Optical Atomic Clocks and Quantum Metrology

Short answer: Optical atomic clocks are described in the research literature as the most accurate sensors developed by humankind, with reported fractional stabilities below 1 part in 10 to the power 18. A 2026 paper says the SI second is still defined by a cesium microwave transition and that redefinition is under active consideration. A 2022 perspective discusses entanglement as a way to push clocks beyond the Standard Quantum Limit, and presents it as a prospect.

The Current State of Precision Timekeeping

A 2015 review in Reviews of Modern Physics calls optical atomic clocks the state of the art in the frontier of modern measurement science, and reviews clocks based on trapped single ions and many neutral atoms. A 2022 perspective in Applied Physics Letters calls optical-transition atomic clocks the most accurate sensors developed by humankind, reaching fractional stabilities below 1 part in 10 to the power 18. It says that corresponds to an uncertainty of less than one second over the age of the universe.

A 2026 Nature Communications paper says that since 1967 the SI second has been defined by 9,192,631,770 cycles of the microwave transition between the hyperfine levels of the ground state of the cesium-133 atom. It says cesium atomic clocks, along with rubidium clocks and hydrogen masers, have kept the world synchronized since then. With optical atomic clocks achieving fractional frequency uncertainties below 1 part in 10 to the power 18, the paper says redefinition of the SI second is under active consideration.

Quantum Limits and Entanglement Strategies

The 2022 perspective says quantum sensors are limited by the Standard Quantum Limit (SQL), which is imposed by the inherent projection noise of a quantum measurement. It says recent developments in atomic physics have enabled the experimental generation of many-body entangled states to boost the performance of quantum sensors beyond the SQL, and it describes commonly used experimental methods to create them.

The perspective focuses on the potential of applying quantum entanglement to state-of-the-art optical atomic clocks. It also presents recently developed time-reversal protocols that make use of complex states with high quantum Fisher information without requiring sub-SQL measurement resolution, and it discusses the prospects for reaching near-Heisenberg limited quantum metrology with them. This is a perspective article about potential, and this article does not claim that entanglement-enhanced clocks are in operation.

Technical Ingredients and Systematic Uncertainty

The 2015 review discusses important technical ingredients for optical clocks, and presents measurement precision and systematic uncertainty for some of the best clocks to date. It ends with an outlook on prospects for clock applications. This article does not reproduce those figures.

The 2026 Nature Communications paper says the precision timekeeping infrastructure enables critical modern applications, including global navigation satellite systems that rely on atomic clocks for positioning accuracy and 5G and 6G networks requiring nanosecond-level synchronization. That sentence is about timekeeping infrastructure in general, not about optical clocks specifically.

Interpretation of Research-Stage Claims

It helps to separate what the sources report as established from what they describe as prospects. The precision figure below 1 part in 10 to the power 18 is reported for optical clocks. Entanglement-enhanced clocks, time-reversal protocols and near-Heisenberg limited metrology are described as potential or prospects. This article summarises abstracts and introductions of three papers and has not checked their experimental details.

FAQ

How precise are optical atomic clocks?

A 2022 perspective says they reach fractional stabilities below 1 part in 10 to the power 18, corresponding to an uncertainty of less than one second over the age of the universe.

How is the SI second defined today?

Since 1967 it has been defined by 9,192,631,770 cycles of the microwave transition between the hyperfine levels of the ground state of the cesium-133 atom, according to a 2026 Nature Communications paper.

Is the SI second going to change?

The 2026 paper says redefinition is under active consideration, given optical clocks reaching fractional frequency uncertainties below 1 part in 10 to the power 18.

What is the Standard Quantum Limit?

The 2022 perspective says it is imposed by the inherent projection noise of a quantum measurement.

Are entanglement-enhanced clocks in use?

The perspective discusses the potential of applying entanglement to optical clocks and the prospects of near-Heisenberg limited metrology. It does not report them in operation.