucose levels were measured. Following exercise in the presence of both active drugs, subjects complained of excessive leg fatique. For the group there was a significant reduction in the total work performed and the maximum heart rate achieved on both drugs. There was a significant correlation between plasma levels of metoprolol, reduction in total work performed and reduction in maximum heart rate. By contrast, after propranolol, there was a wide variation in work performed at a time when the reduction in maximum heart rate was similar for all subjects. This suggests for propranolol that a reduction in heart rate alone is an inappropriate guide to the impairment of work performance. There was a fail in the circulating level of free fatty acids at the end of exercise in the presence of both drugs and it is possible that this biochemical variable contributed to the decrease in work performance.

Inhaled and oral salbutamol in exercise-induced asthma.

Twenty-one asthmatic patients participated in a single blind, crossover study comparing the effects of aerosol and oral salbutamol (a beta-adrenergic bronchodilator), on exercise-induced bronchospasm. By both routes of administration, salbutamol (albuterol) produced significant bronchodilatation at rest, but only the aerosol effectively prevented post-exercise bronchospasm in most patients. Because exercise-induced bronchospasm is a common problem in children and adults with asthma, these observations are relevant to the drug therapy of these patients. The existence of separate mechanisms for bronchodilatation and the prevention of exercise-induced asthma is postulated. It is possible that differing tissue concentrations of salbutamol are responsible for the observed differences between the 2 routes of administration.