Beams Of Light Like A Boomerang

Researchers from the Faculty of Physics at the University of Warsaw have made a discovery that may impact research on the interaction of light with matter. This is a step towards observing quantum backflow. The scientists' research results were published in the journal "Optica".

Scientists from the Faculty of Physics, University of Warsaw superimposed two beams of light "twisted" clockwise, creating twists opposite to this movement in small areas. The research results were published in the journal "Optica". The publication is a step towards observing quantum backflow in two dimensions, which theoretically should be stronger than one-dimensional backflow.

– In classical mechanics, an object has a fixed position. Meanwhile, in quantum mechanics and optics it may be in the so-called superposition of states, which means that a given object can be in two or more positions at the same time - explains Dr. Radek Łapkiewicz, head of the Quantum Imaging Laboratory at the Faculty of Physics, University of Warsaw.

It happens that particles in a superposition can suddenly change direction and move backward for a moment or temporarily spin in the opposite direction to the given direction. Physicists call this phenomenon reverse flow.

Reverse flow
Reverse flow in quantum systems has not yet been observed experimentally. However, it was observed optically. Theoretical scientific work has explored the relationship between backflow in quantum mechanics and the unusual behavior of light beams. Observed, among others : reverse flow, synthesizing a complex wavefront.

Researchers from the group of Dr. Radek Łapkiewicz observed linear reverse flow in a very simple case, i.e. superimposing two light beams. The publication Azimuthal backflow in light carry in g orbital angular momentum presents the effect of backflow in two dimensions.

We superimposed two light beams twisted clockwise and locally observed twists opposite to their motion – explains Dr. Łapkiewicz.

To measure the phenomenon, researchers used a Shack-Hartman wavefront sensor. The system consisting of a microlens array placed in front of a CMOS ( complementary metal-oxide semiconductor ) matrix ensures high sensitivity of two-dimensional spatial measurements.

We examined the superposition of two beams carrying only negative orbital angular momentum, and in the dark regions of the interference pattern we observed positive local orbital angular momentum. This is the azimuthal reverse flow, explains Bernard Gorzkowski, a PhD student at the Quantum Imaging Laboratory at the Faculty of Physics, University of Warsaw.

Superoscillations
As the scientists emphasize, the phenomenon they describe can be interpreted as superoscillations. We talk about superoscillations when, by superimposing waves with frequencies limited in advance, we can find locally higher frequencies in the resulting wave than the highest one used.