A portable atomic clock has been used to conduct field measurements for the first time, reports a study published online in Nature Physics this week. The transportable optical atomic clock was used to measure the height of a specific laboratory on a mountain in the Alps. The study represents a proof-of-principle demonstration for future applications of atomic clocks in gravitational measurements.
Atomic clocks are able to measure time to tremendously precise levels - one part in 1017. This sensitivity enables the detection of the infinitesimally small changes to the flow of time that general relativity predicts should take effect at different heights on Earth (owing to the varying strength of the gravitational field). This means that a very precise clock can be used as a gravity sensor. The high sensitivity of atomic clocks requires carefully controlled conditions, which are very difficult to create outside metrological labs.
Christian Lisdat and colleagues deployed a portable atomic clock fitted within a 2.2 × 3 × 2.2 m car trailer underground in the French Alps, and compared its bearings with those taken at a site in Italy - one roughly 100 km away and 1,000 m lower in height. Working in far from ideal, realistic conditions in the middle of the Frejus road tunnel, the authors used the atomic clock to estimate the height of the mountain laboratory and show that their estimate matches independent measurements made with two other state of the art geodetic (“shape of the earth”) measures: using optical spirit levelling satellite positioning systems combined with gravity field models.
The authors note that, although the precision of their measurement remains much lower than that achievable through conventional geodesy, by overcoming substantial technical challenges this measurement campaign is an important step towards the practical application of atomic clocks for geophysics.