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.

A femtosecond laser-based optical frequency synthesizer is referenced to an optical standard, and we use it to demonstrate the generation and control of the frequency of electromagnetic fields over 100 terahertz of bandwidth with fractional uncertainties approaching 1 part in 10(19). The reproducibility of this performance is verified by comparison of different types of femtosecond laser-based frequency synthesizers from three laboratories.

Optical and radio frequency standards located in JILA and National Institute of Standards and Technology (NIST) laboratories have been connected through a 3.45-km optical fiber link. An optical frequency standard based on an iodine-stabilized Nd:YAG laser at 1064 nm (with an instability of ϳ4 ϫ 10 Ϫ14 at 1 s) has been transferred from JILA to NIST and simultaneously measured in both laboratories. In parallel, a hydrogen maser-based radio frequency standard (with an instability of ϳ2.4 ϫ 10 Ϫ13 at 1 s) is transferred from NIST to JILA. Comparison between these frequency standards is made possible by the use of femtosecond frequency combs in both laboratories. The degradation of the optical and rf standards that are due to the instability in the transmission channel has been measured. Active noise cancellation is demonstrated to improve the transfer stability of the fiber link.

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 (one of the 22 AGs was ultimately withdrawn) 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 nuclear and electronic magnetic dipole moments of 21Fr and the nuclear ground-state spin of 207Fr have been measured directly by on-line atomic-beam magnetic resonance techniques at the ISOLDE facility at CERN. The following results have been obtained: μI(211Fr) = 4.00(8) nm, gJ(Fr, 7s 2S1/2) = 2.00497(9) and I(207Fr) = 9/2. A large deviation of the electronic magnetic moment from the free electron value was found. Using the measured nuclear magnetic moment μI(211Fr), a comparison is made between different theoretical calculations of the magnetic dipole hyperfine constants and with the available experimental data in 211Fr. An analysis of the electric quadrupole interaction is also given. Nuclear magnetic dipole and electric quadrupole moments are deduced for the sequences 207-213Fr and 220-228Fr, using the data on 211Fr as reference values. The nuclear quantities are discussed within the framework of the shell model and the core-quasiparticle model, giving information on the nuclear single-particle structure and on the variation in deformation along the sequences of francium isotopes.

We present measurements and analysis of the currently known relative frequency uncertainty of femtosecond laser frequency combs (FLFCs) based on Kerr-lens mode-locked Ti:sapphire lasers. Broadband frequency combs generated directly from the laser oscillator, as well as octave-spanning combs generated with nonlinear optical fiber are compared. The relative frequency uncertainty introduced by an optically referenced FLFC is measured for both its optical and microwave outputs. We find that the relative frequency uncertainty of the optical and microwave outputs of the FLFC can be as low as 8 10 20 and 1.7 10 18 , with a confidence level of 95%, respectively. Photo-detection of the optical pulse train introduces a small amount of excess noise, which degrades the stability and subsequent relative frequency uncertainty limit of the microwave output to 2.6 10 17 .

A formalism, based on standard least squares techniques, is presented in which analytic expressions can be derived for the study of the influence of hypothetic perturbations in absolute gravimetry. The model function derived from the equation of motion of the test mass and possible perturbations is expanded in an orthogonal basis composed of the shifted Legendre polynomials, and projections onto the model space and the space orthogonal to this serve to isolate the contributions to the model parameters and to the residuals. The part of the perturbation remaining in the orthogonal space can be compared with those retrieved from the residuals of the fit of the model function to the experimental data. The analysis of the structure of these residuals is a major means of gaining information about the character of the perturbations. Finally, some explicit examples are given and a discussion concerning how such an analysis can be made is included.

The Bureau International des Poids et Mesures (BIPM), Sèvres, France, hosted the 7th International Comparison of Absolute Gravimeters (ICAG) and the associated Relative Gravity Campaign (RGC) from August to September 2005.ICAG 2005 was prepared and performed as a metrological pilot study, which aimed:(1) To determine the gravity comparison reference values;(2) To determine the offsets of the absolute gravimeters; and (3) As a pilot study to accumulate experience for the CIPM Key Comparisons.This document presents a complete and extensive review of the technical protocol and data processing procedures. The 1st ICAG-RGC comparison was held at the BIPM in 1980-1981 and since then meetings have been organized every 4 years.In this paper, we present an overview of how the meeting was organized, the conditions of BIPM gravimetric sites, technical specifications, data processing strategy and an analysis of the final results. This 7th ICAG final report supersedes all previously published reports.Readings were obtained from participating instruments, 19 absolute gravimeters and 15 relative gravimeters. Precise levelling measurements were carried out and all measurements were performed on the BIPM micro-gravity network which was specifically designed for the comparison.