Optical clock makes moves on fresh insights into Earth

Gravity has been measured with a transportable optical clock for the first time, opening up the possibility of more accurately mapping Earth’s surface.

optical clock
PTB trailer containing the transportable optical clock (left) with the NPL frequency comb located by the scientist in the middle. The rack immediately in front of the table is the end of the optical fibre link that runs from INRIM in Torin

A European team including researchers from the UK’s National Physical Laboratory carried out the experiment using one of the world’s only transportable optical atomic clocks.

According to Einstein’s Theory of Relativity, clocks closer to massive bodies run slower, meaning a clock at the bottom of a mountain will run more slowly than one at the top.

Optical clocks are now so precise that they are sensitive to these changes, meaning the technique can be used to accurately measure height differences as small as 1cm across Earth’s surface.

Unlike satellite-based measurements, which average the gravity potential over large distances, optical clocks can make measurements at specific locations. And with a transportable clock, these measurements can be taken at points of particular interest, according to Helen Margolis, fellow in optical frequency standards and metrology at NPL.

In this way, the technique could be used to monitor changes to Earth’s gravity potential caused by climate change, for example.

In an experiment described in Nature Physics, the team transported the optical clock to a laboratory located 1,700m below a mountain, in the Frejus road tunnel between France and Italy.

“We took the clock to this particular lab under the Alps because the area is subject to long term uplift, which means that the gravity potential is slowly changing over time, giving us a realistic test bed,” said Margolis. “But you could also imagine placing them in locations where you could monitor changing sea levels that might result from climate change,” she said.

Until now optical clocks have been restricted to laboratories at major research institutes. But researchers at the Physikalisch-Technische Bundesanstalt (PTB) in Germany recently developed a transportable strontium optical lattice clock.

They transported this clock down to the South of France, where it was connected by optical fibre link to another optical clock at the Istituto Nazionale di Ricerca Metrologica (INRIM), 90km away in Turin, at a height distance of around 1000m. The team then measured the gravity potential difference between the two clocks.

Since the portable clock and the laser used to transmit over the fibre operate at different frequencies, NPL researchers provided a frequency comb to relate these different frequencies, allowing the devices to be connected, said Margolis.