A list of standard reference frequency values (LoF) of quantum transitions from the microwave to the optical regime has been recommended by the International Committee for Weights and Measures (Comité international des poids et mesures, CIPM) for use in basic research, technology, and for the metrology of time, frequency and length. The CIPM LoF contains entries that are recommended as secondary representations of the second in the International System of Units, and entries that can be used to serve as realizations of the definition of the metre. The historical perspective that led to the CIPM LoF is outlined. Procedures have been developed for updating existing, and validating new, entries into the CIPM LoF. The CIPM LoF might serve as an entry for a future redefinition of the second by an optical transition.

Lasers from two national metrological institutes (NMIs) were compared in 2016 as part of the CCL-K11 on-going key comparison, initiated by the 13th meeting of the Comité Consultative des Longueurs (CCL) in 2007. The absolute frequency of R(127) 11-5 transitions of molecular iodine was measured for these lasers following the technical protocol for CCL-K11. The results of these measurements are compiled in the present report. The comparison reports, as communicated by each participant, are included as appendices. This document constitutes the eighth final report for the on-going key comparison CCL-K11. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

A project aiming at measuring the von Klitzing constant with an uncertainty of the order of one part in 10 8 is currently ongoing at the BIPM. This measurement will be performed by linking a calculable cross-capacitor to a quantum Hall resistor through a quadrature transfer. The main parts of the overall measuring setup as well as the current status of the project are presented.

A simple method is proposed for the evaluation of comparative measurements when closed measurement loops are present in the data. Using a graph theory representation of the data a basis for the residual subspace is readily identified that together with loop closure values provide the input to a matrix equation of the same rank as the number of independent loops in the graph. As an example, ultra-high-precision frequency ratio measurements including optical standards are considered and small corrections to the experimental data are derived, yielding a list of consistent frequency ratios for work on precision frequency standards and discussions relating to a possible future redefinition of the SI second.

Lasers from two national metrological institutes (NMIs) were compared in 2015 as part of the CCL-K11 ongoing key comparison, initiated by the 13th meeting of the Comité Consultative des Longuers (CCL) in 2007. The absolute frequency of R(127) 11-5 transitions of molecular iodine was measured for these lasers following the technical protocol for CCL-K11. The results of these measurements are compiled in the present report. The comparison reports, as communicated by each participant, are included as appendices. This document constitutes the seventh final report for the ongoing key comparison CCL-K11. Main text To reach the main text of this paper, click on Final Report . Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/ . The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

This paper reports on the new 3-axis interferometer for the velocity measurement for the BIPM watt balance. The frequency of a commercial Nd:YAG 532 nm laser has been stabilized to a hyperfine transition in iodine by saturated absorption techniques. The interferometer was first mounted, aligned and characterized in a separate test bench. The whole experimental set-up was then transferred to the watt balance laboratories. The new interferometer is at present being integrated and aligned inside the watt balance apparatus.

Nous presentons dans le present document une vue d’ensemble du projet de determination directe de la constante de von Klitzing R K, en cours de realisation au Bureau International de Poids et Mesures (BIPM). Cette determination est effectuee dans le contexte general de la revision du Systeme International d’unites qui sera effective a partir de 2018. L’objectif vise est de mesurer avec une incertitude relative reduite a 1.10-8 . Pour cela un nouveau condensateur calculable a ete realise et une chaine de mesure dediee implementee.

Lasers from four national metrological institutes (NMIs) were compared in 2012 as part of the CCL-K11 ongoing key comparison, initiated by the 13th meeting of the Comite Consultative des Longuers (CCL) in 2007. The absolute frequency of R(127) 11-5 transitions of molecular iodine was measured for these lasers following the technical protocol for CCL-K11. The results of these measurements are compiled in this report. The comparison reports, as communicated by each participant, are included as appendices.This document constitutes the fourth final report for the ongoing key comparison CCL-K11.

Lasers from three national metrological institutes (NMIs) were compared in 2013 as part of the CCL-K11 ongoing key comparison, initiated by the 13th meeting of the Comite Consultative des Longuers (CCL) in 2007. The absolute frequency of R(127) 11-5 transitions of molecular iodine was measured for these lasers following the technical protocol for CCL-K11. The results of these measurements are compiled in this report. The comparison reports, as communicated by each participant, are included as appendices.This document constitutes the fifth final report for the ongoing key comparison CCL-K11.

This paper reports the latest progress made on the BIPM watt balance project. The type A uncertainty of the Planck constant measurements was reduced from 1.4 parts in 106 to about 5 parts in 107. The new magnet was successively fabricated and assembled with an expected high uniformity of the flux density of several parts in 104. Several new components are being introduced into the apparatus which we expect to reduce the type B measurement uncertainty significantly.

We present the recent progress on the BIPM (Bureau International des Poids et Mesures) watt balance. The existing apparatus was transferred to the dedicated new laboratory with better thermal and vibrational conditions. The apparatus is fully operational in air. An improvement by a factor of three was achieved on the S/N ratio of both the voltage-to-velocity and force-to-current ratios. The fabrication of the parts of the new magnet is completed and its assembly is finished.

This paper presents progress on the BIPM watt balance project. After a first measurement of the Planck constant in air at the 10-5 level, an improvement of a factor of five of the type A uncertainty has been obtained by refining the velocity measurement, now based on a three-axis interferometer system. Currently, the experiment is being transferred inside a vacuum enclosure located in a dedicated laboratory with improved thermal and vibrational isolation which we expect will further reduce the measurement uncertainty.

