Frequently Asked Questions concerning the Redefinition of the Second

CCTF Task Force on Updating the Roadmap for the redefinition of the second

Table of Contents

• What is the timeline for redefinition?
• Why does the community want to redefine the second?
• What are the options to redefine the second?
• What will happen to the caesium standards after the redefinition?
• What actions might I need to take prior to the redefinition?
• Will I need to replace my caesium standards after the redefinition?
• Will caesium standards still be accepted into UTC after the redefinition?
• What are the criteria for deciding if we are ready for the redefinition?

What is the timeline for redefinition?

The timeline for the proposed redefinition of the second is set by meetings of the General Conference on Weights and Measures (CGPM). These are held every four years, with the next meeting scheduled for 2026. Two steps are required at the CGPM:

1. Presentation and consideration of the proposal (earliest date 2026)
2. Ratification of the new definition (earliest date 2030)

Implementation of the new definition would be expected to follow shortly after ratification.

To present the proposal to the CGPM in 2026, the CCTF must have a draft version of the proposal ready for the CCTF meeting in September 2025.

Why does the community want to redefine the second?

From a metrological point of view, we have reached the point where the best realizations of the current definition (i.e. caesium cold atom fountain clocks) have been surpassed by several optical frequency standards already constituting secondary representations of the second. Some of these have achieved two orders of magnitude lower systematic uncertainties and correspondingly lower frequency instabilities than caesium fountain clocks, with continuing progress being made.

The accuracy of frequency measurements is, therefore, limited by the current definition, rather than being limited by the technological level reached by the most advanced scientific teams. This challenges the recognition of the SI second as the universal common time unit and may hinder scientific and technological progress. As in 1967, this prompts the community to examine alternative options and choose a new definition, in-line with contemporary technological capacities, and able to remain on top of the metrological pyramid for a long time.

What are the options to redefine the second?

The SI units are currently defined using seven constants of nature that have fixed numerical values. The defining constant for the second, Δν_Cs, is a microwave frequency that is a characteristic of the Cs atom. Two options are being considered for redefining the second to be based on an optical transition and, therefore, realize improved accuracy:

• Option 1: Select a single optical atomic transition in place of Δν_Cs and set its frequency (ν_Xy) as the new definition of the second.
• Option 2: Use a weighted geometric average of optical frequencies from specific atomic transitions to create a new defining constant.

A third option based on fundamental constants has been also considered:

• Option 3: Fix the numerical value of another fundamental constant, such as the electron mass (m_e). However, this option is currently considered impractical in the current state of science and technology. As a matter of fact, the values of fundamental constants are currently known with uncertainties (1.9 part in 10¹² for the Rydberg constant or 3.0 parts in 10¹⁰ for m_e) that are several orders of magnitude larger than the present realizations of the unit of time of the current SI system (few parts in 10¹⁶) and even further away from the capabilities of optical frequency standards (10⁻¹⁸ or better).

Each of these options will impact the definitions of other base units, except for the mole. For more technical details, you can refer to https://iopscience.iop.org/article/10.1088/1681-7575/ad17d2/pdf.

What will happen to the caesium standards after the redefinition?

After the redefinition, caesium standards will have an additional uncertainty (of the order of 1 x 10⁻¹⁶), but this will only be significant in the uncertainty assigned to the very best caesium standards such as fountains. The caesium fountains will become a secondary representation of the second.

What actions might I need to take prior to the redefinition?

Depending on how legal time is defined in your national legislation, it may be necessary to work with your legislators to revise this definition. The current timetable allows at least four years for this to take place.

Education of the user community and stakeholders about the redefinition is most effectively done by their local NMI. The CCTF plans to provide resources to help with this responsibility.

The time laboratories are encouraged to consider realizing the SI second using the new definition in their forward planning. Improvements to time-transfer links may also be needed to fully realize the benefits of better clocks.

Will I need to replace my caesium standards after the redefinition?

No. Although optical frequency standards will become the primary representation of the second, caesium will remain a secondary standard. Traceability for caesium standards will still be provided through Circular-T.

Will caesium standards still be accepted into UTC after the redefinition?

Yes. Their exact status will depend on the chosen redefinition option, but after the redefinition the former caesium primary standards will be considered as secondary standards and be included in UTC just like secondary standards are currently used to steer EAL.

The pool of ~ 450 clocks contributing to UTC already mixes commercial thermal beam caesium clocks with hydrogen masers, with its resulting time scale EAL being steered by the primary and secondary frequency standards measurements to get TAI and finally UTC. The redefinition will have no immediate impact on this situation.

What are the criteria for deciding if we are ready for the redefinition?

A roadmap towards redefining the SI second to be based on an optical transition was adopted at the 21st meeting of the CCTF in 2017 and was refined and updated by a CCTF taskforce established in 2020. To choose the best new definition and establish a timeline, criteria and conditions were defined as part of the roadmap to assure that the redefinition:

• offers an immediate improvement in accuracy by 10 – 100×, with a possible larger improvement in the long term;
• ensures continuity with the definition based on caesium;
• ensures continuity of the availability of the new SI second, enabling an immediate improvement of the quality of TAI and UTC(k);
• is acceptable to NMIs and stakeholders, enabling broad dissemination of the unit to users.

Along these lines, eight mandatory criteria were established that must be met before a redefinition. The fulfilment of the mandatory criteria relies on the progress of high-performance and reliable optical frequency standards and time and frequency transfer techniques required for the realization of the new definition and its dissemination to users, including the contributions of optical frequency standards to International Atomic Time (TAI).

Additionally, six ancillary conditions were established that call out essential work that must have achieved a reasonable amount of progress at the time of redefinition, with a commitment of stakeholders to continue their efforts on the associated activities.

Fulfilment indexes were also defined to evaluate the fulfilment level of the mandatory criteria, to draw attention to the remaining work to fulfil the criteria and ultimately, to decide when it is time to change the definition.

For more technical details, you can refer to https://iopscience.iop.org/article/10.1088/1681-7575/ad17d2/pdf.