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Transportable optical clock used to measure gravitation for the first time

For the first time, researchers measure gravitation with a transportable optical clock, paving the way for standardised measurement of the Earth’s surface


A European collaboration involving clock experts from the National Physical Laboratory (NPL), the Physikalisch-Technische Bundesanstalt (PTB) and the Istituto Nazionale di Ricerca Metrologica (INRIM) has used PTB's transportable optical atomic clocks to measure gravitation for the first time. The results of the experiment were published in Opens external link in new windowNature Physics.

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PTB's transportable optical atomic clock in the Modane Underground Laboratory (LSM). (Photo: Lisdat/PTB)

The vibration-damped and temperature-stabilised trailer with the transportable atomic clock in the Modane Undergound Laboratory. (Photo: Lisdat/PTB)

Inside view into the trailer with the transportable optical atomic clock. (Photo: Physikalisch-Technische Bundesanstalt (PTB))

Until now, such delicate clocks have been restricted to laboratories at a few major research institutions, however, researchers at PTB Opens external link in new windowhave developed a transportable strontium optical lattice clock, opening up the possibility of performing measurements in the field.

The transportable clock was driven in a vibration-damped and temperature-stabilised trailer to the French Modane Underground Laboratory (LSM). Operated by Centre National de la Recherche Scientifique and Grenoble-Alpes University, the multidisciplinary lab is located in the middle of the Fréjus road tunnel between France and Italy.

There, the team measured the gravity potential difference between the exact location of the clock inside the mountain and a second clock at INRIM– located 90 km away in Torino, Italy, at a height difference of about 1,000 m.

The accurate comparison of the two clocks was made possible using a 150 km long optical fibre link, set up by INRIM, and a frequency comb from NPL, to connect the clock to the link. Researchers from Leibniz Universität Hannover also determined the gravity potential difference using conventional geodetic techniques, and the two measurements were shown to be consistent.

With improvements to the accuracy of the transportable optical clock, this technique has the potential to resolve height differences as small as 1 cm across the Earth’s surface. The advantage of using optical clocks is that they can make measurements at specific points, in contrast to satellite-based measurements, such as GRACE and GOCE, which average the gravity potential over length scales of about 100 km.

This novel method could lead to higher resolution measurements of the Earth’s gravity potential, allowing scientists to monitor, with unprecedented accuracy, continental height changes related to sea levels and the dynamics of ocean currents. It will also lead to more consistent national height systems.

Currently, different countries measure the Earth’s surface in the same way, but relative to different reference levels. This has led to problems – one such being the Hochrhein Bridge between Germany and Switzerland, where construction on each side used different sea level calculations, leading to a 54 cm discrepancy between the two sides.

Achieving consistency between national height systems will help to prevent costly mistakes from happening in engineering and construction projects. Improved measurements of gravity potential may also help to improve our understanding of geodynamic effects associated with mass changes under the Earth’s surface. This type of measurement of height will also help us to monitor changing sea levels in real-time, allowing us to track seasonal and long-term trends in ice sheet masses and overall ocean mass changes. Such data provides critical input into models used to study and forecast the effects of climate change.

Christian Lisdat, Leader of the group ‘Optical Lattice Clocks’, at PTB, said:
“Optical clocks are deemed to be the next generation atomic clocks – operating not only in laboratories but also as mobile precision instruments. "
“This cooperation proves again how disciplines such as physics or metrology, geodesy and climate impact research can mutually benefit each other.”

Helen Margolis, Fellow in Optical Frequency Standards and Metrology at NPL, said:
“Our proof-of-principle experiment demonstrates that optical clocks could provide a way to eliminate discrepancies and harmonise measurements made across national borders. "
“One day such technology could help to monitor sea level changes resulting from climate change.”

