The study demonstrated the backflow of optical light

This observation is relevant to any physical system supporting coherent waves.


Quantum backflow is a counterintuitive phenomenon in which a forward-propagating quantum particle propagates locally backward. The actual counter-propagation property associated with this delicate interference phenomenon has not been observed to date in any field of physics.

In a new study, scientists from Tel Aviv University have demonstrated the backflow of optical light propagating forward.

Dr. Alon Bahabad of the Department of Physical Electronics at TAU’s School of Electrical Engineering said, “This ‘backflow’ phenomenon is quite delicate and requires precise control over the state of a particle, so its demonstration was hindered for half a century.”

“This phenomenon reveals an unintuitive behavior of a system comprised of waves, whether it’s a particle in quantum mechanics or a beam of light. Our demonstration could help scientists probe the atmosphere by emitting a laser beam and inducing a signal propagating backward toward the laser source from a given point in front of the laser source. It’s also relevant for cases in which fine control of light fields is required in small volumes, such as optical microscopy, sensing and optical tweezers for moving small particles.”

During the experiment, scientists split and reassembled a laser beam in the form of light waves that propagated at positive angles concerning an axis. The different light beams must be developed cautiously, with exact values for their strength and delay. When the superposition was made, a small slit was set and moved perpendicularly to the beam, too, as a result, measure the direction of the beam in various areas.

The light escaping from the slit was revealed in most locations as moving at a positive angle. But in some locations, the light escaping the slit propagated at a negative angle, even though the light hitting the other side of the slit was comprised of a superposition of beams all propagating at a positive perspective.

Dr. Bahabad said, “We used holography to create a clear manifestation of the backflow effect. We realized at some point that we could utilize a previous study of ours, where we discovered the mathematical phenomenon known as suboscillation, to help us design a beam of light with backflow.”

“To conclude, if interfering waves, all going in one direction, are constructed especially, and you were to measure the direction of propagation of the overall wave at specific locations and times, you might find the wave going backward. This wave can describe a particle using quantum mechanics. This surprising behavior violates any intuition that we gained from our daily experience with the movement of macroscopic objects. Nevertheless, it still obeys the laws of nature.”

The study is published in the journal Optica.

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