For Precision, Two Clocks Are Better Than One

The optical lattice clocks use lasers to create “egg box” structures that contain single atoms, leading to unprecedented precision.

AsianScientist (Feb. 10, 2015) – Researchers at the RIKEN Center for Advanced Photonics in Japan, led by Dr Hidetoshi Katori, have developed a pair of cryogenic atomic clocks that can measure time to unprecedented accuracy. This research was published in Nature Photonics.

In an atomic clock, a laser is used to excite a transition between two excited states of a cold gas of atoms, measuring the frequency of the laser then provides the “ticks” of the clock. While an ordinary wristwatch might tick once a second, atomic clocks use light with frequencies of up to 1015 hertz, allowing for an incredibly precise measurement of time. Such atomic clocks have the potential to measure intervals of time to 18 digits’ worth of accuracy—equivalent to measuring the age of the Earth (4.5 billion years) with an accuracy of a tenth of a second.

However, the atoms of an atomic clock must be isolated from environmental influences which might affect the frequency of the atomic transition being measured, posing serious technical challenges to the building of atomic clocks.

“As such, in 2001, we proposed the novel concept of an ‘optical lattice atomic clock’, in which trapping lasers build an interference pattern that can hold and measure a few thousand atoms simultaneously,” explained Dr. Hidetoshi Katori, corresponding author of the present study. “Even then, the atoms can be affected by the thermal radiation of the measurement chamber.”

“To overcome this, we took two complementary approaches,” Katori continued. “Firstly, we designed an atomic clock, operating with atoms of Strontium-87, to run inside a chamber that could be cryogenically cooled to 95 kelvins, or -180°C. As the thermal radiation decreases rapidly with temperature, we were able to drastically reduce the effect of thermal radiation on the atoms we were measuring.”

“Secondly, we built two atomic clocks side-by-side to run simultaneously. Synchronizing the two clocks allowed us to further reduce the uncertainty by two-thirds, ultimately achieving a precision of two parts in 1018 after a measurement time of only an hour.”

The two optical lattice clocks running side by side. Credit: Dr. Hidetoshi Katori.
The two optical lattice clocks running side by side. Credit: Dr. Hidetoshi Katori.

Building on this research, the RIKEN team ultimately hopes to develop optical lattice clocks that measure 18-digit-accurate time within a few minutes. These rapid, ultra-precise time measurements would open up many new possibilities in the field of precision measurements, such as the possibility of measuring relativistic effects in the course of everyday life.

Relativity predicts that time speeds up minutely when one is at the top of a building as compared to the ground floor, or when one is standing still as compared to walking; a rapid, ultra-precise time measurement would thus provide us a new type of quantum sensor to measure relativistic space-time.

“We are now operating our cryogenic clocks at two remote sites 15km apart, namely RIKEN and the University of Tokyo, to see if we can detect the gravitational potential differences between the clocks. Exciting new endeavours are just starting,” Katori told Asian Scientist Magazine.

Also, increasing the precision of time measurements would allow for increased precision in the measurement of other natural constants such as the speed of light or the charge of the electron, and even check whether these “constants” are truly constant, or whether they show minute changes over time. As such, these advances in atomic clock technology may ultimately shed light on questions that are at the very heart of physics itself.

The article can be found at: Ushijima et al. (2015) Cryogenic Optical Lattice Clocks.


Copyright: Asian Scientist Magazine; Photo: RIKEN.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Shern Ren is studying towards a PhD degree in physics at the National University of Singapore. When he isn't working on the statistical mechanics of nanomachines and single-molecule systems, you may find him scratching his head over politics, education and the mathematics of Threes.

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