For the first time, a team of scientists led by Davide Calonico (INRIM), Christian Lisdat (PTB) and Helen Margolis (NPL), performed an outstanding geodesy experiment, measuring the light red shift between a reference point and a “precision measurement unfriendly” location in the middle of the Alps. The remote test site for the chronometric levelling with optical clocks has been the Fréjus tunnel in the Laboratoire Souterrain de Modane at the University of Grenoble. Comparing the frequency ratios of the first transportable Strontium lattice frequency standard (PTB) at the remote site and at the metrological institute of Italy with respect to the clock transition frequency of a stationary Cesium fountain clock (INRIM), the gravitational potential difference induced frequency shift was measured. Moreover, a measurement of the optical frequency ratio between the stationary Ytterbium lattice frequency standard at INRIM and the Strontium lattice frequency standard was performed, in order to qualify the accuracy of the transportable optical clock acting essentially as a quantum sensor.

Schematic representation of the measurement campaign. For more information see Nature Physics (2018) doi:10.1038/s41567-017-0042-3

In the experiment, two Menlo Systems Optical Frequency Combs, a stationary unit sitting in the laboratory in Torino and a transportable unit located in the Fréjus tunnel, provided and qualified by the team of Helen Margolis team, served as clockwork. The first comb was used to connect the radio-frequency of the Cesium clock to the optical frequencies of the Ytterbium stationary lattice clock and to the transportable Strontium lattice clock, allowing the measurement of the frequency ratios. The second comb has been used for the transfer of the SI-defined reference frequency via the optical link between the two remote locations.

The outstanding results have been published in an open-access paper issued in Nature Physics by J. Grotti et. al.: “Geodesy and metrology with a transportable optical clock”.