The unit of time: present and future directions

Abstract

Some 50~years ago, physicists, and after them the entire world, started to found their time reference on atomic properties instead of motions of the Earth that have been in use since the origin. Far from being an arrival point, this decision marked the beginning of an adventure characterized by a 6 orders of magnitude improvement in the uncertainty of realization of atomic frequency and time references. Ever progressing atomic frequency standards and time references derived from them are key resources for science and for society. We will describe how the unit of time is realized with a fractional accuracy approaching $10^{-16}$ and how it is delivered to users via the elaboration of the international atomic time. We will describe the tremendous progress of optical frequency metrology over the last 20~years which led to a novel generation of optical frequency standards with fractional uncertainties of $10^{-18}$. We will describe work toward a possible redefinition of the SI second based on such standards. We will describe existing and emerging applications of atomic frequency standards in science.

16 pages, 199 references, part of special issue of Comptes Rendus Physique "The new International System of Units"

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• The Confrontation between General Relativity and Experiment
• Systematic evaluation of an atomic clock at 2e-18 total uncertainty
• Photonic microwave signals with zeptosecond level absolute timing noise
• Quantum State Engineering and Precision Metrology using State-Insensitive Light Traps
• Ultra-stable optical clock with two cold-atom ensembles
• 'Designer atoms' for quantum metrology
• Progress in Atomic Fountains at LNE-SYRTE
• Search for ultralight scalar dark matter with atomic spectroscopy
• Real-time geopotentiometry with synchronously linked optical lattice clocks
• A transportable optical lattice clock with $7\times 10^{-17}$ uncertainty
• Simultaneous remote transfer of accurate timing and optical frequency over a public fiber network
• Long-distance frequency transfer over an urban fiber link using optical phase stabilization
• High accuracy correction of blackbody radiation shift in an optical lattice clock
• Improved tests of Local Position Invariance using 87Rb and 133Cs fountains
• An atomic clock with $1\times 10^{-18}$ room-temperature blackbody Stark uncertainty
• Hyperpolarizability and operational magic wavelength in an optical lattice clock
• Evaluation of Doppler Shifts to Improve the Accuracy of Primary Atomic Fountain Clocks
• Evaluating and Minimizing Distributed Cavity Phase Errors in Atomic Clocks
• Non-Destructive Probing of Rabi Oscillations on the Cesium Clock Transition near the Standard Quantum Limit
• Geophysical applicability of atomic clocks: direct continental geoid mapping
• Determination of a high spatial resolution geopotential model using atomic clock comparisons
• Demonstration of a Dual Alkali Rb/Cs Atomic Fountain Clock
• Evaluation of Long Term Performance of Continuously Running Atomic Fountains
• Atomic Clock Ensemble in Space (ACES) data analysis
• Phase locking a clock oscillator to a coherent atomic ensemble
• All-polarization-maintaining, single-port Er:fiber comb for high-stability comparison of optical lattice clocks

Cited by in corpus (3)

• Long term measurement of the ${}^{87}$Sr clock frequency at the limit of primary Cs clocks
• Optical Atomic Clock Comparison through Turbulent Air
• Doppler Compensated Cavity For Atom Interferometry