It has been recommended that the relative standard uncertainty of the numerical value of the Planck constant required for the redefinition of the kilogram should not exceed 2 x 10(-8). To reach this goal using experiments based on a watt balance, the free-fall acceleration (g) traceable to the SI, at a given point and a given time, needs to be known with a sufficiently small uncertainty well below 2x10(-8). Reducing the uncertainty in g allows the other uncertainties related to the watt balance to be increased. Instead of a simultaneous operation of an absolute gravimeter with a watt balance, we propose an alternative approach and demonstrate that a standard uncertainty below 5 mu Gal(12) (relative uncertainty of 5 x 10(-9)) is reachable under the conditions at BIPM. Further decreasing the uncertainty could significantly increase commitments in terms of personnel and equipment and would not significantly improve the uncertainty targeted for the BIPM watt balance experiment. A 5 mu Gal uncertainty might also satisfy the needs of other watt balance experiments underway or planned.In our approach we combine the following information: (1) the Key Comparison Reference Values obtained from the CCM. G-K1, a key comparison carried out in the frame of the International Comparison of Absolute Gravimeters in 2009 (ICAG2009); (2) the accurate gravity network established using the qualified absolute and relative gravimeters; (3) temporal gravity variations based on observed Earth-tide parameters and modelled effects of polar motion and atmospheric mass redistribution; (4) uncertainty estimates that account for non-modelled effects; (5) the option to carry out absolute gravity measurements once every one or two years with two or more gravimeters for monitoring the stability of the gravity field at the BIPM.

We have validated the laser-induced fluorescence technique to measure the quality of iodine cells whose transitions are used as a reference in laser standards for the realization of the definition of the metre. In this technique, the change in induced fluorescence is measured for different values of iodine pressure in the cell: for an iodine cell contaminated with foreign gases the fluorescence is more sensitive to iodine pressure changes than for a pure one due to collisional quenching of the excited states. We have measured the fluorescence parameter K-o for a total of 96 iodine cells, and for each cell we correlated K-o with the measured absolute frequency when the cell is inserted into laser standards at wavelengths 515 nm, 532 nm and 633 nm. We observed that the distribution of the quality of the produced iodine cells has a large dispersion and we believe that this also significantly affects the frequency distribution of the iodine-based laser standards that are used to realize the definition of the metre.

The 8th International Comparison of Absolute Gravimeters (ICAG2009) took place at the headquarters of the International Bureau of Weights and Measures (BIPM) from September to October 2009. It was the first ICAG organized as a key comparison in the framework of the CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA) (CIPM 1999). ICAG2009 was composed of a Key Comparison (KC) as defined by the CIPM MRA, organized by the Consultative Committee for Mass and Related Quantities (CCM) and designated as CCM.G-K1. Participating gravimeters and their operators came from national metrology institutes (NMIs) or their designated institutes (DIs) as defined by the CIPM MRA. A Pilot Study (PS) was run in parallel in order to include gravimeters and their operators from other institutes which, while not signatories of the CIPM MRA, nevertheless play important roles in international gravimetry measurements. The aim of the CIPM MRA is to have international acceptance of the measurement capabilities of the participating institutes in various fields of metrology. The results of CCM.G-K1 thus constitute an accurate and consistent gravity reference traceable to the SI (International System of Units), which can be used as the global basis for geodetic, geophysical and metrological observations of gravity. The measurements performed afterwards by the KC participants can be referred to the international metrological reference, i.e. they are SI-traceable.The ICAG2009 was complemented by a number of associated measurements: the Relative Gravity Campaign (RGC2009), high-precision levelling and an accurate gravity survey in support of the BIPM watt balance project. The major measurements took place at the BIPM between July and October 2009. Altogether 24 institutes with 22 absolute gravimeters(32) and nine relative gravimeters participated in the ICAG/RGC campaign.This paper is focused on the absolute gravity campaign. We review the history of the ICAGs and present the organization, data processing and the final results of the ICAG2009.After almost thirty years of hosting eight successive ICAGs, the CIPM decided to transfer the responsibility for piloting the future ICAGs to NMIs, although maintaining a supervisory role through its Consultative Committee for Mass and Related Quantities.

The International Comparison of Absolute Gravimeters 2009 (ICAG2009) was carried out in September 2009 at the International Bureau of Weights and Measures (BIPM). The ICAG2009 and its data processing were performed in accordance with the agreed Technical Protocol which was approved by the Steering Committee. The pilot laboratory (BIPM) evaluated the final results of the key comparison from the measurements made at the BIPM in the framework of ICAG2009. A pilot study run in parallel allowed evaluation of the offsets for absolute gravimeters from institutes not fulfilling the requirements for participating in the key comparison. The gravity values at the five stations, and the offsets for each absolute gravimeter, were obtained by a weighted least squares fit. The results indicate that all participating gravimeters are consistent within their expanded uncertainties.

This paper describes the progress on the implementation of a calculable cross capacitor for the measurement of RK at the BIPM. The main electrodes are installed and the alignment process has been demonstrated. The optical interferometer required to measure the guard displacement is operational. The chain of bridges for the transfer from RK to 1 pF exists, and first measurements can be expected during 2012.