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PHYSICS

The effects of quantum turbulence

Disturbing a cloud of cold rubidium atoms produces a wave phenomenon similar to light

When subjected to the specific conditions of quantum systems, the same atoms that form everything from stars to living beings, to a piece of paper, cease to behave as particles and begin to manifest as waves. In this state, matter can exhibit effects that contradict traditional assumptions, and atoms can cross barriers previously thought impassable. In recent experiments, a team coordinated by researchers from the São Carlos Institute of Physics at the University of São Paulo (IFSC-USP) found that a supercooled and confined rubidium atom cloud preserves aspects of its undulatory behavior even after being disturbed by the generation of vortices, progressing into a state of quantum turbulence. Atoms have already been well studied under these conditions, but the effects of introducing a major disturbance into this type of system were previously unknown.

In an article published in the journal PNAS in October, the researchers report that a speckle pattern emerges as the cloud expands, which is characteristic of wave interference, such as when laser light is projected onto a screen, which creates a two-dimensional speckle; in the cooled rubidium atom cloud, the speckles were three-dimensional. “This is the first observation of three-dimensional speckles,” explains physicist Vanderlei Bagnato from IFSC-USP, one of the authors of the article and coordinator of the Optics and Photonics Research Center (CEPOF), one of the FAPESP Research, Innovation, and Dissemination Centers (RIDC).

The USP group studied a trapped superfluid called Bose-Einstein condensate, formed by a cloud of hundreds of thousands of rubidium atoms enclosed in a magnetic trap. Quantum phenomena begin to manifest when the matter is cooled to temperatures of roughly 1 millionth of a degree above absolute zero, known as zero Kelvin, or -273.15 degrees Celsius. Under this condition, the grouped atoms lose all viscosity and become a superfluid, which is another state of matter. To introduce quantum turbulence into the cloud, the physicists used a magnetic field to induce the formation of vortices. They then turned off the trap and observed the condensate as it expanded. It was during this moment of instability that the speckle was generated.

The São Carlos team has pioneered the introduction of turbulence into a Bose-Einstein condensate. In partnership with colleagues from the University of Florence, Italy, the researchers from São Paulo showed in 2009 that condensate offers a simpler means of studying turbulence in superfluids than the traditionally used liquid helium. The new study provides opportunities for further investigation into the behavior of speckles and quantum turbulence. “This is a new situation in physics, which could reveal as yet unknown effects,” says Pedro Ernesto Tavares, the lead author of the PNAS article. The two-dimensional granular pattern currently generated by lasers is widely used for studying material surfaces.

Project
CEPOF – Optics and Photonics Research Center (No. 13/07276-1); Grant Mechanism Research, Innovation, and Dissemination Centers (RIDC); Principal investigator Vanderlei Salvador Bagnato (USP); Investment R$28,014,802.21 (for the entire project).

Scientific article
TAVARES, P. E. S. et al. Matter wave speckle observed in an out-of-equilibrium quantum fluid. Proceedings of the National Academy of Sciences (PNAS). V. 114, No 8, pp. 12691–95. 28 Nov. 2017.