9:00 a.m. - 10:15 a.m.	Introduction to Optical Clocks	Dr. Pierre Dubé, National Research Council (NRC), Canada
10:15 a.m. - 10:30 a.m., Break
10:30 a.m. - 11:45 a.m.	Optical frequency combs: from principles of operation to metrological applications	Dr. Helen Margolis, National Physical Laboratory (NPL), UK
11:45 a.m. - 1:30 p.m., Lunch is on your own
1:30 p.m. - 2:45 p.m.	Realization of a UTC(k) time scale	Dr. Daniele Rovera, LNE-SYRTE Observatoire de Paris - CNRS, France
2:45 p.m. - 3:00 p.m., Break
3:00 p.m. - 4:15 p.m.	Global Navigation Satellite System (GNSS) Overview with a Focus on Precise Time Disseminations and Standards	Ed Powers, The Aerospace Corporation, USA
4:15 p.m. - 4:30 p.m., Break
4:30 p.m. - 5:45 p.m.	Optical fiber and its use for time and frequency transfer	Dr. Sven-Christian Ebenhag, RISE Research Institute, Sweden

Introduction to Optical Clocks

Time: Tuesday, January 21, 9:00 a.m. - 10:15 a.m.

In the first part of this tutorial the fundamental principles that led to the development of modern optical clocks will be presented. Optical clocks based on trapped neutral atoms in optical lattices or single ions in rf traps currently realize the most accurate frequency standards. Their performances are such that a re-definition of the SI second using an optical clock is expected to happen within the next decade. They outperform the accuracy of cesium fountain clocks that currently define the SI second by two orders of magnitude. An overview of these optical clocks and a comparison of their respective performances will be given. The primary focus of the second part will be on single-ion optical clocks. The optical clock developed at NRC, based on a trapped and laser-cooled single ion of 88Sr+, will be presented in more detail to provide a concrete example of the steps involved in achieving very low uncertainties. Many important topics will be covered, from the basic operation of our standard to a discussion of key systematic shifts that must be well-understood and controlled to achieve high accuracy. At present, the 88Sr+ ion clock at NRC has an evaluated fractional uncertainty of 10-17. Methods for reducing it to the 10-18 level will be discussed briefly. The tutorial will conclude with examples of optical clock applications in fundamental physics and relativistic geodesy.

Dr. Pierre Dubé received his Ph.D. in physics from the University of Toronto. He went on to be a postdoctoral fellow at JILA in Boulder, Colorado, with John Hall, and a postdoctoral fellow at TRIUMF, in Vancouver, Canada, with Otto Haüsser. Since 2000, he has been working at the National Research Council of Canada, primarily on high-accuracy single-ion optical clocks.

---

10:15 a.m. - 10:30 a.m., Break

---

Optical frequency combs: from principles of operation to metrological applications

Time: Tuesday, January 21, 10:30 a.m. - 11:45 a.m.

Femtosecond optical frequency combs are highly versatile tools for precision measurements. Providing phase coherent frequency references across the entire optical spectrum, they offer users a unique combination of broad spectral coverage, high spectral resolution and a calibrated frequency scale. As a result, they have found numerous applications in frequency metrology and beyond. This tutorial will explain how femtosecond optical frequency combs work, covering both the fundamental principles and some practical details. The construction and major components of several of the most common types of frequency comb technology will be described. Differentiating factors that may influence the choice of technology for a particular application will be discussed. Finally, case studies will illustrate how optical frequency combs are applied in precision frequency metrology.

Dr. Helen Margolis is an NPL Fellow in Optical Frequency Standards and Metrology. She leads NPL's research activities in optical frequency metrology using femtosecond combs, which are part of the research programme to develop a new generation of high accuracy optical atomic clocks based on laser-cooled trapped ions and atoms.

---

11:45 a.m. - 1:30 p.m., Lunch is on your own

---

Realization of a UTC(k) time scale

Time: Tuesday, January 21, 1:30 p.m. - 2:45 p.m.

Many different approaches can be found in literature for the realization of a time scale. Most of them consider the realization of an autonomous time scale without considering the specific needs of a UTC(k). This tutorial suggests an unconventional approach, oriented to the practical implementation of a physically available robust UTC(k) time scale. This tutorial is based on the assumption that an UTC(k) is a physical representation of UTC with the best availability and the lowest possible departure from UTC, taking into account the fact that the number and the quality of the available clock is limited by external constraint. The first part will be devoted to a description of UTC and of the expected characteristics of an UTC(k) time scale. The second part will describe the steering of an UTC(k) to UTC, with examples provided. The analysis of performances of some UTC(k) will be discussed.

Dr. Daniele Rovera has been a researcher at LNE-SYRTE Observatoire de Paris, (formerly LPTF) since 1989 where his current interests include atomic clocks, optical frequency measurements, time comparisons and time scales.

---

2:45 p.m. - 3:00 p.m., Break

---

Global Navigation Satellite System (GNSS) Overview with a Focus on Precise Time Disseminations and Standards

Time: Tuesday, January 21, 3:00 p.m. - 4:15 p.m.

GPS has provided an operational Position Navigation and Timing (PNT) services for more 25 years. The vast majority of PNT applications use GPS as its fundamental source for PNT data. GPS operates using very precise synchronized ranging signals, where in general every nanosecond of synchronization error can lead to one foot of navigation error. Because of this exquisite timing synchronization, GPS provides a precise timing service used to support many important user communities ranging from power grid, telecommunication networks, science and the banking industry. Today GPS is no longer the only GNSS system, there are now several other GNSS systems in either operations and/or system development. This tutorial will provide an overview of how GPS operates, discuss the other GNSS systems and the reference standard that underlay each system.

Ed Powers received his BS and MS degrees in Electronic Engineering and Instrumental Science from the University of Arkansas. Previously, he has worked at NRL on GPS clock development and at the USNO as the GPS Operations Division Chief. Ed joined the Aerospace Corp in October 2018 as Senior Project Engineer, GNSS Engineering & Technology.

---

4:15 p.m. - 4:30 p.m., Break

---

Optical fiber and its use for time and frequency transfer

Time: Tuesday, January 21, 4:30 p.m. - 5:45 p.m.

The need to know the time in present and future society is increasing, whether it is in a time stamp of a financial transaction, or the data handover between two base stations in a 5G network. In many occasions it is satisfied with time through radio transmission, but with synchronization needs related to national security or international regulations, the security of an optical fibers is essential. The comparison of frequency from an optical clock is also fundamental for enhanced accuracy in measurements. While techniques for fiber optic communication are in constant development, with data rates exceeding 400 Gbit/s, the evolution of techniques for time or frequency transfer over fiber must follow a different path. 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. Sven-Christian Ebenhag received his PhD from Chalmers University of Technology in Sweden. Since 2002 has he worked at the SP Technical Research Institute of Sweden, where he is one of the senior scientists in the implementations of time and frequency transfer over a national fiber communication network.

---