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Apr. 19, 2019 Research Highlight Physics / Astronomy

Laser intensity fine-tuned to boost atomic clock’s precision

A lattice of strontium atoms could ‘tick’ with unprecedented accuracy

Image of two optical lattice clocks Figure 1: These two optical lattice clocks use strontium atoms to keep time with incredible accuracy. © 2019 RIKEN Chief Scientist Laboratories

A significant step toward building an optical lattice clock that will set a new record for accuracy has been taken by RIKEN researchers1. Such clocks could be used to measure millimeter-level differences in height.

The astonishing precision with which atomic clocks keep time sees them used in applications as diverse as global positioning systems and telecommunications. Conventional atomic clocks contain a thin gas of atoms, such as cesium, which act as pendulums. The atoms absorb and emit energy at a specific microwave frequency, and this frequency is used to define the duration of 1 second.

In contrast, optical clocks use visible light to make atoms ‘tick’ at frequencies tens of thousands of times higher than microwave frequencies. “That makes optical clocks far more precise than conventional microwave atomic clocks,” notes Hidetoshi Katori of the Quantum Metrology Laboratory.

Optical clocks are so accurate that they lose less than 1 second in 16 billion years—longer than the age of the Universe. They thus have a fractional uncertainty of roughly 10−18.

But further improvements in accuracy could open up new applications. For example, according to the general theory of relativity, two clocks at different heights above the Earth’s surface will tick at slightly different rates. If those clocks operated with an uncertainty of 10−19, it should be possible to measure height differences between them with millimeter precision, which may lead to applications such as detecting tiny anomalies in the Earth’s gravitational field to improve earthquake-warning systems.

The leading method to achieve this uses lasers to confine thousands of atoms in a lattice. The laser would normally affect the atoms’ ticking, but researchers can eliminate most of this perturbation by using a carefully chosen ‘magic frequency’ of laser light.

Katori’s team has now developed a way to fine-tune the intensity of this laser light to further reduce any perturbations of the atoms. This ‘magic intensity’ could improve the accuracy of the optical lattice clock to unprecedented levels.

The team cooled strontium atoms to a fraction of a degree above absolute zero (−273.15 degrees Celsius) and loaded them into an optical lattice trap. They then carefully monitored how very tiny shifts in the frequency of the atoms varied with the laser intensity and frequency. These measurements allowed the team to calculate the precise laser intensities and frequencies that should enable them to reduce the clock’s uncertainty down to 10−19.

The researchers are now building two optical lattice clocks so that they can apply these predicted magic frequency and intensity conditions, and measure how closely the clocks agree. “This will lead to exciting applications in the future,” predicts Katori.

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  • 1. Ushijima, I., Takamoto, M. & Katori, H. Operational magic intensity for Sr optical lattice clocks. Physical Review Letters 121, 263202 (2018). doi: 10.1103/PhysRevLett.121.263202