By IQT News posted 29 Dec 2021

( Today novel developments in methods using ultracold atoms and laser technologies open enhanced prospects for applying quantum physics in both satellite and terrestrial geodesy—the science of measuring the shape, rotation, and gravity of Earth—and for improving measurement reference systems. Such methods have great potential for more accurately monitoring how the Earth system is responding to natural and human-induced forcing, from the planet’s solid surface shifting in response to tectonic and magmatic movements to sea level rising in response to melting glaciers.
The past few years have seen new efforts to develop such technologies for many uses. In 2018, for example, the European Commission began a long-term research and innovation initiative called Quantum Flagship. For geodetic applications, efforts are being coordinated and supported largely through the Novel Sensors and Quantum Technology for Geodesy (QuGe) program, a worldwide initiative organized under the umbrella of the International Association of Geodesy and launched in 2019.

QuGe emphasizes three pillars of development.
1 The first focuses on investigations of ultracold-atom technologies for gravimetry on the ground and in space.
Measuring Earth’s gravity field from space requires precisely monitoring the changing distance between paired orbiting satellites—as in the GRACE-FO mission—which accelerate and decelerate slightly as they are tugged more or less by the gravitational pull exerted by different masses on Earth.
The performance of these traditional accelerometers is thus challenged by quantum sensors, which have already demonstrated improved long-term stability and lower noise levels on the ground. In addition, hybrid systems combining the benefits of quantum accelerometers with electrostatic accelerometers, which still provide higher measurement rates, could cover a wider range of slower and faster accelerations and could greatly support navigation and inertial sensing on the ground and in space. Quantum accelerometers will also serve as a basis for developing the next generation of gravity-monitoring missions.

2 The second pillar of QuGe focuses on improving technology for laser interferometric ranging between spacecraft to achieve nanometer-scale accuracy, which will become the standard for future geodetic gravity-sensing missions. This method involves comparing the difference in phase between two laser beams: a reference beam and a test beam received back from the second satellite. Such optical measurements are much more precise than similar measurements using microwave ranging or mechanical devices, allowing intersatellite distances to be observed with an accuracy of tens of nanometers or better compared with micrometer accuracies achieved with microwaves.

3 QuGe’s third pillar of development focuses on applying general relativity and optical clocks to improve measurement reference systems. Einstein told us that gravity distorts space and time. In particular, a clock closer to a mass—or, say, at a lower elevation on Earth’s surface, closer to the planet’s center of mass—runs slower than one farther away. Hence, comparing the ticking rates of accurate clocks placed at different locations on Earth informs us about height differences, a technique called chronometric leveling. This technique has been achieved by comparing outputs from highly precise optical clocks connected by optical links over distances on the order of 1,000 kilometers.

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