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Einstein's principle of equivalence passed a quantum test

Published in Nature Communications the experiment realized at the University of Florence
Einstein's principle of equivalence is of fundamental importance for the understanding of gravity and space-time. The principle implies that the inertial mass and the gravitational mass of any object are equivalent and hence that all bodies fall in the same way when subject to gravity. Thus far, the principle has been tested only for systems in a well-defined, classical state of their total mass. Now an article in Nature Communications reports the first test of the Einstein’s principle of equivalence for atoms whose total mass was in a quantum superposition state. 

The famous relativistic relation  E=mc2  means that the total mass of a system depends on its energy, and is thus called the mass-energy. In quantum theory, a system can occupy two or more different energy states at the same time — in a quantum superposition — not possible in classical physics. As a result, a quantum system can occupy different mass-energy states in superposition which has been used in the new test reported in Nature Communications. 

The experiment realised by the international team led by Prof. Guglielmo Tino at the Department of Physics and Astronomy of the University of Florence and INFN can be seen as a quantum analogue of the apocryphal test of 16th century Italian scientist Galileo Galilei who realized that various bodies fall with the same gravitational acceleration considering dropping spheres of different mass from the leaning tower of Pisa. The team of Prof. Tino measured the gravitational acceleration of rubidium atoms prepared in quantum superpositions of different internal energies and cooled to temperatures close to the absolute zero with laser light. To realise their measurements they used a new scheme developed by the group of Florence, based on Bragg atom interferometry. The experiment confirmed the validity of the Einstein’s equivalence principle for quantum superpositions with a relative precision of a few parts per billion.  The collaboration includes also experimentalists from the University of Bologna and the European Space Agency and theorists from the University of Vienna, Austrian Academy of Sciences and the University of Queensland.
 
The new scheme used in the experiment is based on quantum technologies that will lead to the development of new sensors with a variety of applications on Earth and in space: studies of volcanic eruptions and earthquakes, search for mineral deposits, geodesy, inertial navigation, high-precision measurements of time.
01 June 2017