Monday, January 25
6:00 a.m. - 7:15 a.m. PST	Fundamentals of Time and its Measurement	Dr. Elisa Felicitas Arias
7:30 a.m. - 8:45 a.m. PST	Time Transfer Protocols	Francisco Girela
9:00 a.m. - 10:15 a.m. PST	Microwave Oscillators	Dr. Patrick Berthoud
Tuesday, January 26
6:00 a.m. - 7:15 a.m. PST	Optical Oscillators	Dr. David R. Leibrandt
7:30 a.m. - 8:45 a.m. PST	Optical Transmission	Dr. Per Olof Hedekvist

Fundamentals of Time and its Measurement

Time: Monday, January 25, 6:00 a.m. - 7:15 a.m.

The action of measuring relies on a clock at its very endpoint but involves a complex process where the fundamental element is the time scale. Requisites on a time scale depend on its applications; this means not only its metrological quality represented by the minimum required levels of stability and accuracy, but also other factors as traceability to national/international standards, security, resilience, robustness, accessibility. The elements to consider are the clocks and the algorithms for their connection; the link to a reference; the infrastructure supporting its maintenance and dissemination; the eventual legal implications. This tutorial will present the fundamental elements for time measurement and discuss the various time scales designed for different applications.

Dr. Felicitas Arias directed the BIPM Time Department during 18 years. She retired end of 2017, but continues contributing to time metrology and space reference frames programs at Paris Observatory.

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Time Transfer Protocols

Time: Monday, January 25, 7:30 a.m. - 8:45 a.m.

Time has provided technology a common reference to define when something has happened, schedule tasks or synchronize actions. However, a time reference is not useful if it cannot be accessed periodically by those devices that require time-awareness. Time transfer is key to propagating a global reference and needs to fulfill the specific requirements for each application in terms of reliability, accuracy, simplicity and cost among others. Those requirements have led to a heterogeneous ecosystem of time transfer technologies.

In this talk, the state-of-art time transfer mechanisms that are broadly used in the industry are introduced, including examples of their adoption in the industry and references to the latest public results that have been released. This presentation focuses on time transfer protocols to distribute time information from the reference to the time consumers, how they make it and their adoption and limitations.

Francisco Girela is the Americas Tech Responsible at Seven Solutions. He holds a Master's degree in Telecommunications Engineering and is working on his PhD. He has specialized in ultra-accurate time transfer systems and the development and integration of the White Rabbit technology in real life industrial applications.

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Microwave Oscillators

Time: Monday, January 25, 9:00 a.m. - 10:15 a.m.

Microwave oscillators have been the first atomic clocks developed in the early 1950’s which have paved the way to the present definition of the SI second in 1967 using the microwave transition of the 133 Cs atom. Beside cesium beam primary clocks, numerous secondary standards are using microwave transitions such as rubidium standards and hydrogen masers. From the early developments, numerous improvements have been made for the atomic preparation and detection (optical pumping, laser cooling, coherent population trapping) while still probing their reference microwave transition.

In this tutorial we will first address the theoretical description of microwave interactions (Bloch equations). Then we will describe the differences and advantages of each clock types (Rb cell, H- maser, Cs thermal beam clock, atomic fountains, chip scale clocks …). Finally we will conclude on some challenges we must address at our industrial level to turn such clock technology from a laboratory prototype to an industrial turn-key product.

Dr. Patrick Berthoud received his Ph.D. in physics from the University of Neuchâtel, Switzerland in 2000 on laser cooling of atoms. After a postdoctoral fellowship at JILA in Boulder, CO, USA, in 2001, he has developed several microwave clock prototypes for ground and space applications at the Observatory of Neuchâtel, Switzerland. In 2008 he joined Oscilloquartz SA, Switzerland as Chief Scientist focussing on the development and industrialization of Cesium thermal beam clocks.

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Optical Oscillators

Time: Tuesday, January 26, 6:00 a.m. - 7:15 a.m.

Taking advantage of advances in low noise laser local oscillators and femtosecond frequency combs, optical frequency standards have surpassed the performance of the cesium microwave frequency standards upon which the SI second is based, and they are on the cusp of transitioning from scientific experiments to mainstream tools for metrology and fundamental physics. These standards are based on optical transitions in trapped and laser cooled atoms or ions and they derive their performance advantages from high transition frequencies, narrow transition linewidths, and small fractional systematic frequency shifts. State-of-the-art optical frequency standards are now reaching fractional frequency instabilities below 10 -16 at 1 s and systematic uncertainties near 10 -18.

This tutorial will cover cavity-stabilized laser local oscillators and optical frequency standards, including both optical lattice and trapped-ion clocks. After an introduction to the basic operating principles and building blocks, performance limitations and techniques to achieve high stability and accuracy will be presented. Finally, the tutorial will conclude with a discussion of the planned redefinition of the SI second based on optical frequency standards, and applications ranging from relativistic geodesy to tests of fundamental physics.

Dr. David Leibrandt received his Ph.D. in Physics from the Massachusetts Institute of Technology. Since 2009, he has been with the Time and Frequency Division of the National Institute of Standards and Technology in Boulder, Colorado, where he currently leads the trapped-ion optical atomic clock and precision measurement experiments within the Ion Storage Group. Highlights of his research include the development of optical atomic clocks based on quantum-logic spectroscopy of aluminum ions with record systematic uncertainty below 10 -18 as well as laser frequency stabilization based on compact, portable Fabry-Perot cavities and spectral-hole burning.

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Optical Transmission

Time: Tuesday, January 26, 7:30 a.m. - 8:45 a.m.

High precision timing is not only based on new clocks, but also on the techniques to transfer the time signal to the user. While wireless radio transmission has been used extensively, both terrestrial and via satellite, the utilization of optical properties though fiber has several advantages. The performance of optical time transfer can be tuned to accurately detect the phase of an optical signal which makes it the ultimate choice for comparison of optical clocks, but also at lower requirements of precision, the advantage of optical fiber lies in the robustness and security. While the necessary bandwidth is low, the requirements for stability and symmetry are huge, and not easily handled. The difference in transfer of frequency in comparison to time is also fundamental, since it relates either to the momentary change in phase or delay, or the accumulated changes. This tutorial will demonstrate the background of fiber optics, from the physics of light confinement in glass fibers to the limitations that must be overcome, including attenuation, dispersion, polarization, and environmental variations. It will conclude with the presentation of the latest state-of-the-art time and frequency transfer results, which have been experimentally demonstrated around the globe.

Dr. Per Olof Hedekvist received his PhD 1998 from Chalmers University of Technology in Sweden. After a post-doc at Caltech in California and a few years as Associate Professor at Chalmers, he joined RISE Research Institutes of Sweden in 2005, working mainly on fiber-based techniques for time transfer. Presently he is also Sr. Scientist working with the NMI management in Sweden.

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