When Photons Collide: Crashing Photons Create Vortices — And a Possible New Paths to Quantum Computing

Insider Brief

• Weizmann Institute of Science researchers say when photons interact, they create vortices — and possibly new ways to improve quantum computing.
• Vortices are characterized by circular flow around a stationary axis.
• The team reports that the next step is to see if the photons could be used as qubits.
• Image: A vortex ring and lines created by the influence of three photons on one another. The color describes the phase of the electric field, which completes a 360-degree rotation around the vortex core. Credit — Weizmann Institute of Science

PRESS RELEASE — Vortices are a common physical phenomenon. You find them in the structure of galaxies, tornadoes and hurricanes, as well as in a cup of tea, or water as it drains from the bathtub. Normally, vortices are formed when very fast-flowing air, water or another substance encounters an area with a slower flow. They are characterized by circular flow around a stationary axis. The role of vortices, therefore, is to bridge the tension between adjacent areas with different speed flows.

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

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

When the gas cloud was at its densest and the photons were close to each other, they exerted the highest level of mutual influence

“When the photons pass through the dense gas cloud, they send a number of atoms into electronically excited states known as Rydberg states,” Firstenberg explains. “In these states, one of the electrons in the atom starts moving in an orbit that is 1,000 times wider than the diameter of an unexcited atom. This electron creates an electric field that influences a huge 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 the first photon has created and, in response, it alters its speed – as if it has passed through glass. So, when two photons pass relatively close to each other, they move at a different speed than they would have if each had been traveling 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 the use of photons in quantum computing, the positions of the peaks and valleys become completely inverted relative to one another, owing to the influence the photons have on each other – a phenomenon known as a 180-degree phase shift.

The direction that the research took was as unique and extraordinary as the paths of the photons in the gas cloud. The study, which also included Dr. Eilon Poem and Dr. Alexander Poddubny, began eight years ago and has seen two generations of doctoral students pass through Firstenberg’s laboratory. Over time, the Weizmann scientists managed to create a dense, ultracold gas cloud, packed with atoms. As a result, they achieved something unprecedented: photons that underwent a phase shift of 180-degrees – and sometimes more. When the gas cloud was at its densest and the photons were close to each other, they exerted the highest level of mutual influence. But when the photons moved away from each other or the atomic density around them dropped, the phase shift weakened and disappeared.

The prevalent assumption was that this weakening would be a gradual process, but 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 as in the dark center we know from other vortices.