Vortex Power: Spinning Light Revolutionizing Quantum Computing

A new study of photons in quantum computing made a surprising discovery: When photons collide, they create vortices.

Researchers at the Weizmann Institute of Science discovered a new type of vortex formed by the photo interactions, which can progress quantum computing.

Vortex phenomena

Eddy currents are a widespread natural phenomenon, observed in swirling formations of galaxies, tornadoes and hurricanes, as well as in simpler environments such as a stirring cup of tea or water spiraling down a bathtub drain. Typically, eddies arise when a fast-moving substance such as air or water meets a slower-moving area, creating a circular motion around a fixed axis. Essentially, eddies serve to reconcile differences in flow velocity between adjacent regions.

Discovery of a new kind of Vortex

A previously unknown type of vortex has now been discovered in a study, published in science, directed by Dr. Lee Drori, Dr. Bankim Chandra Das, Tomer Danino Zohar and Dr. Gal Winer from the laboratory of Prof. Ofer Firstenberg at the Department of Complex Systems Physics of the Weizmann Institute of Science. Researchers began looking for an efficient way to use photons to process data in quantum computers and found something unexpected: They realized that in the rare case that two photons interact, they create vortices. Not only does this discovery add to the fundamental understanding of vortices, but it may ultimately contribute to the study’s original goal of improving data processing in quantum computing.

Photon interactions and quantum computing

The interaction between photons – particles of light that also behave as waves – is possible only in the presence of matter that serves as an intermediary. In their experiment, the researchers forced the photons to interact by creating a unique environment: a 10-centimeter glass cell that was completely empty, except for rubidium atoms that were packed so tightly in the center of the container that they formed a tiny gas. and dense. re about 1 millimeter long. The researchers fired more and more photons through this cloud, examined their condition after passing through it, and looked to see if they affected each other in any way.

When the gas cloud was denser and the photons were closer together, they exerted the highest level of mutual influence.

Dynamical interactions in dense gas clouds

“When photons pass through the dense gas cloud, they send a number of atoms into electronically excited states known as Rydberg states,” explains Firstenberg. “In these states, one of the electrons in atom begins to move in an orbit that is 1000 times wider than the diameter of an unexcited atom. This electron creates an electric field that affects a large number of adjacent atoms, turning them into a kind of imaginary ‘glass ball’.

The image of a glass ball reflects the fact that the second photon present in the area cannot ignore the environment created by the first photon and, in response, it changes its speed – as if it had passed through the glass. So when two photons pass relatively close to each other, they move at a different speed than they would if each had traveled alone. And when the speed of the photon changes, so does the position of the peaks and valleys of the wave it carries. In the optimal case for using photons in quantum computing, the positions of the peaks and valleys become completely inverted relative to each other, due to the influence the photons have on each other – a phenomenon known as a 180-degree phase shift.

Pioneering Research in Photon Dynamics

The direction the search took was as unique and extraordinary as the paths of photons in the gas cloud. The study, which also included Drs. Eilon Poem and Dr. Alexander Poddubny, started eight years ago and has seen two generations of PhD students pass through Firstenberg’s lab.

Over time, the Weizmann scientists managed to create a dense, ultra-cold gas cloud filled with atoms. As a result, they achieved something unprecedented: photons that underwent a phase shift of 180 degrees – and sometimes even more. When the gas cloud was denser and the photons were closer together, they exerted the highest level of mutual influence. But when the photons moved away from each other or when the atomic density around them dropped, the phase shift weakened and disappeared.

Surprising behavior of photon vortices

The prevailing assumption was that this weakening would be a gradual process, but the researchers were in for a surprise: A pair of vortices developed when two photons were a certain distance apart. In each of these vortices, the photons completed a 360-degree phase shift and, at their center, there were almost no photons at all—just like the dark center we know from other vortices.

The scientists found that the presence of a single photon affected 50,000 atoms, which in turn affected the movement of a second photon.

Insights into Photon Vortex Dynamics

To understand photon vortices, think about what happens when you pull a plate held vertically through water. The fast movement of water pushed off the plate complements the slower movement around it. This creates two eddies that, when viewed from above, appear to move together across the surface of the water, but in fact, they are part of a three-dimensional configuration known as an eddy ring: The submerged part of the plate creates half rings. , which connects the two visible vortices at the surface, forcing them to move together.

Another well-known example of vortex rings are smoke rings. In the final stages of the study, the researchers observed this phenomenon when they introduced a third photon, which added an additional dimension to the findings: The scientists discovered that the two vortices observed during the two-photon measurement are part of a three-dimensional vortex ring. created by the mutual influence of three photons. These findings show how similar the newly discovered eddies are to those known from other environments.

Advances toward quantum data processing

Vortices may have stolen the show in this study, but researchers are continuing to work toward their goal of quantum data processing. The next phase of the study will be to fire the photons at each other and measure the phase shift of each photon individually. Depending on the strength of the phase shifts, photons can be used as qubits – the basic units of information in quantum computing. Unlike regular computer memory units, which can be either 0 or 1, quantum bits can represent a range of values ​​between 0 and 1 simultaneously.

Reference: “Quantum vortices of strongly interacting photons” by Lee Drori, Bankim Chandra Das, Tomer Danino Zohar, Gal Winer, Eilon Poem, Alexander Poddubny, and Ofer Firstenberg, 13 July 2023, science.
DOI: 10.1126/science.adh5315