8:30 a.m. - 9:45 a.m. PST	Time Scales - Continuous UTC, Redefinition of the SI Second and Plans for a Lunar Time	Dr. Elisa Felicitas Arias
9:45 a.m. - 11:00 a.m. PST	GNSS Time and Frequency Transfer Techniques and Alternate PNT Methods to GNSS	Dr. Jeff Sherman
11:00 a.m. - 11:15 a.m. Refreshment break
11:15 a.m. - 12:30 p.m. PST	Chip-Scale Atomic Devices	Dr. John Kitching
12:30 p.m. - 1:30 p.m. Buffet luncheon for tutorial attendees
1:30 p.m. - 2:45 p.m. PST	Deep-Space Positioning, Navigation, and Timing	Dr. Todd Ely
2:45 p.m. - 3:00 p.m. Refreshment break
3:00 p.m. - 4:15 p.m. PST	Optical Atomic Clocks and Frequency Combs	Dr. Cort Johnson
4:15 p.m. - 5:30 p.m. PST	Nuclear Clocks and Tests of Fundamental Physics	Dr. David Leibrandt

Time Scales - Continuous UTC, Redefinition of the SI Second and Plans for a Lunar Time

Time: Monday, January 26, 8:30 a.m. - 9:45 a.m.
Room: Ballroom AB

This tutorial will detail the future evolution of Coordinated Universal Time (UTC) and their consequences; and will report on the activity around the establishment of a time reference for the Moon.

UTC, the international reference time scale derived from International Atomic Time (TAI), is on the way to suffer changes that will give it long-term uniformity and optimized accuracy.

• Its present non-continuity, coming from the insertion of leap seconds at irregular intervals to limit its departure from the time derived from Earth’s Rotation (UT1) within 0.9 s. The artificial steps of UTC have negative impact on the modern systems of time transmission and dissemination, and on associated technologies that became essential to human activities. The work and discussions finally converged to an increase of the maximum value of UT1 – UTC that will preserve the continuity of UTC, may be for ever.
• Its accuracy, represented by the accuracy of the second of the International System of Units (SI) and its realizations. The present definition of the second is based on the period of a Caesium 133 transition, realized with an accuracy of parts in 1016. Work in time metrology laboratories made possible to develop frequency standards based on different species and transitions, operating frequencies higher than that of the Caesium. Some of these standards proved their capacity to reach some 10-18 accuracy, representing a challenge to physics and metrology. Some of these new transitions have been recommended as secondary representations of the second. One, or an ensemble of them will provide in the term of 5 – 10 years a new definition of the second.

Space agencies started projects and programmes to enhance the exploration of the Moon, demanding the establishment of references for positioning on the lunar surface (emulating the geodetic references), and of a time reference for events on and around the Moon, and communication also between the Moon and the Earth.

Dr. Elisa Felicitas Arias, was director of the Time Department at the Bureau International des Poids et Mesures (1999-2017), responsible for the computation of Coordinated Universal Time UTC. She retired in the end of 2017 but continues her professional activities in astronomy and time and frequency metrology at Paris Observatory.

---

GNSS Time and Frequency Transfer Techniques and Alternate PNT Methods to GNSS

Time: Monday, January 26, 9:45 a.m. - 11:00 a.m.
Room: Ballroom AB

GNSS, like GPS, offer wide and inexpensive dissemination of time signals with nanosecond-level precision and accuracy. By far the most successful use of atomic clocks, GNSS supports billions of dollars of economic activity monthly in the United States alone. We will review the physical basis and common techniques of GNSS time dissemination and time transfer by using GNSS signals as a transfer standard. We highlight the few weaknesses of GNSS: low signal amplitude at Earth's surface admits accidental or purposeful interference; space-borne assets in general are vulnerable to space weather or other failures. What alternatives exist for nanosecond-level time transfer? We will review protocols and techniques for time transfer over computer networks, terrestrial broadcasts and point-to-point transmissions, other space-borne constellations, and direct fiber optic links.

Dr. Jeff Sherman currently leads the Time Realization and Distribution Group at the National Institute of Standards and Technology. The group maintains and improves NIST systems that produce and distribute official U.S. time, time-interval, and frequency signals based on an ensemble of atomic clocks and frequency references.

---

11:00 a.m. - 11:15 a.m. Refreshment break

---

Chip-Scale Atomic Devices

Time: Monday, January 26, 11:15 a.m. - 12:30 p.m.
Room: Ballroom AB

Chip-scale atomic clocks are now successful commercial products and research within this scientific field continues to be highly active. This tutorial will cover the design, fabrication and performance of chip-scale atomic devices including frequency stability, size, weight, power and manufacturability. The key physics elements that underlie these instruments will be discussed, as well as the most important application spaces in which these devices are used. Current trends in the area of chip-scale atomic devices will be presented and some speculation for the future will be discussed.

