NRC Metrology makes time and builds atomic fountain clock

No time like the present

Time flies; time heals all wounds; time is money. We save time, lose time, and even waste time. Time marks the passage of events in our lives and allows us to synchronize our daily schedules. However, for all of the practical and philosophic impacts of time on our lives, we don't often give much thought to how time is actually measured.

Humankind has used several methods to measure time accurately throughout history. The ability to mark the passage of years, seasons, days, and finer intervals of time were critical to many aspects of peoples' lives including commerce and navigation. This drove the technological advancements of clocks through the forms of sundials, water clocks, astronomical observations, and vibrating quartz crystals that increased the accuracy of timekeeping by orders of magnitude.

Today, time is no longer tied to the rotation of the Earth, but to the quantum mechanical behaviour of atoms.

As Canada's official time keeper, the NRC's Frequency and Time team from the Metrology Research Centre takes pride in advancing their research to achieve measurement capabilities for time at all levels of precision. The second is the unit of measure of time and is 1 of 7 base units that form the International System of Units (SI). Due to the accuracy of atomic clocks, in 1967 the second was changed from an astronomical definition to one based on the oscillations of isolated caesium-133 atoms, and this definition still holds today.

A long-term goal of Metrology has been to develop a caesium fountain clock with 1 of the most accurate measurements of the SI second, which could serve as a primary frequency standard (PFS) for Canada, meaning that it would require no external calibration.

Although there are several labs across the world evaluating primary frequency standards, very few report to the International Bureau of Weights and Measures (BIPM). It is the role of the BIPM to compute and distribute International Atomic Time (TAI); the world's most stable timescale. By integrating data from several international PFSs, the BIPM is able to steer TAI to keep this global consensus timescale consistent with the definition of the second. With help from collaborators, the NRC has achieved a major milestone by building and characterizing a caesium fountain clock, and passing the rigorous peer review that enables Canada to contribute to the steering of TAI on a world-class level.

Like clockwork

The NRC's caesium fountain PFS (NRC-FCs2) consists of 4 sub-systems comprising of:

1. a physics package,
2. a microwave system,
3. a laser system, and
4. control electronics and software.

During the project's early stages, the NRC looked for a collaborative partner who could design a physics package (a vacuum system in the fountain that contains the microwave cavity and the caesium atoms along with other components), which is a key sub-system of the fountain clock. The NRC reached out to the National Physical Laboratory (NPL) in the United Kingdom, where they have well-established expertise in the development and operation of caesium fountain clocks. In collaboration with Professor Kurt Gibble of Pennsylvania State University, researchers at NPL developed a proven design for a microwave cavity and physics package.

The NRC and NPL entered into a scientific and commercial collaboration, where the NRC purchased a physics package from NPL and provided scientific personnel to assist in its assembly and testing. In parallel, NRC researchers Scott Beattie, Bin Jian, John Alcock and Marina Gertsvolf also worked to develop their own microwave, laser, and control systems for NRC-FCs2. Upon completion of the assembly and testing of the physics package at NPL, the delicate equipment was shipped to the NRC, where it was integrated with the other NRC fountain subsystems.

With continued collaboration, the NRC Frequency and Time group embarked on the long and demanding process of completing a full evaluation of the frequency standard. This work requires the painstaking characterization and correction of the myriad of external effects that can influence the performance of the clock. Upon completion, the results of the evaluation were published in the scholarly journal Metrologia. This publication confirms that the international community of time and frequency experts agrees with the NRC result and its claimed level of uncertainty of only 2.3 × 10^-16; this corresponds to the clock neither gaining nor losing a second in over 100 million years!

Standing the test of time

The NRC has now joined the club of national metrology institutes offering the best realizations of the SI second in the world and has begun contributing to the BIPM for the steering of TAI. NRC-FCs2 will continue to provide an accurate local timescale in Canada and facilitate the NRC's continuing improvement of next-generation optical clocks.

Marina Gertsvolf, Team Lead of the Frequency and Time team, shared, "This project succeeded thanks to a tight network of National Metrology Institutes around the world all working together on the support and the betterment of the SI system of units, along with the superior technical expertise of the NRC's fountain team and their focus on delivering a state-of-the-art apparatus on time, and with excellent performance characteristics."

Although NRC-FCs2 is now performing at a world-class level, researchers in the Frequency and Time group are always looking for ways to improve its performance.

What will be the ultimate limit? Only time will tell.