The SI unit of temperature will soon be redefined in terms of a fixed value of the Boltzmann constant k derived from an ensemble of measurements worldwide. We report on a new determination of k using acoustic thermometry of helium-4 gas in a 3 l volume quasi-spherical resonator. The method is based on the accurate determination of acoustic and microwave resonances to measure the speed of sound at different pressures. We find for the universal gas constant R = 8.314 4614(50) J.mol(-1).K-1. Using the current best available value of the Avogadro constant, we obtain k = 1.380 648 78(83) x 10(-23) J.K-1 with u(k)/k = 0.60 x 10(-6), where the uncertainty u is one standard uncertainty corresponding to a 68% confidence level. This value is consistent with our previous determinations and with that of the 2014 CODATA adjustment of the fundamental constants (Mohr et al 2016 Rev. Mod. Phys. 88 035009), within the standard uncertainties. We combined the present values of k and u(k) with earlier values that were measured at LNE. Assuming the maximum possible correlations between the measurements, (k(present)/< k> -1) = 0.07 x 10(-6) and the combined u(r)(k) is reduced to 0.56 x 10(-6). Assuming minimum correlations, (k(present)/< k> -1) = 0.10 x 10(-6) and the combined u(r)(k) is reduced to 0.48 x 10(-6).

The Boltzmann constant k has been determined from a measurement of the speed of sound in helium gas in a quasi-spherical resonator (volume 0.5 l) maintained at a temperature close to the triple point of water (273.16 K). The acoustic velocity c is deduced from measured acoustic resonance frequencies and the dimensions of the quasi-sphere, the latter being obtained via simultaneous microwave resonance. Values of c are extrapolated to the zero pressure limit of ideal gas behaviour. We find k = 1.380 6487(14) x 10(-23) J.K-1, a result consistent with previous measurements in our group and elsewhere. The value for k, which has a relative standard uncertainty of 1.02 ppm, lies 0.02 ppm below that of the CODATA 2010 adjustment.

In the French watt balance experiment, in order to have a single point of application of a weight and an electromagnetic force balancing this weight, a system of double gimbals with. flexure pivots was made out of copper-beryllium alloy. This allows the articulation of the suspension devices of the mass (generating the weight) and coil (on which the electromagnetic force acts) around four coplanar axes which intersect at one point (equivalent to a virtual centre of rotation). The study of the free dynamic behaviour under vacuum of this device dealt with resonance, damping due to internal friction of the material and coupling between the two systems of gimbals. The results obtained show that this novel device presents suitable characteristics for concentrating two different forces at a single point and aligning them around a common vertical axis. It can be used not only for watt balance experiments but also for any other mechanical applications where alignment and concentration of two forces are necessary.

The correlated photons source is obtained from the parametric down conversion of photons generated in a non linear crystal. When associated with a system of coincidences counting, this source allows one to measure the detection efficiencies of detectors working at photon counting levels, without the need for reference sources or detectors. At the Institut National de Metrologie ( France), this method was implemented with the aim of realising a new standard detector for the absolute measurement of very weak radiations. The relative standard uncertainty of the detection efficiency measurements is 1.1%.

The parametric down conversion of photons generated in a non-linear crystal gives rise to two correlated photons. When associated with a system of coincidence counting, this phenomenon allows one to measure the quantum efficiencies of detectors working at a normal rate of photon counting, without the need for reference sources or detectors. At the Institut National de Metrologie (INM) (France), this method was implemented with the aim of developing a new standard detector for the absolute measurement of very weak radiation. The validation of this method is presented from an international comparison between the laboratories of the INM (France) and the Istituto Elettrotecnico Nazionale Galileo Ferraris (IENGF) (Italy) and from a comparison of methods at INM using the French national standard detector, the cryogenic radiometer.

We have measured the frequency, isotope shift and hyperfine structure intervals of the 4d105s2Sfrac12 rarr 4d9 5s2 2D3/2 two-photon transition in 107Ag and 109Ag by laser spectroscopy of a thermal atomic beam. Frequencies were linked to the hyperfine component a3 of the line R111(18-1) in 127 I2 near 520 THz

We demonstrate an original solution to obtain a single-frequency ring laser coupled to an external passive nonresonant ring cavity, which plays the role of an optical diode. This system provides more output power than systems with an intracavity unidirectional device. To the best of our knowledge, this work marks the first demonstration of a unidirectional planar ring laser at 1.3 microm. Using 12 W at 797 nm to pump a Nd:YLF laser, combined with intracavity second-harmonic generation, we achieve yields of 440 mW at 661.3 nm and 340 mW at 656.0 nm.

. The frequencies of the hyperfine components of the transition 4d^105s ^2S_1/2→ 4d^95s^2 2 D 3/2 in 107 Ag and 109 Ag have been determined using Doppler-free two-photon laser spectroscopy of a thermal atomic beam and heterodyne calibration with respect to a molecular iodine line 111R(18-1) near 520 THz. For the centre of gravity of a mixture of natural abundance, we deduce the value 1 040 706 327(3) MHz. For the isotope shift, we obtain ν(^109 Ag)-ν(^107 Ag) = + 599.6(2) MHz. We find the magnetic hyperfine splitting constants of the excited state to be A ( 107 Ag ( 2 D 3/2 )) = -315.9(2) MHz and A ( 109 Ag ( 2 D 3/2 )) = -363.3(2) MHz, an order of magnitude improvement over previous authors [W. Fischer et al. , Z. Phys. 238 , 249 (1970)].

