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Quantum acceleration

Clouds of cold atoms can be used to measure subtle variations in the force of gravity

Bose_frutaLÉO RAMOSPhysicist Philippe Courteille and his colleagues at the São Carlos Physics Institute (IFSC) of the University of São Paulo (USP) are building an instrument to measure the effect of Earth’s gravitational force on what is known as Bose-Einstein condensate—microscopic clouds composed of about 100,000 strontium atoms maintained at temperatures close to absolute zero (-273.15°C)—with high precision. This device—an atomic gravimeter—should allow real-time calculation of the intensity of the gravitational force on a microscopic scale, something that has not yet been measured well. There are other, similar instruments in the world, also with sensitivity good enough to measure gravitational forces on this scale. But existing devices only reconstruct the motion of atoms afterwards and cannot follow them in real time, as the researchers in São Carlos promise to do. They believe that the new gravimeter will have practical and fundamental physics applications.

Other experiments with atomic gravimeters—some already carried out, others in progress—have measured the gravitational force on microscopic scales. Even so, they have still not achieved the same degree of precision obtained for the other fundamental physical forces. “There are theories that predict that Newton’s law of gravity may not apply to distances under a few micrometers,” says Courteille. The law of gravity states that the force of attraction between two bodies is inversely proportional to the square of the distance between them and explains observations in the macroscopic world very well. “We may need to modify this law of attraction to explain what occurs at the microscopic level,” says the physicist.

The practical applications of the new gravimeter will depend on its sensitivity. If it is quite high, the device can be used to map oil and ore reserves. Courteille is not yet able to establish the exact degree of sensitivity that his instrument will be able to achieve, but estimates that it should be able to surpass the best commercial high-precision gravimeters, which use laser beams to measure the acceleration of gravity acting on a small mirror in free fall in a vacuum. Geophysicists use this type of equipment to map underground reserves with economic value. Tiny variations in Earth’s gravitational acceleration allow detection of differences in subterranean rock densities, indicating the presence of ore.

060-061_Bose-Einstein_233Courteille has already finished the most important part of the gravimeter: the annular optical cavity. It consists of three small, special mirrors placed at the vertices of a triangle, about 2 centimeters apart from each other. It is these carefully designed and arranged mirrors that should ensure the success of the future device, according to articles published in the journals Optics Express and Laser Physics Letters. Computer simulations performed by Courteille and Romain Bachelard, of IFSC, in partnership with Marina Samoylova and Nicola Piovella of the University of Milan, Italy, and Gordon Robb of the University of Strathclyde in the UK, indicate that the optical cavity should improve gravimeter operation for two reasons. The first is that the cavity should prevent the destruction of the condensate by the laser beam that interacts with it to measure its displacement. The second is that it should stabilize the condensate’s oscillations, making them more regular and predictable. The researchers submitted a patent application for the device to the Brazilian Industrial Property Institute (INPI) in 2015.

In free fall
Physicists have been performing experiments using cold atoms as gravimeters since the late 1990s. When cooled to temperatures close to absolute zero, some types of atoms can coalesce and form what is called a Bose-Einstein condensate. In the condensate, the atoms no longer act as individual particles and begin to move together, forming a cloud of identical atoms—physicists say that they behave like a single wave of matter. Several atomic gravimeters built so far measure how the properties of this cloud of atoms change as it moves exclusively under the influence of gravity. To analyze the action of just the gravitational force, physicists generate this cloud of atoms inside a vacuum chamber and let it move vertically toward the ground. In this experiment, similar to an elevator in free fall, the cloud falls with nothing to slow it down; the only force acting is gravity.

Courteille’s gravimeter, however, works differently, similar to the device developed in 2005 by the team of physicist Massimo Inguscio at the University of Florence, Italy. In the experiment performed by the Italian, the Bose-Einstein condensate falls freely up to a certain point. When gravitational acceleration makes the condensate reach a certain speed, it interacts with a wave of light created by the intersection of two laser beams. At that moment, the condensate is hit by an impulse from the light wave and starts to move upwards, a process that is repeated indefinitely. “It’s as if the wave of matter is jumping on a trampoline,” explains Courteille. “The frequency of the jumps depends on Earth’s gravitational acceleration.”

By using three mirrors to create an optical cavity in which the laser beams remain trapped, circulating almost indefinitely, Courteille was able to eliminate some of the drawbacks of the Italian experiment. Inguscio’s gravimeter used a third laser to measure the displacement of the condensate, which destroyed it. In Courteille’s set-up, the environment is controlled and the light of the third laser, even if it interacts with the condensate, does not disorder it. Under Courteille’s supervision, the physicist Raul Teixeira, who is a post-doctoral researcher at IFSC, is building the gravimeter’s vacuum chamber and preparing the assembly of the lasers and the optical cavity. “It’s a huge technical challenge,” says Courteille. “We won’t have scientific results for at least two years.”

1. Development of quantum sensors based on ultracold atoms (nº 2013/04162-5); Grant Mechanism Thematic Project; Principal Investigator Philippe Wilhelm Courteille (IFSC-USP); Investment R$1,988,250.00 (FAPESP – for the entire project).
2. Continuous monitoring of Bloch oscillations of ultracold atoms for application in gravimetry (nº 2014/12952-9); Grant Mechanism Scholarships in Brazil – Postdoctoral-; Recipient Raul Celestrino Teixeira; Principal Investigator Philippe Wilhelm Courteille (IFSC-USP); Investment R$177,860,00 (FAPESP).

Scientific Articles
SAMOYLOVA, M. et al. Synchronization of Bloch oscillations by a ring cavity. Optics Express. V. 23, No. 11. May 28, 2015.
SAMOYLOVA, M. et al. Mode-locked Bloch oscillations in a ring cavity. Laser Physics Letters. V. 11, No. 12. Nov. 12, 2014.