Researchers at the University of Warsaw's Faculty of Physics have superposed two light beams twisted in the clockwise direction to create anti-clockwise twists in the dark regions of the resultant superposition. The results of the research have been published in Optica. This discovery has implications for the study of light-matter interactions and represents a step towards the observation of a peculiar phenomenon known as a quantum backflow.

"Imagine that you are throwing a tennis ball. The ball starts moving forward with positive momentum. If the ball doesn't hit an obstacle, you are unlikely to expect it to suddenly change direction and come back to you like a boomerang," notes Bohnishikha Ghosh, a doctoral student at the University of Warsaw's Faculty of Physics. "When you spin such a ball clockwise, for example, you similarly expect it to keep spinning in the same direction."

However, everything gets complicated when, instead of a ball, we are dealing with particles in . "In , an object has a known position. Meanwhile, in quantum mechanics and optics, an object can be in the so-called superposition, which means that a given particle can be in two or more positions at the same time," explains Dr. Radek Lapkiewicz, head of the Quantum Imaging Laboratory at the Faculty of Physics, University of Warsaw.

Quantum particles can behave in quite the opposite way to the aforementioned —they may have a probability to move backwards or spin in the opposite direction during some periods of time. "Physicists call such a phenomenon backflow," Bohnishikha Ghosh specifies.

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