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When a particle of light sees the light

The institute's scientists were looking for how to establish an efficient method for processing information in quantum computers - and unexpectedly discovered a new type of vortices that are created when two photons collide

The institute's scientists were looking for how to establish an efficient method for processing information in quantum computers - and unexpectedly discovered a new type of vortices that are created when two photons collide
A ring and vortex lines formed by the interaction between three photons. The color describes the phase of the electric field, which makes a complete rotation (360 degrees) around the vortex

Vortices are a well-known physical phenomenon that can be found in the structure of galaxies, in tornadoes and hurricanes, in a cup of tea or in the water draining from the bathtub. Most often, eddies are formed at a sudden meeting between an area of ​​very fast movement and an area of ​​slow movement, and are characterized by a circular flow around a stationary focus. Therefore, the role of the eddies is to bridge the tension between close environments that each have a different flow velocity.

Vortices of a type that was not known until now were revealed in the study of Dr. Lee Drori, Dr. Bankim Chandra Des, Tomer Danino Zohar and Dr. Gal Viner fromThe laboratory of Prof. Ofer Furstenberg from the Department of Physics of Complex Systems at the Weizmann Institute of Science. The researchers set out with the aim of establishing an efficient method for processing information in quantum computers using photons - and unexpectedly discovered eddies that are created by the meeting, rare in itself, between two photons.

Interaction between photons - light particles that are also characterized by the properties of waves - is possible only through the mediation of matter. As part of the experiment, the researchers brought photons together by creating a unique environment: a glass container about 10 cm long that was completely empty, except for rubidium atoms compressed in the center into a small, dense cloud of gas about a millimeter long. The researchers shot more and more photons into the compressed gas cloud, measuring their condition after passing through it and looking for signs of mutual influence between them.

Prof. Firstenberg explains: "The photons that enter the dense gas cloud excite a series of atoms into exotic states called Rydberg states. In these situations, one of the electrons in the atom moves in a circle whose diameter is more than a thousand times the diameter of the atom. This electron produces an electric field that affects many atoms around it and turns them into a sort of imaginary 'glass ball'." The image of a glass ball is meant to reflect the fact that a second photon caught in the scene cannot ignore the situation created around it by the first photon, and in response it changes its speed as if it had passed through glass. Therefore, when two photons pass relatively close to each other, their speed is different from what it would have been had each of them moved forward alone. With the speed of the photon, the rate at which it reaches its peak and trough also changes. In the ideal case, the positions of the high and low points of the photons are completely reversed due to their mutual influence, a phenomenon known as a 180 degree phase shift.

The path of the photons in the gas cloud is unique, and the path of the research itself was also unusual. The research, in which Dr. Elon Foum and Dr. Alexander Podobny also participated, began eight years ago, during which two generations of doctoral students changed in Prof. Furstenberg's laboratory. Gradually, the institute's scientists succeeded in creating a very cold gas cloud, compressed and denser in atoms - and reached the achievement on a global scale of photons undergoing a phase change of 180 degrees, and even beyond that. At the peak of the density of the gas cloud and when the photons are close to each other - the intensity of the effect between them is maximum. But when the photons move away from each other or when the density of the atoms around them decreases - the phase change weakens and disappears. One could assume that this decrease in intensity would be gradual and nothing more, but then the unexpected result was discovered: a pair of vortices that develop when the two photons are at a certain distance from each other. In each of the vortices, the photons complete a full phase change of 360 degrees, and in their center there are almost no photons - as in the dark focus known from other vortices.

These photon eddies are similar to the situation created when a plate is pushed through water, and the fast movement of the water as a result of pushing the plate meets the slow movement around it. Then two eddies are created that are visible above the surface of the water as they move together. In the final stages of the research, the institute's scientists added a third photon to the experiment, which showed how similar the eddies they found were to those known from other environments. The third photon added a third dimension to the findings, and thus it was discovered that the two vortices observed in the measurement of two photons are actually part of a well-known phenomenon of a vortex ring. For example, the part of the plate submerged in the water forms a half ring that connects the two eddies that are visible above the surface of the water and causes them to move together. Another familiar case of a ring of vortices is revealed in the bloom of a smoke ring, which actually consists of many vortices moving together in the air.

The institute's scientists were looking for how to establish an efficient method for processing information in quantum computers - and unexpectedly discovered a new type of vortices that are created when two photons collide
The institute's scientists were looking for how to establish an efficient method for processing information in quantum computers - and unexpectedly discovered a new type of vortices that are created when two photons collide

Although the vortices unexpectedly stole the show, the researchers continue towards the goal of quantum information processing. In follow-up experiments, they plan to send the photons in front of each other to measure the phase change of each individual photon. Depending on the intensity of the phase change of the photon, it could be used as a qubit - a memory unit of a quantum computer, which, unlike a normal memory that is only in a state of 0 or 1, can also be in intermediate states.

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