Optical atomic clocks

In plain language

An atomic clock keeps time by counting the very regular oscillation that an atom produces when it changes energy level. The current international definition of the second is based on caesium, which oscillates at microwave frequencies. An optical clock uses a transition that oscillates at the much higher frequency of visible light. More cycles per second means time can be divided into finer, more consistent steps.

In practice, a laser is locked to the atomic transition. Researchers trap either many neutral atoms, for example strontium or ytterbium held in a laser-made lattice, or single ions held in electromagnetic traps. In 2024, a strontium lattice clock was reported with a total systematic uncertainty of 8.1 × 10⁻¹⁹, which means the clock's known sources of error are very small in that laboratory setup. Clocks of this kind are being carried between laboratories, and they can also detect tiny differences in gravitational potential, which is why they are of interest to surveyors and geodesists.

Technical explanation

Optical clocks come in two main families. Lattice clocks trap a large number of neutral atoms in an optical lattice, a standing wave of laser light that holds the atoms in place. Averaging over many atoms improves stability. Ion clocks trap one or a few charged atoms, which are isolated from many environmental disturbances. Aeppli and colleagues (2024) report a strontium-87 lattice clock that probes the ¹S₀ to ³P₀ transition in a dilute, shallow, vertically oriented one-dimensional lattice, with a total systematic uncertainty of 8.1 × 10⁻¹⁹ in fractional frequency units and all other systematic effects below 1 × 10⁻¹⁹. The paper's figure is a laboratory evaluation of the clock's error budget, not a measure of performance in the field. The 2019 aluminium-27 ion clock of Brewer and colleagues uses quantum-logic spectroscopy, in which a magnesium-25 ion is trapped alongside the aluminium ion to cool it and read out its state. It reported a systematic uncertainty of 9.4 × 10⁻¹⁹ and a stated stability of 1.2 × 10⁻¹⁵/τ, where τ is averaging time.

A nuclear clock is a further development. Zhang and colleagues (2024, Nature) directly measured the frequency ratio between the nuclear isomeric transition of thorium-229 and the strontium-87 clock, using a frequency comb locked to the strontium clock. The abstract does not give a numerical value for the ratio, and the nuclear transition is not yet used as a working clock. The key international step is the planned redefinition of the second. The BIPM frequency-roadmap FAQ describes the options as: fixing the frequency of a single optical transition; defining a fictive frequency from a weighted geometric mean across several transitions; or fixing another fundamental constant, which the BIPM describes as less favoured. The CGPM Draft Resolution B, to be considered in 2026, concerns continued work toward a proposal at the 2030 CGPM. The BIPM FAQ refers to eight mandatory criteria that must be met before redefinition. This profile does not set them out in full.

For geodesy, a clock at a different gravitational potential runs at a slightly different rate. Comparing two clocks can therefore reveal a height difference, a method called chronometric levelling. A preprint from the International Clock and Oscillator Networking collaboration (2024) reports that transportable optical clocks from Japan and Europe, compared across countries, determined geopotential height offsets at the level of 4 cm. The authors state that these comparisons need neither a high-performance frequency link nor prior knowledge of the geopotential difference between sites. This result has not been reviewed in a journal in the sources checked.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• The uncertainty figures quoted for the best systems come from laboratory evaluations of specific clocks. They describe the evaluated error budget, not long-term performance in an operational setting.
• Transportable clocks are used in field comparisons, but the sources reviewed do not give an uncertainty budget for the clocks used in the 4 cm geodesy result.
• The 4 cm result comes from a preprint whose authors describe it in general terms. The sources reviewed do not give the measurement conditions in detail, and it has not been checked against a peer-reviewed version.
• The species or definition for the redefined second is not yet chosen. The BIPM sets out options but does not state a decision, and the outcome of the 2026 CGPM was not confirmed in the sources reviewed.
• The thorium-229 nuclear clock is at an early stage. The sources reviewed show a measured frequency ratio but do not give its numerical value, and the nuclear transition is not yet a working clock.

Possible future uses

Timeline

1. Jun 2015Ludlow and colleagues publish the review 'Optical atomic clocks' in Reviews of Modern Physics, covering clocks based on single trapped ions and on many neutral atoms. Sources: [3]
2. Jul 2019Brewer and colleagues report an aluminium-27 ion quantum-logic clock with a systematic uncertainty of 9.4 × 10⁻¹⁹ in Physical Review Letters. Sources: [2]
3. 2024Aeppli and colleagues report a strontium lattice clock with a total systematic uncertainty of 8.1 × 10⁻¹⁹ in Physical Review Letters. Sources: [1]
4. 2024Zhang and colleagues publish a direct measurement of the frequency ratio between the thorium-229 nuclear transition and the strontium-87 clock in Nature. Sources: [4]
5. 20 Feb 2024BIPM reports progress towards the redefinition of the second and states that adoption by the CGPM is anticipated in 2030 or later. Sources: [5]
6. 30 Oct 2024The International Clock and Oscillator Networking collaboration posts a preprint reporting geopotential height offsets at the level of 4 cm from transported optical clocks. Sources: [7]

Futures this could help bring about

How far away?

Redefining the second with optical clocks

Engineering prototypes · 4 of 10 steps done

How far away?

Time travel

Science demonstrated · 4 of 8 steps done

Related breakthroughs

• Strontium optical lattice clock reports 8.1 × 10⁻¹⁹ total systematic uncertainty (10 Jul 2024)

Glossary

Optical clock

A clock whose tick is an oscillation at optical (visible light) frequency, which is much higher than the microwave frequency used by caesium standards.

Lattice clock

An optical clock that traps many neutral atoms in a standing wave of laser light, called an optical lattice, and averages their signal.

Systematic uncertainty

The bound on errors that can shift a clock's frequency in a consistent way, such as electric fields, temperature or laser light effects.

Chronometric levelling

Measuring height differences by comparing clocks, since clocks at different gravitational potentials run at slightly different rates.

Sources

1. peer reviewed paper
   A clock with 8×10⁻¹⁹ systematic uncertainty
   Alexander Aeppli, Kyungtae Kim, William Warfield et al.. Physical Review Letters, 2024. DOI: 10.1103/PhysRevLett.133.023401. Accessed 21 Oct 2026.
2. peer reviewed papercorrected
   Al⁺²⁷ Quantum-Logic Clock with a Systematic Uncertainty below 10⁻¹⁸
   S. M. Brewer, J.-S. Chen, A. M. Hankin et al.. Physical Review Letters, 15 Jul 2019. DOI: 10.1103/PhysRevLett.123.033201. Accessed 21 Oct 2026.
3. review article
   Optical atomic clocks
   Andrew D. Ludlow, Martin M. Boyd, Jun Ye et al.. Reviews of Modern Physics, 26 Jun 2015. DOI: 10.1103/RevModPhys.87.637. Accessed 21 Oct 2026.
4. peer reviewed paper
   Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock
   Chuankun Zhang, Tian Ooi, Jacob S. Higgins et al.. Nature, 2024. DOI: 10.1038/s41586-024-07839-6. Accessed 21 Oct 2026.
5. other
   Progressing towards the redefinition of the second
   Bureau International des Poids et Mesures, 20 Feb 2024. Accessed 21 Oct 2026.
6. other
   Frequently Asked Questions concerning the Redefinition of the Second
   Bureau International des Poids et Mesures, 13 Oct 2026. Accessed 21 Oct 2026.
7. preprint
   International comparison of optical frequencies with transportable optical lattice clocks
   Anne Amy-Klein, Erik Benkler. arXiv, 30 Oct 2024. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.