Researchers use light to observe quantum backflow

Researchers at the University of Warsaw’s Faculty of Physics have ingeniously superimposed two light beams, inducing anti-clockwise twists within the dark regions of their resulting combination. Published in Optica, this breakthrough bears significance for light-matter interactions, marking a stride toward unraveling the elusive quantum backflow phenomenon.

Picture throwing a tennis ball, expecting it to maintain its forward momentum. Yet, in the quantum realm, particles defy this predictability. Dr. Radek Lapkiewicz, head of the Quantum Imaging Laboratory, elucidates the complexity of quantum mechanics, where particles exist in superposition, occupying multiple positions simultaneously.

In contrast to the straightforward trajectory of a tennis ball, quantum particles exhibit backflow—a peculiar phenomenon where they may probabilistically move backward or spin in the opposite direction. This research sheds light on the intriguing behaviors within the quantum realm, challenging classical expectations.

Backflow in optics

The elusive phenomenon of backflow in quantum systems has remained elusive in experimental observations. Classical optics, however, has successfully demonstrated backflow using light beams. Theoretical contributions by Yakir Aharonov, Michael V. Berry, and Sandu Popescu explored the intriguing link between quantum backflow and the anomalous behavior of optical waves at local scales.

Pioneering the observation of optical backflow, Y. Eliezer et al. synthesized a complex wavefront. Dr. Anat Daniel and colleagues in Dr. Radek Lapkiewicz’s group later showcased this phenomenon in one dimension through the simple interference of two beams.

Dr. Anat Daniel emphasizes the fascinating oddities encountered in local scale measurements, shedding light on the peculiarities within this realm. The recent paper from the Faculty of Physics at the University of Warsaw, titled “Azimuthal backflow in light carrying orbital angular momentum,” extends the understanding of backflow into two dimensions. Dr. Lapkiewicz elaborates on the methodology, describing the superposition of two clockwise-twisted light beams leading to locally observed counterclockwise twists.

Utilizing a Shack-Hartman wavefront sensor for observation, the researchers detected positive local orbital angular momentum in the dark regions of the interference pattern. Bernard Gorzkowski, a doctoral student, clarifies this as the azimuthal backflow, showcasing the intricate nature of light’s behavior.

The roots of this exploration trace back to 1993 when Marco Beijersbergen et al. experimentally generated light beams with azimuthal phase dependence using cylindrical lenses. These beams, carrying orbital angular momentum, have since found applications in various fields, including optical microscopy and optical tweezers.

Optical tweezers, invented by Arthur Ashkin, a Nobel laureate in Physics in 2018, play a crucial role in manipulating objects at the micro- and nanoscale. Currently employed in studying the mechanical properties of cell membranes, DNA strands, and interactions between healthy and cancer cells, optical tweezers continue to contribute significantly to diverse scientific endeavors.

When physicists play Beethoven

The scientists underscore that their current demonstration can be understood as superoscillations in phase, a concept initially linked to backflow in quantum mechanics by Professor Michael Berry in 2010.

Superoscillation, a phenomenon wherein local oscillations in a superposition exceed the speed of their fastest Fourier component, was predicted in 1990 by Yakir Aharonov and Sandu Popescu. This entails specific combinations of sine waves producing regions in the collective wave that oscillate faster than any of the individual components.

In his publication “Faster than Fourier,” Michael Berry showcased the potency of superoscillation by theoretically illustrating the possibility of playing Beethoven’s Ninth Symphony using only sound waves with frequencies below 1 Hertz—frequencies imperceptible to the human ear due to their low amplitude.

Bohnishikha Ghosh emphasizes that the presented backflow is a manifestation of rapid phase changes, holding potential significance in applications involving light–matter interactions, such as optical trapping or the design of ultra-precise atomic clocks. Despite its impracticality, the study sheds light on the intricate dynamics of superoscillation.

Beyond its immediate applications, the Faculty of Physics, University of Warsaw’s publication marks a stride towards observing quantum backflow in two dimensions, theoretically deemed more robust than its one-dimensional counterpart. This exploration opens avenues for further understanding and harnessing these phenomena in diverse scientific realms.

Source: University of Warsaw

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