Dr. John Kitching is the leader of the Atomic Devices and Instrumentation Group in NIST’s Physical Measurements Laboratory and a NIST Fellow. He and his group pioneered the development of microfabricated “chip-scale” atomic devices for use as frequency references, magnetometers and other sensors. He is a Fellow of the American Physical Society, the IEEE and the National Academy of Inventors.

---

12:30 p.m. - 1:30 p.m. Buffet luncheon for tutorial attendees

---

Deep-Space Positioning, Navigation, and Timing

Time: Monday, January 26, 1:30 p.m. - 2:45 p.m.
Room: Ballroom AB

This tutorial covers the basics of deep space navigation including its measurements, methods, and the typical performance realized in the different phases of a space mission. Focus will be on navigating to the Moon and Mars, in orbit about them, and, when on their surface, positioning. Key factors contributing to a reliable trajectory solution such as geometric diversity, complimentary data types, accurate error modeling/compensation will be highlighted. Throughout, the role of clocks and the navigation capabilities that could be enabled by the ready availability of stable space clocks will be discussed.

Dr. Todd Ely has been at the Jet Propulsion Laboratory since 1999 where he has been developing and implementing navigation systems and architectures for many projects; his current focus is on lunar and Mars navigation, space based VLBI, and the advancement of atomic clocks and autonomous navigation technologies. Dr. Ely is the recipient of JPL’s Magellan Award and NASA’s Outstanding Public Leadership Medal for his work on DSAC.

---

2:45 p.m. - 3:00 p.m. Refreshment break

---

Optical Atomic Clocks and Frequency Combs

Time: Monday, January 26, 3:00 p.m. - 4:15 p.m.
Room: Ballroom AB

Optical atomic clocks can greatly improve the performance of precision timing systems compared to those that rely on microwave-based atomic clocks. Practical optical clock implementations are only possible when the optical output is divided to the microwave regime using optical frequency comb technology. This tutorial will present the fundamental principles that led to the development of both technologies. It will include a comparison of optical clocks designed for portability versus those optimized for precision and review the key systematic shifts that must be well-understood and well-controlled to achieve high accuracy. It will also provide a survey of applications beyond timekeeping that utilize the unique spectral properties of optical frequency combs.

Dr. Cort Johnson is a staff scientist at Draper in the quantum applications group. Over the past decade he has led efforts to design, fabricate, and test compact optical atomic clocks. Prior to joining Draper in 2013, he was a member of the technical staff at Sandia National Laboratories.

---

Nuclear Clocks and Tests of Fundamental Physics

Time: Monday, January 26, 4:15 p.m. - 5:30 p.m.
Room: Ballroom AB

Clocks based on hyperfine and electronic transitions in laser-cooled atoms, with fractional inaccuracy and instability now reaching below 1e-18, have revolutionized positioning, navigation, and timekeeping (PNT) and serve as one of the experimental foundations on which the Standard Model of particle physics was built. A new type of clock based on the internal transitions of atomic nuclei, dubbed nuclear clocks, was proposed by Peik and Tamm in 2003. Among nuclei, the thorium-229 nucleus is unique in having a transition at low enough energy to be accessible with present-day laser technology, and laser spectroscopy of the thorium-229 nuclear isomer transition was first demonstrated by three groups nearly simultaneously in 2024. The race is now on to build the first nuclear clocks, which have the potential for transformative advances in PNT and searches for physics beyond the Standard Model (BSM).

In this tutorial, I will discuss the two types of thorium nuclear clocks that are being pursued. Solid-state thorium clocks are based on spectroscopy of thorium doped into a crystal or thin film. This is uniquely possible with nuclear transitions because atomic nuclei are shielded from the solid-state environment by their core electrons, and enables clocks based on Avogadro's number of atoms with quantum-projection-noise-limited instability orders of magnitude below atomic clocks typically based on less than one million laser-cooled atoms. Due to their relative simplicity, it might also be possible to make low size, weight, and power (SWaP) solid-state nuclear clocks that can be deployed outside the laboratory. Trapped-ion thorium clocks are based on well-isolated trapped and laser cooled thorium ions. Because nuclei are much smaller than atoms, nuclear transitions are much less sensitive than hyperfine and electronic transitions to environmental electromagnetic fields that limit atomic clock accuracy. Trapped-ion nuclear clocks may thus reach inaccuracies well beyond the best atomic clocks.

I will conclude by discussing the fundamental physics reach of thorium nuclear clocks. Due to the higher energy scales and additional fundamental interactions present in the nucleus, nuclear transitions are much more sensitive to small deviations from the predictions of the Standard Model than atomic transitions. Beyond their applications in PNT, thorium nuclear clocks may offer insights about the nature of dark matter or other hints about what lies beyond the Standard Model.

Dr. David Leibrandt is a professor in the Department of Physics & Astronomy at UCLA. Prior to moving to UCLA in 2022, he was an associate professor adjoint at the University of Colorado and led the trapped-ion optical atomic clock and precision measurement experiments within the Ion Storage Group at the National Institute of Standards and Technology in Boulder, CO.

---