The frequencies of the hyperfine components of the transition \(4d^{10}5s ^{2}S_{1/2} \rightarrow 4d^{9}5s^{2}\) 2 D 3/2 in 107Ag and 109Ag have been determined using Doppler-free two-photon laser spectroscopy of a thermal atomic beam and heterodyne calibration with respect to a molecular iodine line 111R(18-1) near 520 THz. For the centre of gravity of a mixture of natural abundance, we deduce the value 1 040 706 327(3) MHz. For the isotope shift, we obtain \(\nu(^{109}{\rm Ag})-\nu(^{107}{\rm Ag}) = + 599.6(2)\) MHz. We find the magnetic hyperfine splitting constants of the excited state to be A(107Ag (2D 3/2)) = -315.9(2) MHz and A(109Ag (2D 3/2)) = -363.3(2) MHz, an order of magnitude improvement over previous authors [W. Fischer et al. , Z. Phys. 238, 249 (1970)].

We describe single-frequency operation of a diode-pumped Nd:YLF laser in the range 1311.9-1317.2 nm. It can be used for the interrogation of the clock transition in calcium (1314.0 nm) or spectroscopy in hydrogen and metastable singly ionized helium (1312.6 nm). By using a twisted-mode cavity, we have obtained output powers of 830 and 970 mW at 1312.6 and 1314.0 nm, respectively, in a single longitudinal mode.

We report advances towards the development of frequency standards based on narrow two-photon transitions in the silver atom at 576 and 661 nm from the ground 4d105s2S1/2 state to the 2D3/2,5/2 levels of the configuration 4d95s2. We describe detection schemes for metastable atoms, localisation of the position of the metastable 2D5/2 level and diode-pumped Nd:YLF lasers around 1.3 μm destined to replace dye lasers for both cooling and spectroscopy of the clock transition. Our result for the position of the 2D5/2 level is more than a factor of two improvement over previous vales.

We describe the efficient cw operation of two Nd:YLF lasers at 1312.0 and 1322.6 nm for the development of a silver atom optical clock. For a simple linear cavity laser configuration investigated at these wavelengths, we have obtained an output power of 3.6 W at 1312.0 nm for 13.8 W of absorbed pump power (λ=806 nm) and 4.8 W output at 1322.6 nm with 16.3 W pump. At 1312.0 nm, using a twisted-mode cavity, a single-frequency output power of 750 mW has been obtained. Single-frequency operation (450 mW) at 1322.6 nm was achieved using an intra-cavity solid etalon.

We describe single-frequency operation of diode-pumped Nd :YLF lasers providing 1.5 W or more output around 1.3 µm. Their harmonics are destined for laser cooling of atomic silver, and interrogation of clock transitions in silver and calcium. Using a ring laser, we have obtained ≅ 400 mW at 661.3 nm with intra-cavity frequency doubling in type-I BBO. Other applications include Doppler-free two-photon spectroscopy of hydrogen and singly-ionized helium for tests of QED and the determination of the Rydberg constant.

Summary form only given. We are developing an optical frequency standard based on the narrow 4d/sup 10/ 5s /sup 2/S/sub 1/2//spl rarr/4d/sup 9/ 5s/sup 2/ /sup 2/D/sub 5/2/ transition in atomic silver. We have realized a two-photon spectrometer and are now engaged in a measurement of the frequency of the broader but easier-to-detect two-photon transition to the 4d/sup 9/ 5s/sup 2/ /sup 2/D/sub 3/2/ level.

In metrology, a mass is measured by comparison with a standard. When calibrating a one kilogram in stainless steel with respect to a standard in platinum-iridium, the correction to the mass difference (about 95 mg) arising from air buoyancy is considerable in regard to the accuracy sought (some /spl mu/g). The main uncertainty on this term is associated with the measurement of air humidity in the chamber of the balance. Experimental observations show that a dew-point hygrometer must be used carefully when the measurement involves a relatively small volume of air. This paper, focus on monitoring water vapour in an enclosure chamber at atmospheric pressure. It give proof of the sensitivity and make advantage of the optical sensor based on molecular absorption, for real-time control of the variations of the air moisture content in mass metrology.

The frequency of the center of gravity of the transition 4d(9)5s(2) D-2(5/2) --> 4d(10)6p P-2(3/2) in Ag I has been determined using laser heterodyne spectroscopy of a collimated thermal metastable atomic beam. We find the value nu = 547 376 425(12) MHz, five times more accurate than our earlier published result. For the isotope shift, we obtain nu(Ag-109) -nu(Ag-107)=-978.1(0.5) MHz, a ten-fold improvement over our previous measurement.

The frequency of the center of gravity of the transition 4d 9 5s 2   2 D 5/2 → 4d 10 6p  2 P 3/2 in Ag I has been determined using laser heterodyne spectroscopy of a collimated thermal metastable atomic beam. We find the value ν = 547376425(12) MHz, five times more accurate than our earlier published result. For the isotope shift, we obtain ν(109Ag)-ν(107Ag)=-978.1(0.5) MHz, a ten-fold improvement over our previous measurement.

The frequency of the centroid of the transition 4d(9)5s(2) D-2(5/2) --> 4d(10)6p P-2(3/2) in Ag I has been determined by laser spectroscopy of a collimated metastable thermal atomic beam. We find nu = 547 376 388(60) MHz. The isotope shift nu(Ag-109) - nu(Ag-107) = -982.6(5.4) MHz. For the magnetic hyperfine structure constant of the 4d(10)6p, P-2(3/2) state, assuming IJ coupling, we find, A(Ag-107) = -8.4(6) MHz and A(Ag-109) = -10.2(6) MHz.

The frequency of the centroid of the transition \(\) in Ag I has been determined by laser spectroscopy of a collimated metastable thermal atomic beam. We find \(\) MHz. The isotope shift \(\) MHz. For the magnetic hyperfine structure constant of the \(\) state, assuming IJ coupling, we find, \(\) MHz and \(\) MHz.