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.

arXiv comment16 pages, 199 references, part of special issue of Comptes Rendus Physique "The new International System of Units"

References in corpus (106)

• Quantum sensing
• The Confrontation between General Relativity and Experiment
• Search for New Physics with Atoms and Molecules
• The String Dilaton and a Least Coupling Principle
• An Optical Lattice Clock with Accuracy and Stability at the $10^{-18}$ Level
• Modern tests of Lorentz invariance
• Frequency Comparison of Two High-Accuracy Al+ Optical Clocks
• An atomic clock with $10^{-18}$ instability
• Systematic evaluation of an atomic clock at 2e-18 total uncertainty
• A sub-40 mHz linewidth laser based on a silicon single-crystal optical cavity
• Varying constants, Gravitation and Cosmology
• Atomic clock performance beyond the geodetic limit
• Single-Ion Atomic Clock with $3\times 10^{-18}$ Systematic Uncertainty
• 1.5 $\mu$m lasers with sub 10 mHz linewidth
• Implementation of Cavity Squeezing of a Collective Atomic Spin
• Geodesy and metrology with a transportable optical clock
• A clock network for geodesy and fundamental science
• 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
• A Fermi-degenerate three-dimensional optical lattice clock
• The MICROSCOPE mission: first results of a space test of the Equivalence Principle
• Prospects for a mHz-linewidth laser
• 'Designer atoms' for quantum metrology
• Optical two-way time and frequency transfer over free space
• A Search for Variations of Fundamental Constants using Atomic Fountain Clocks
• Progress in Atomic Fountains at LNE-SYRTE
• 8E-17 fractional laser frequency instability with a long room-temperature cavity
• Atomic Clocks for Geodesy
• Search for ultralight scalar dark matter with atomic spectroscopy
• Experimenting an optical second with strontium lattice clocks
• Real-time geopotentiometry with synchronously linked optical lattice clocks
• A transportable optical lattice clock with $7\times 10^{-17}$ uncertainty
• Searching for an oscillating massive scalar field as a dark matter candidate using atomic hyperfine frequency comparisons
• Test of special relativity using a fiber network of optical clocks
• Frequency ratio of Yb and Sr clocks with $5\times 10^{-17}$ uncertainty at 150 s averaging time
• A gravitational redshift test using eccentric Galileo satellites
• Comparison of Two Independent Sr Optical Clocks with 1e-17 Stability at 10^3 s
• Imaging optical frequencies with 100 $\mu$Hz precision and 1.1 $\mu$m resolution
• Orientation-Dependent Entanglement Lifetime in a Squeezed Atomic Clock
• Ultra-low noise microwave generation with fiber-based optical frequency comb and application to atomic fountain clock
• Entanglement-assisted atomic clock beyond the projection noise limit
• Synchronization of Distant Optical Clocks at the Femtosecond Level
• 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
• Theoretical Aspects of the Equivalence Principle
• Optical frequency transfer via 146 km fiber link with 10^{-19} relative accuracy
• Ultra-stable long distance optical frequency distribution using the Internet fiber network
• High accuracy correction of blackbody radiation shift in an optical lattice clock
• A high-performance optical lattice clock based on bosonic atoms
• Improved accuracy of the NPL-CsF2 primary frequency standard: evaluation of distributed cavity phase and microwave lensing frequency shifts
• Hyper-Ramsey Spectroscopy of Optical Clock Transitions
• Lattice Induced Frequency Shifts in Sr Optical Lattice Clocks at the $10^{-17}$ Level
• Realization of a time-scale with an accurate optical lattice clock
• 0.75 atoms improve the clock signal of 10,000 atoms
• An atomic clock with $1\times 10^{-18}$ room-temperature blackbody Stark uncertainty
• Improved tests of Local Position Invariance using 87Rb and 133Cs fountains
• Trapped-Ion Quantum Simulator: Experimental Application to Nonlinear Interferometers
• High accuracy measure of atomic polarizability in an optical lattice clock
• Spectral purity transfer between optical wavelengths at the $10^{-18}$ level
• Frequency-stabilization to 6x10^-16 via spectral-hole burning
• Controlling the cold collision shift in high precision atomic interferometry
• Non-destructive measurement of the transition probability in a Sr optical lattice clock
