In this photograph, clusters of about ten thousand cesium atoms are seen floating in a vacuum chamber, levitated by crossed laser beams that create a stable optical lattice. A cylindrical tungsten weight and its support are visible at the top. Credit: Cristian Panda, UC Berkeley
Once upon a time, to study gravity, it was sufficient to let some object fall from above, as in the case of the famous experiment, attributed to Galileo, of the fall of heavy bodies from the Tower of Pisa. Today the fundamental physics questions still unanswered – and there are many – around this which remains the most irreducible of forces, the only one that still resists a theory of everything, require enormously more complex experiments. Experiments like the one prepared by a team of physicists from the University of California, Berkeley, reported this week in the pages of Nature, to search for tiny deviations from the commonly accepted theory of gravity. Deviations which, if found, could offer clues, for example, to understanding more about the nature of dark energy. Although the researchers found no deviation from Newton’s theory of gravity, the expected improvements in the experiment’s precision promise to uncover evidence to support – or refute – theories such as the one about a hypothetical particle-mediated “fifth force” so-called “chameleon”, or “symmetrons”, candidates to explain dark energy.
The experiment, carried out in the wake of other analogues that we have already written about Media Inafcombines an atomic interferometer, which allows to measure gravity precisely, with an optical lattice capable of holding small groups of atoms – in this case, groups of about ten thousand cesium atoms – in position, cooling them and trapping them with a system of laser beams, for relatively very long times, up to 70 seconds. Thus allowing to arrive at a measurement of the gravitational attraction exerted on the atoms by a small mass – a tungsten cylinder – five times more precise than the best currently available.
Schematic of the experiment performed at UC Berkeley. Small clusters of cesium atoms (pink) were immobilized in a vertical vacuum chamber, then each atom was split into two wave packets (white and blue) so that they found themselves in a quantum superposition of two “heights,” the upper “half” (white) closer to the tungsten mass (the shiny cylinder) and the other “half” (blue) lower. When the wave packets recombine, they give rise to an interference that allows the difference in gravitational attraction between the two “halves” to be measured. Credits: Cristian Panda/UC Berkeley
But how does it work? “In a first phase, the cesium atoms are cooled with laser light to a temperature close to absolute zero and trapped in luminous ‘holes’ near a small tungsten cylinder”, explains Media Inaf one of the co-authors of the study, Guglielmo Maria Tino of the University of Florence. «An atomic interferometer is then created: each atom is brought for a few seconds into a quantum state in which it is simultaneously in two different positions in which the values of the gravitational field generated by the source mass are different. When the two parts are superimposed again, a quantum interference effect is observed from which the gravitational attraction exerted on the atoms by the tungsten mass can be measured».
«Compared to previous experiments based on atomic interferometry for the study of gravitational effects, such as those conducted by my group in Florence for about twenty years now, the peculiarity of this work», continues Tino, «is in the small source mass used, hence the need to optimize the sensitivity of the atomic interferometer while controlling possible systematic effects».
The main purpose of these experiments, as we said, is to seek answers to the great unsolved problems of fundamental physics, from the nature of dark energy to the search for a quantum formulation of gravity. “Most theorists agree that gravity is quantum, but no one has ever observed an experimental signature in this regard,” recalls another of the study’s authors in this regard, Holger Müller of UC Berkeley. “If we could hold our atoms 20 or 30 times longer than ever before, we might have a 400,000 to 800,000 times greater chance of finding evidence that gravity is indeed quantum.”
The atomic grating interferometer can also be used, as a quantum sensor, for more “everyday” applications that require precision measurements of gravity. «Atom interferometry is particularly sensitive to gravity or inertial effects. It is possible to exploit it to build gyroscopes and accelerometers”, underlines the first author of the study, Cristian Pandaof UC Berkeley. “This gives a new direction to atom interferometry, where quantum sensing of gravity, acceleration, and rotation could be done with atoms held together by optical lattices in a compact structure that resists environmental imperfections or noise.”
«These devices», concludes Tino, «could be used, for example, in the search for underground cavities and mineral resources, in the monitoring of active volcanoes and in the study of earthquakes».
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