For the first time ever, scientists from the University of Ottawa have demonstrated that twisting light in a vacuum can slow it down, putting its speed below the speed of light established by Einstein's theory of relativity and opening up new opportunities for storing information in quantum computing and communications applications.

The study reveals that twisted light pulses in a vacuum travel up to 0.1 percent slower than the speed of light constant, which is 299,792,458 meters per second. While light also slows down when it travels through clear, dense material such as glass or water, this study marks the first time that the phenomenon has been observed through twisting it around its axis like a corkscrew.

"Anyone who wants to use twisted light for quantum communication should be aware of this effect," said Ebrahim Karimi, who headed the research. "If they don't compensate for the slow-light effect, information coded on twisted light might not arrive in the right order. Propagation speeds can significantly affect many protocols related to quantum communication."

The team first noticed the slower nature of twisted light during a previous experiment that compared twisted light to Gaussian laser beams.

"We realized that the two beams didn't arrive at the detector at the same time," Karimi said. "The twisted light was slower, which was surprising until we realized that the twists make the beam tilt slightly as it propagates. This tilt means that the twisted light beam doesn't take the straightest, and thus fastest, path between two points."

After the finding, the team used a modified frequency-resolved optical gating (FROG) approach to reveal that increasing the number of twists in the light further slowed it, measuring delays of up to 23 femtoseconds for twisted light beams.

"The type of precision that can be measured using FROG was not previously used in the quantum optics community, and thus scientists in this area were not aware that twisted light traveled slower than the speed of light," Karimi said.

In today's electronics, light is used to encode information using two different methods: varying the number of photons emitted and alternating between the two polarization states of light. Twisted light offers advantages over these two methods due to the fact that each twist can encode a different value or letter, meaning it can be used to encode more information with less light. 

The findings were published in the March 23 issue of the journal Optica.