• Frequency measurements of superradiance from the strontium clock transition
• Hyperpolarizability and operational magic wavelength in an optical lattice clock
• A coherent optical link through the turbulent atmosphere
• Optical to microwave clock frequency ratios with a nearly continuous strontium optical lattice clock
• Development of a strontium optical lattice clock for the SOC mission on the ISS
• Interrogation of caesium atoms in a fountain clock by a femtosecond laser microwave oscillator
• Noise and instability of an optical lattice clock
• First international comparison of fountain primary frequency standards via a long distance optical fiber link
• High Performance Clocks and Gravity Field Determination
• Comparing a mercury optical lattice clock with microwave and optical frequency standards
• Contributing to TAI with a Secondary Representation of the SI Second
• Scattering of Cold Atom Coherences by Hot Atoms: Frequency Shifts from Background Gas Collisions
• Observation and cancellation of the dc Stark shift in strontium optical lattice clocks
• Evaluating and Minimizing Distributed Cavity Phase Errors in Atomic Clocks
• Evaluation of Doppler Shifts to Improve the Accuracy of Primary Atomic Fountain Clocks
• Controlling systematic frequency uncertainties at the $10^{-19}$ level in linear Coulomb crystals
• 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
• Quantum coherence between two atoms beyond Q=10^15
• Determination of a high spatial resolution geopotential model using atomic clock comparisons
• Demonstration of a Dual Alkali Rb/Cs Atomic Fountain Clock
• Atomic clocks as a tool to monitor vertical surface motion
• Analysis of thermal radiation in ion traps for optical frequency standards
• First industrial-grade coherent fiber link for optical frequency standard dissemination
• Direct comparison of optical lattice clocks with an intercontinental baseline of 9 000 km
• A noise-immune cavity-assisted non-destructive detection for an optical lattice clock in the quantum regime
• Evaluation of Long Term Performance of Continuously Running Atomic Fountains
• Atomic Clock Ensemble in Space (ACES) data analysis
• SI-traceable measurement of an optical frequency at low $10^{-16}$ level without a local primary standard
• Testing General Relativity with Atomic Clocks
• A Large Area Fiber Optic Gyroscope on multiplexed fiber network
• Phase locking a clock oscillator to a coherent atomic ensemble
• Relativistic Corrections for Time and Frequency Transfer in Optical Fibres
• Least-squares analysis of clock frequency comparison data to deduce optimized frequency and frequency ratio values
• Optical Stabilization of a Microwave Oscillator for Fountain Clock Interrogation
• All-polarization-maintaining, single-port Er:fiber comb for high-stability comparison of optical lattice clocks
• Locking Local Oscillator Phase to the Atomic Phase via Weak Measurement
• Spectrally narrow, long-term stable optical frequency reference based on a Eu${}^{3+}$:Y${}_2$SiO${}_5$ crystal at cryogenic temperature
• Feasibility of giant fiber-optic gyroscopes
• Preparation of spin squeezed atomic states by optical phase shift measurement
• Chronometric geodesy: methods and applications
• Feshbach resonances in Cesium at Ultra-low Static Magnetic Fields
• Towards Adoption of an Optical Second: Verifying Optical Clocks at the SI Limit

Cited by in corpus (11)

• 113 km Free-Space Time-Frequency Dissemination at the 19th Decimal Instability
• Quantum-limited optical time transfer for future geosynchronous links
• Long term measurement of the ${}^{87}$Sr clock frequency at the limit of primary Cs clocks
• Optical frequency ratio of a ${}^{171}{\mathrm{Yb}}^{+}$ single-ion clock and a ${}^{87}\mathrm{Sr}$ lattice clock
• Direct comparisons of European primary and secondary frequency standards via satellite techniques
• Optical Atomic Clock Comparison through Turbulent Air
• Fundamental accuracy-resolution trade-off for timekeeping devices
• Deterministic generation of hybrid high-N00N states with Rydberg ions trapped in microwave cavities
• Accurate laser frequency locking to optical frequency combs under low-signal-to-noise-ratio conditions
• Accurate Determination of Blackbody Radiation Shifts in a Strontium Molecular Lattice Clock
• Doppler Compensated Cavity For Atom Interferometry