Davide Calonico, Leader of the group ‘Optical Lattice Clocks and Fibre links’, at INRIM, said:
“We demonstrated that optical clocks are valuable quantum sensors, and their quantum technology is beneficial outside primary metrology, in geodesy.”
“Together, optical clocks and optical fiber links offer the possibility to access new and fascinating scientific investigation”

Heiner Denker, Principal Investigator for Relativistic Geodesy and Gravimetry within geo-Q, at Leibniz Universität Hannover, said:
“The newly developed optical clocks have the potential to revolutionise geodetic height determination, as they can overcome some of the limitations of classical geodetic techniques."
“Optical clocks could help to establish a unified world height reference system with significant impact on geodynamic and climate research.”

‘Geodesy and metrology with a transportable optical clock’ Grotti et al., Nature Physics, DOI: Opens external link in new windowhttp://dx.doi.org/10.1038/s41567-017-0042-3

About NPL
NPL is the UK’s National Measurement Institute, providing the measurement capability that underpins the UK's prosperity and quality of life. From new antibiotics to tackle resistance and more effective cancer treatments, to secure quantum communications and superfast 5G, technological advances must be built on a foundation of reliable measurement to succeed. Building on over a century’s worth of expertise, our science, engineering and technology provides this foundation and helps to make the impossible possible. We save lives, protect the environment and enable citizens to feel safe and secure, as well as support international trade and commercial innovation. As a national laboratory, our advice is always impartial and independent, meaning consumers, investors, policymakers and entrepreneurs can always rely on the work we do. Based in Teddington, south-west London, NPL employs over 600 scientists. NPL also has regional bases across the UK, including at the University of Surrey, the University of Strathclyde, the University of Cambridge and the University of Huddersfield's 3M Buckley Innovation Centre. (Opens external link in new windowwww.npl.co.uk, Opens external link in new windowhttps://twitter.com/NPL, Opens external link in new windowhttps://www.facebook.com/npldigital, Opens external link in new windowhttps://www.linkedin.com/company/national-physical-laboratory)

About PTB
The Physikalisch-Technische Bundesanstalt, Germany's national metrology institute, is a scientific and technical higher federal authority falling under the competence of the Federal Ministry for Economic Affairs and Energy. In Braunschweig and Berlin, time is measured with atomic clocks, length is measured – even deeply in the nano-world, scientists investigate fundamental questions related to physical units, and we calibrate measurement devices to satisfy highest requirements on accuracy. Physikalisch-Technische Bundesanstalt thus belongs to the first addresses in the international world of metrology. As national metrology institute, PTB is Germany's highest authority when it comes to correct and reliable measurements. PTB is a higher federal authority of the Federal Ministry for Economic Affairs and Energy and employs about 1900 staff. (www.ptb.de)

About Leibniz Universität Hannover
Founded in 1831 by the scholar Karl Karmarsch, the “Higher Trade School of Hannover” started with only 64 students. Today, Leibniz Universität Hannover is the second largest university in Lower Saxony with around 28,700 students. More than 3,000 scientists work on nine faculties with more than 160 institutes. The university offers a wide range of study opportunities with around 90 study subjects and more than 180 study and part-study courses. The spectrum ranges from engineering and natural sciences, architecture and environmental planning, law, social sciences, economics and business administration to the humanities. Only few universities in Germany are able to offer a comparable range of subjects. In research, international and interdisciplinary aspects of mechanical engineering, physics and biomedical engineering are continuously being expanded. (Opens external link in new windowhttps://www.uni-hannover.de/en/)

The Istituto Nazionale di Ricerca Metrologica (INRIM), based in Turin, is a public research centre acting as Italy's national metrology institute (NMI) and employs 200 people. INRIM realises, maintains and develops the national reference standards of the measurement units of the International System (SI). Basic and applied research in different fields - such as materials science, nanoscience, quantum optics, studies on the fundamental constants of physics - as well as the development of new measurement technologies and instruments enhance the metrology activity. INRIM promotes education, in particular through a PhD course in Metrology. In order to meet the needs of industry, the institute has a dedicated department at direct contact with the world of production and providing consultancy, calibration, and testing services. (Opens external link in new windowwww.inrim.it)