Humanity may soon have access to a 'nuclear clock' that is far more accurate than atomic clocks.



Atomic clocks , which are used to measure standard time and calculate GPS, boast extremely high accuracy, with an error of just one second per several hundred thousand years. Researchers from the University of Colorado Boulder, the National Institute of Standards and Technology (NIST), the Swiss Federal Institute of Technology Lausanne, Vienna University of Technology, Ludwig Maximilian University Munich, and IMRA America have jointly announced the possibility of developing a ' nuclear clock' that is even more accurate than atomic clocks.

Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock | Nature
https://www.nature.com/articles/s41586-024-07839-6

'Nuclear clock' breakthrough paves the way for super-precise timekeeping
https://www.nature.com/articles/d41586-024-02865-w

The First Nuclear Clock Will Test if Fundamental Constants Change | Quanta Magazine
https://www.quantamagazine.org/the-first-nuclear-clock-will-test-if-fundamental-constants-change-20240904/

What Is an Atomic Clock? - NASA
https://www.nasa.gov/missions/tech-demonstration/deep-space-atomic-clock/what-is-an-atomic-clock/

Since ancient times, time has been defined by the rotation and revolution of the Earth. However, to accurately define time using this astronomical standard requires long-term observation. Therefore, humans began to define time based on the properties of atoms rather than the movement of celestial bodies.

An atom consists of a nucleus made of electrons and neutrons, with electrons flying around it. It is known that the orbits of these electrons vary depending on their energy state, and when the orbits of the electrons change, the atom absorbs or emits light at a specific frequency. The frequency of this light is specific to the atom and has a very precise and stable value. Since frequency (Hz) is the reciprocal of time (s), it is possible to define time from the frequency of light. A clock that uses this principle is called an 'atomic clock.'

In the International System of Units , one second was defined in 1967 as 9,192,631,770 times the radiation period corresponding to the transition between the two hyperfine levels in the ground state of the cesium-133 atom. The world's first atomic clock was developed in 1949 using ammonia molecules, and an atomic clock using cesium-133 was put into practical use in the UK in 1955.



The Deep Space Atomic Clock, launched by NASA in 2019, was developed with the aim of realizing a future where we live on planets other than Earth by installing it on spacecraft. This Deep Space Atomic Clock observes the orbital transition of mercury ions held in an electromagnetic trap, and is designed to minimize the effects of environmental factors such as temperature changes. Its error is an astounding 1 second every 9 million years, which is about 50 times more accurate than the atomic clocks installed on GPS satellites.

At the time of writing, the most accurate atomic clock is the 'Low-temperature strontium optical lattice clock'

developed in July 2024 by JILA, a joint research institute of the University of Colorado Boulder and NIST. Its accuracy is 8.1 x 10-19 , meaning the error is one second per 40 billion years. This low-temperature strontium optical lattice clock works by capturing laser-cooled strontium atoms in a lattice created by interfering lasers, and measuring the frequency of the light by shining laser light on the atoms.



And 'nuclear clocks' are said to be able to measure time with even greater precision than atomic clocks. While regular atomic clocks use the movement of electrons in cesium-133 or rubidium, nuclear clocks use the change in state of the nucleus of thorium-229.

The nucleus of thorium-229 has a special state that can be excited with very low energy. The energy required for this change is about 1/10,000th of that required for other atoms. Therefore, the key point is that the state of the nucleus can be changed using an ordinary ultraviolet laser. Because nuclear clocks use nuclei instead of electrons, they are less susceptible to external influences than ordinary atomic clocks and optical lattice clocks, making it possible to measure time more stably.

Previous studies had not determined the exact laser frequency required for the nuclear transition of thorium-229. This is because the low energy required for the transition makes it extremely difficult to measure. This is the third study to observe the nuclear transition of thorium-229, and a major achievement has been reported in this study, which has been able to measure the frequency with precision millions of times higher than previous studies.



In addition to its role in measuring time accurately, it is hoped that this nuclear clock may also be able to verify the theories of modern physics.

In the thorium-229 nucleus, the balance between the two fundamental interactions, the electromagnetic force and the strong force , is very delicate, and these two forces almost completely cancel each other out within the nucleus. Therefore, if the strength of the fundamental interactions themselves changes, it may be possible to detect the effect. For example, it may be possible to detect whether dark matter has a very slight effect on the interaction of atomic nuclei. In addition, it is expected that the question of 'whether the fundamental constants of physics change over time' in superstring theory can be verified by using nuclear clocks in combination with atomic clocks.

However, while the accuracy of the low-temperature strontium optical lattice clock is on the order of 10 -19 , the accuracy of the measurement of the nuclear transition frequency of thorium-229 this time is on the order of 10 -12 . Therefore, in order for nuclear clocks to surpass atomic clocks in accuracy, the measurement accuracy needs to be improved by several million to tens of millions of times.

The research team said, 'It will be many years before we overcome this hurdle.' Nevertheless, José Crespo López Urrutia, a physicist at the Max Planck Institute for Nuclear Physics, said, 'My first thought when I heard the report was not, 'Oh, the accuracy is still insufficient,' but, 'The research team has finally made the nuclear clock work.' Although technical challenges with the laser system remain, I am convinced that they will be overcome within a few years and that nuclear clocks will overtake atomic clocks in precision and accuracy.'

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