At this level of precision, an error of one second would accumulate only after more than 260 billion years. For comparison, the Universe is less than 14 billion years old.
An atomic clock does not measure time with gears. It uses a very precise change in the state of an atom, caused by light of a specific frequency. This change serves as a regular reference, much as a pendulum provides a rhythm for a mechanical clock.
Illustration of lutetium used as a reference in an optical atomic clock.
Credit: Melanie Lee / Centre for Quantum Technologies.
Researchers at the National University of Singapore chose lutetium-176. Each of their two clocks confines a single ion, meaning an atom carrying an electric charge. A laser is tuned to the frequency specific to this ion. The resulting uncertainty approaches 1 × 10-19, corresponding to verification down to the 19th decimal place.
Such precision is not enough, however, if it is achieved with only one device. The researchers therefore built two independent clocks and compared their rates for approximately 200 hours. Their measured difference remained compatible with zero, with an uncertainty of 5.7 × 10-19.
Lutetium offers a practical advantage here. The frequency used to measure time is barely affected by changes in temperature or magnetic fields. This reduces several corrections required with other atoms. The two devices can therefore operate at room temperature while retaining their precision.
At this level, gravity itself becomes measurable. According to general relativity, time passes very slightly differently depending on altitude and the gravitational field. The two clocks were sensitive enough to detect the effect corresponding to about a 5 mm difference in height on the same table.
This sensitivity opens up uses that go beyond simply measuring time. Such clocks could compare the elevation of different locations on Earth with exceptional precision, track certain variations in the gravitational field, or test fundamental laws of physics. They could also be used to search for possible variations in physical constants.
The official second is still based on a transition in cesium-133, used as an international reference since the 1960s. Optical clocks use much higher frequencies and now achieve greater precision. International organizations are therefore studying a future redefinition of the second based on this new generation of clocks.
The next step will be to take the system out of the laboratory. The researchers are working to make the lutetium clock transportable without sacrificing its precision. A mobile device would make it possible to directly compare clocks installed in distant laboratories and test their performance under real-world conditions.